US20100244127A1 - Bandgap engineered mos-gated power transistors - Google Patents

Bandgap engineered mos-gated power transistors Download PDF

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US20100244127A1
US20100244127A1 US12/795,157 US79515710A US2010244127A1 US 20100244127 A1 US20100244127 A1 US 20100244127A1 US 79515710 A US79515710 A US 79515710A US 2010244127 A1 US2010244127 A1 US 2010244127A1
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gate
trench
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transistor
sige
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Ihsiu Ho
Qi Wang
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Semiconductor Components Industries LLC
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Fairchild Semiconductor Corp
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    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
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    • H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
    • H01L29/7802—Vertical DMOS transistors, i.e. VDMOS transistors
    • H01L29/7813—Vertical DMOS transistors, i.e. VDMOS transistors with trench gate electrode, e.g. UMOS transistors
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    • H01L29/1025—Channel region of field-effect devices
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    • H01L29/1033—Channel region of field-effect devices of field-effect transistors with insulated gate, e.g. characterised by the length, the width, the geometric contour or the doping structure
    • H01L29/1054—Channel region of field-effect devices of field-effect transistors with insulated gate, e.g. characterised by the length, the width, the geometric contour or the doping structure with a variation of the composition, e.g. channel with strained layer for increasing the mobility
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    • H—ELECTRICITY
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    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
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    • H01L29/49—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET
    • H01L29/4916—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET the conductor material next to the insulator being a silicon layer, e.g. polysilicon doped with boron, phosphorus or nitrogen
    • H01L29/4925—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET the conductor material next to the insulator being a silicon layer, e.g. polysilicon doped with boron, phosphorus or nitrogen with a multiple layer structure, e.g. several silicon layers with different crystal structure or grain arrangement
    • H01L29/4933—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET the conductor material next to the insulator being a silicon layer, e.g. polysilicon doped with boron, phosphorus or nitrogen with a multiple layer structure, e.g. several silicon layers with different crystal structure or grain arrangement with a silicide layer contacting the silicon layer, e.g. Polycide gate
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    • H01L29/66007—Multistep manufacturing processes
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    • H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
    • H01L29/66409—Unipolar field-effect transistors
    • H01L29/66477—Unipolar field-effect transistors with an insulated gate, i.e. MISFET
    • H01L29/66674—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
    • H01L29/66712—Vertical DMOS transistors, i.e. VDMOS transistors
    • H01L29/66734—Vertical DMOS transistors, i.e. VDMOS transistors with a step of recessing the gate electrode, e.g. to form a trench gate electrode
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    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/778—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface
    • H01L29/7782—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with confinement of carriers by at least two heterojunctions, e.g. DHHEMT, quantum well HEMT, DHMODFET
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    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates generally to power transistors, and more specifically to metal-oxide-semiconductor-gated (MOS-gated) power transistors with silicon-germanium (SiGe) sources, wells, channels, poly silicon-germanium gates, or a combination thereof.
  • MOS-gated metal-oxide-semiconductor-gated
  • Trench-gated power MOSFET devices are popular choices for use in several demanding applications, such as DC-to-DC converters. These applications can be very harsh, putting a tremendous strain on these transistors. For example, sourcing and sinking large currents into an inductive load can result in large voltage transients at one or more of the device's terminals. Specifically, a large voltage excursion seen by a trench-gated power MOSFET can activate a parasitic npn transistor, leading to a destructive failure. A less catastrophic, but still performance-degrading event, can occur when a large transient forward-biases a device's body diode, slowing the transistor's reverse recovery.
  • embodiments of the present invention provide devices, methods, and processes that improve immunity to transient voltages and reduce parasitic impedances.
  • Embodiments of the present invention provide transistors having improved immunity to unclamped inductive switching and other transient voltage events.
  • an exemplary embodiment of the present invention provides a trench-gated power MOSFET device having a SiGe source.
  • a SiGe source reduces the gain of a parasitic npn transistor by reducing hole current in the body or well region, thereby decreasing the likelihood of a latch-up condition following an unclamped inductive switching event.
  • the body tie on this device can also be eliminated in order to reduce transistor cell size.
  • Another exemplary embodiment of the present invention provides a trench-gated power MOSFET device having a SiGe body or well region.
  • a SiGe body reduces hole current when the body diode is turned on, thereby reducing the power lost during its reverse recovery.
  • Embodiments of the present invention may also improve device characteristics. For example, an exemplary embodiment of the present invention reduces parasitic gate impedance by incorporating a poly SiGe gate. Another embodiment provides a channel having reduced channel resistance through the use of a SiGe layer near the device's gate.
  • Improvements that boost immunity to voltage transients and those that enhance device performance are not exclusive.
  • the use of a SiGe body decreases channel impedance, while a SiGe layer in the body region improves body diode reverse recovery.
  • These embodiments may be used to improve n-channel or p-channel devices.
  • Embodiments of the present invention may incorporate one of more of these or the other features described herein.
  • FIG. 1 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention
  • FIG. 2 illustrates a cross section of an n-channel trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention
  • FIG. 3 illustrates the energy band structure through the source-to-well region of the device illustrated in FIG. 2 ;
  • FIG. 4 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention
  • FIG. 5 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention
  • FIG. 6 is a cross section of an n-channel trench-gate power MOSFET having a SiGe well according to an embodiment of the present invention
  • FIG. 7 illustrates the net doping and germanium mole fraction of the MOSFET shown in FIG. 6 ;
  • FIG. 8 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe well region according to an embodiment of the present invention
  • FIG. 9 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention.
  • FIG. 10 is a cross section of an n-channel trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention.
  • FIG. 11 illustrates material resistivity as a function of boron concentration for various materials
  • FIG. 12 illustrates a DC-DC converter output including a p-channel high-side power-MOSFET device and an n-channel low-side power-MOSFET device;
  • FIG. 13 illustrates the increase in efficiency provided by the incorporation of an embodiment of the present invention
  • FIG. 14 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention.
  • FIG. 15 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
  • FIG. 16 shows the hole mobility measured of two different Si 1-x Ge x films grown on relaxed Si 1-x Ge x virtual substrate
  • FIG. 17 illustrates a change in mobility as a function of germanium concentration in a channel region
  • FIG. 18 illustrates a valence band offset for a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention
  • FIGS. 19A-19C illustrate a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
  • FIG. 20 is a flowchart of a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
  • FIG. 1 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention.
  • This device includes n-type source regions 110 , a body formed by p-well 120 , n-type drain region 130 , substrate 160 , gates 140 , and metal contact 150 .
  • a parasitic npn bipolar transistor is inherent in this structure. Specifically, the parasitic device's emitter is the source region 110 , its base is the body or well region 120 , while the parasitic device's collector corresponds to the epi region 130 . Destructive failure can occur if this parasitic transistor is biased into its forward active mode of operation. This can occur, for example, during avalanche breakdown induced by an unclamped inductive switching (UIS) event. Holes generated by impact ionization can flow through the base formed by the P-Well 120 causing an ohmic voltage drop. If this voltage drop exceeds about 0.6V, the turn-on voltage for the well-to-source junction diode can become forward biased and inject electrons over the potential barrier, thus initiating forward active operation and potential failure.
  • UAS unclamped inductive switching
  • an unclamped inductive switching event can pull the source low (1). This causes holes to flow into the body or well 120 (2).
  • the well charges up, or increases in voltage compared to the source 110 , turning on the p-well 120 to source 110 junction (3).
  • the result is electron current to flow into the source 110 (4). This causes the parasitic npn to turn on (5), which can lead to device failure.
  • This propensity for this series of events to occur can be minimized by providing a low resistance path for the removal of holes from the parasitic base and by reducing the current gain of the parasitic npn 160 . This may be achieved by contacting the body or p-well with the source connection 150 .
  • a solution to this problem is to employ band-gap engineering techniques to narrow the energy gap of the source region with respect to the well region. This can be achieved by forming the source region using an alloy of Si 1-x Ge x , where x is the mole fraction of germanium in the alloy, with typical values 0.1 ⁇ x ⁇ 0.3. In other embodiments, other concentrations can be used, for example the concentration of germanium can be less than 10 percent, or greater than 30 percent.
  • the SiGe source region can be fabricated by standard techniques such as epitaxial growth or by ion implantation.
  • FIG. 2 illustrates a cross section of an n-channel trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention.
  • This device includes an n-type source region 210 that is formed from silicon germanium, a body formed by p-well 220 , n-type drain region 230 , gates 240 , metal contact 250 , and n substrate 260 .
  • n-well contact allows a significant reduction in cell pitch. This has the additional benefit of reducing on-state resistance and improving current conduction capability by increasing the size of the source region relative to the device of FIG. 1 .
  • n-channel transistor is shown, embodiments of the present invention can similarly be used to improve p-channel devices.
  • the p-well 220 may also be SiGe, while the gate may be poly SiGe.
  • SiGe in the source 210 allows for holes to flow to the source, reducing the hole current in the body or p-well 220 .
  • the presence of germanium in the alloy creates a narrowing of the energy gap (Eg) that is roughly proportional to the germanium mole fraction. Most of this band offset occurs in the valence band. This allows holes to easily flow to the source region, providing a path for hole removal reducing the npn gain. Now when an unclamped inductive event pulls the source low (1), the holes flow to the source 210 , and the well does not charge (2).
  • FIG. 3 illustrates the energy band structure through the source-to-well region of the device illustrated in FIG. 2 .
  • FIG. 3 was calculated for a germanium mole fraction of 30%. The data show approximately a 0.21 eV reduction of the energy gap of the source with respect to the well, that is, 0.9 eV in the source compared with 1.1 eV in the well or body region. In theory, this leads to an increase in the hole current crossing the junction by a factor ⁇ exp(Eg/kT). In practice, the improvement is somewhat less than this, since not all the energy band offset occurs in the valence band.
  • the source 210 has a germanium content of 30 percent by mole fraction.
  • a comparison between devices having a silicon source and this SiGe source shows an increase in hole current and reduction in the npn current gain for a device using a SiGe source region. Specifically, at a typical bias of 0.75V, the hole current is increased by about 100 ⁇ and the parasitic current gain is reduced by a factor of about 500. This data should be considered worst case, since in practice the well region would not be left entirely floating, but would be remotely contacted in the third dimension (perpendicular to the illustrated cross-section of FIG. 2 ). This reduction in parasitic current gain makes it less likely that the parasitic transistor 160 would conduct current leading to a catastrophic failure during a UIS event.
  • FIG. 4 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention.
  • a SiGe source is formed in order to achieve a greater immunity to failure caused by an unclamped inductive switching event.
  • an epitaxial layer is formed.
  • a SiGe layer is formed in the epitaxial layer in act 420 .
  • a trench is cut in the epitaxial layer in act 430 , while in act 440 , a gate is formed in the trench. The gate is contacted with metal, and the SiGe layer is contacted to form a source region in acts 450 and 460 .
  • trench-gate power MOSFETs are particularly useful in low-voltage DC-to-DC converter applications due to their low conduction losses and fast switching time. But the source being pulled low is not the only inductive effect these devices are exposed to. During normal operation, the MOSFET source can become positively biased with respect to the drain, leading to turn-on of the parasitic body-to-drain diode (also commonly referred to as the body diode). It should also be noted that while this and the other embodiments of the present invention are well suited to DC-to-DC power converters, other types of systems may incorporate devices that are improved by the incorporation of embodiments of the present invention.
  • FIG. 5 illustrates this mechanism.
  • This device includes n-type source regions 510 , p-type body region 520 , n-type drain region 530 , gates 540 , and metal contact 550 .
  • the source can be pulled high (1). This causes the body or well diode to turn on (2). When the source returns low (3), holes continue to be injected into the epi 530 or drain. This hole current slows the reverse recovery (4). This leads to energy losses due to both the forward conduction of the diode and also the reverse recovery of the diode.
  • the dissipated power can be roughly approximated as follows:
  • V f is the body-diode forward voltage
  • Iout is the diode forward current
  • Fsw is the switching frequency
  • trr is the reverse recovery time
  • Irr is the reverse recovery current.
  • the reverse recovery characteristics are primarily governed by holes injected from the body region (p-type) into the drain region (n-type).
  • Embodiments of the present invention employ band-gap engineering techniques to narrow the energy gap of the body region with respect to the drain epitaxial region. This can be achieved by forming the transistor body region to include an alloy of Si 1-x Ge x where x is the mole fraction of germanium in the alloy, with typical values 0.1 ⁇ x ⁇ 0.3. In other embodiments, other concentrations can be used, for example the concentration of germanium can be less then 10 percent, or greater than 30 percent.
  • the body region comprising Si 1-x Ge x can be fabricated by standard techniques such as epitaxial growth or by ion implantation.
  • FIG. 6 is a cross section of an n-channel trench-gate power MOSFET having a SiGe well according to an embodiment of the present invention.
  • This device includes n-type source regions 210 and p-type body region 220 , which comprises silicon germanium, n-type drain region 230 , and gates 240 .
  • the p-type body region 220 to metal 250 contact area can be omitted, as it was in FIG. 2 above.
  • the gate may be polysilicon germanium, or poly SiGe.
  • germanium in the alloy creates a narrowing of the energy gap (Eg) that is roughly proportional to the germanium mole fraction.
  • the effect of the smaller bandgap in the body region is to increase electron injection at a given forward bias. This in turn implies reduced hole injection for a given current density.
  • FIG. 7 illustrates the net doping and germanium mole fraction of the MOSFET shown in FIG. 6 .
  • the net doping 710 and germanium mole fraction 720 are plotted along the Y-axis as a function of depth along the X-axis.
  • the net doping is typical of that in the trench-gate MOSFET body and drain regions.
  • the germanium profile has a peak mole fraction of 0.15 and width of 100 nm at its peak.
  • FIG. 8 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe well region according to an embodiment of the present invention.
  • a trench-gate power transistor having a SiGe body region is formed.
  • an epitaxial layer is formed.
  • a SiGe well is formed in act 820 .
  • This layer can be grown on the epitaxial layer formed in act 810 , or it may be implanted into the epitaxial layer formed in act 810 .
  • a source is formed in the well in act 830 .
  • a trench is formed or cut in the epitaxial layer in act 840 , and a gate is formed in the trench in act 850 .
  • the gate is contacted, and the source is contacted in acts 860 and 870 .
  • the use of silicon germanium results in a significant decrease in hole current as well as a small reduction in the forward voltage.
  • the hole current is decreased by a factor of approximately 5 as compared to a silicon well structure. This results in a significant improvement in the reverse recovery characteristics of the transistor. Further, unlike conventional techniques wherein improved reverse recovery is obtained at the expense of increased forward voltage, the improvement in reverse recovery is achieved along with a reduction in the forward voltage.
  • trench-gate power MOSFETs can be used in DC-to-DC converter circuits to achieve voltage conversion with a high efficiency, that is, with a minimum loss of power.
  • Some improvements that can be made to these devices to achieve this goal include lowering device on resistance (Rds on ), input capacitance (C iss ), gate charge (Q g and Q gd ), gate impedance (ESR or R g ), body diode reverse recovery (T rr ), or a combination of the above.
  • gate impedance reduction is commonly achieved by saturating gate doping or by adding silicide.
  • large concentrations of dopant in the gate can enhance dopant penetration into the channel (or well or bulk), particularly when gate oxide is thin. This dopant penetration results in a shift in device threshold voltage (V th ).
  • V th device threshold voltage
  • FIG. 9 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention.
  • This cross section includes a device having a gate 940 , source 910 , body or well region 920 , and drain or epi region 930 .
  • the gate 940 includes a silicide layer 942 .
  • a small gate dimension is desirable for low voltage, low Rds on , and low cost. But the narrow gate dimension limits the effectiveness of the silicide layer 942 . That is, the portion of the gate 940 below the silicide layer 942 retains the characteristics of polysilicon: it does not benefit from the presence of the silicide layer 942 .
  • embodiments of the present invention provide a lower gate resistance to enable faster switching speeds for MOS-gated power transistors without the above drawbacks and limitations.
  • embodiments of the present invention provide a lower gate resistance by using a polysilicon germanium (Si 1-x Ge x ) gate for MOS-gated power transistors such as power trench-gate MOSFET transistors. Incorporating a poly Si 1-x Ge x gate offers improved performance as compared to devices utilizing a conventional polysilicon and silicided gate.
  • the smaller bandgap of Si 1-x Ge x results in a larger number of carriers at a given temperature and doping concentration. This in turn reduces gate depletion that shifts the threshold voltage V th and also lowers gate impedance.
  • the carrier mobility in poly Si 1-x Ge x is over 3 times higher than that of poly silicon. This further lowers gate impedance by 67% and allows the transistor to switch faster.
  • FIG. 10 is a cross section of an n-channel trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention.
  • This cross section includes a device having a gate 1040 , source 1010 , body or well region 1020 , and drain or epi region 1030 .
  • the entirety of the gate 1040 benefits from the lower resistance of the poly SiGe, as opposed to the narrower improvement provided by the silicide layer 942 in FIG. 9 .
  • the p-type body region 1020 to metal 1050 contact area can be omitted, as it was in FIG. 2 above.
  • the source 1010 , p-well 1020 , or both, may be formed using SiGe.
  • Poly SiGe gates can also be used to improve electrostatic discharge devices.
  • a Zener device (n + p) built using a Si 1-x Ge x poly gate is able to withstand higher electrostatic discharge power than that of a conventional poly silicon gate device.
  • High carrier mobility of poly Si 1-x Ge x reduced series resistance, particularly in the low doped p-type regions of the Zener, resulting in less ohmic loss and a cooler operation.
  • p-channel devices can also be improved. In p-channel devices, boron penetration from a Si 1-x Ge x poly gate to body or well via the gate oxide is reduced. This allows the use of thin oxide without threshold voltage instability.
  • Gate series resistance is greatly reduced using a SiGe poly gate.
  • a silicided gate only lowers gate resistance by a factor of two in a conventional trenched gate device. This limited decrease is due to the small geometry available for silicide formation over the gate material in a narrow trench.
  • Using poly Si 1-x Ge x as gate material reduces the gate impedance by more than 67% under the same gate cross section, thus offering an even better performance without the extra expense and difficulty of using a silicide process.
  • poly SiGe remains stable at 1000° C. while the use of a silicided gate limits subsequence process temperature to less than 850° C. due to possible agglomeration.
  • the use of the poly SiGe gate 1040 eliminates the need for a silicide layer 942 on the gate.
  • FIG. 11 illustrates material resistivity as a function of boron concentration for various materials.
  • the resistivity of SiGe poly is much less than conventional polysilicon.
  • the concentration of germanium is 35 percent by mole fraction at boron concentration of 2 ⁇ 10 ⁇ 19, where ⁇ is a symbol for “to the power of.” In other embodiments, the concentration may be higher than 35 percent, or less than 35 percent.
  • FIG. 13 illustrates the increase in efficiency provided by the incorporation of an embodiment of the present invention.
  • the results in FIG. 13 indicate that the peak efficiency increases from 83% to 88%. This equates to a saving of 30% power that would otherwise be lost.
  • the peak efficiency for a conventional poly gate 1310 is lower than the peak efficiency for a SiGe gated device 1320 .
  • Data in FIG. 13 also suggests that this converter efficiency is approaching to that with n-channel MOSFETs in both the high- and low-side (curve 1330 ). This significantly reduces the power consumption of gate driver and will enable designers to simplify driver circuits but at the expense of 1-2% efficiency loss at high output current.
  • Table 1 lists electrical properties for various materials that could be used in forming a MOSFET gate. Again, SiGe poly has a lower resistivity than conventional polysilicon. Using this material as a gate reduces gate series resistance and increases circuit efficiency.
  • FIG. 14 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention.
  • a poly SiGe gate is incorporated into a trench-gate power MOSFET device.
  • an epitaxial layer is formed.
  • a well layer is formed in the epitaxial layer.
  • a source is formed in the well in act 1430 .
  • a trench is formed in the epitaxial layer.
  • a poly SiGe gate is formed in the trench in act 1440 .
  • the poly SiGe gate and source are contacted in acts 1450 and 1460 .
  • channel impedance limits the performance of trench-gated power MOSFET devices. That is, it is desirable to reduce the on-resistance (Rdson) of MOS-gated power transistors such as n-channel and p-channel power trench-gate MOSFET devices, particularly in low voltage applications.
  • Rdson on-resistance
  • MOS-gated power transistors such as n-channel and p-channel power trench-gate MOSFET devices
  • the gate driver for a p-channel MOSFET consumes less power than a driver for an n-channel device.
  • R ch The channel resistance (R ch ) is:
  • R ch L Z ⁇ ⁇ ⁇ p ⁇ C ox ⁇ ( V G - V T )
  • L is the channel length
  • Z is the channel width
  • C ox is the gate oxide capacitance per unit area
  • V G is the gate voltage
  • V T is the threshold voltage.
  • the series on-resistance is dominated by the resistivity of the well region 120 , particularly the resistivity of the portion of the well region 120 near the gates 140 . If this resistivity is reduced, the series on-resistance of the device is reduced.
  • One way to reduce this resistivity is to utilize a material having a higher mobility. Specifically, if the portion of the channel that begins to conduct first, that is the portion near the gates 140 , is made of a higher mobility material, the series on-resistance can be reduced. Accordingly, embodiments of the present invention employ strained Si 1-x Ge x in its channel area to reduce channel resistance.
  • FIG. 15 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
  • This device includes p-type source regions 1510 , an n-well or body 1520 , p-type drain region 1530 , gates 1540 , a SiGe layer 1534 .
  • the SiGe layer 1534 is isolated from the gates 1540 by a silicon layer 1532 .
  • This silicon layer 1532 provides a proper interface to the polysilicon gate.
  • the n-type body region 1520 to metal 1550 contact area can be omitted, as the p-type body region contact was removed in FIG. 2 above.
  • the source 1510 may be formed using SiGe
  • the gate 1540 may be poly SiGe.
  • a layer of Si 1-x Ge x layer 1534 is grown commensurately along the sidewall of a silicon trench. Because the lattice constant of Si 1-x Ge x is larger than that of silicon, depending on the germanium mole fraction, the Si 1-x Ge x layer is under biaxial compressive strain in [100]/[010] directions.
  • a strained Si 1-x Ge x planar layer has increased hole mobility compared to that of bulk silicon under either compressive or tensile biaxial strains. This increased mobility results in reduced resistivity in the SiGe layer 15341 .
  • FIG. 16 shows the hole mobility measured of two different Si 1-x Ge x films grown on relaxed Si 1-x Ge x virtual substrate.
  • the bulk silicon universal mobility curve 1630 is included as a basis of comparison.
  • the hole mobility in Si 1-x Ge x channel layer is greatly enhanced due to valence band splitting and shape change of valence band. This enhancement also depends on the germanium concentration relative to that in Si 1-x Ge x virtual substrate; indicating a strain related phenomena.
  • the improved hole mobility with 20 at % Ge 1640 and 84 at % Ge 1650 are shown.
  • FIG. 17 illustrates a change in mobility as a function of germanium concentration in a channel region.
  • a mobility enhancement of a factor of two is observed in Si 1-x Ge x film with a germanium concentration of 50 at %.
  • the device using Si 50 Ge 50 as channel possesses a channel resistance only half of that of the same device using bulk silicon.
  • This mobility enhancement also significantly improves sub-threshold characteristics, which is important for low voltage applications.
  • FIG. 18 illustrates a valence band offset for a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. This offset equals:
  • FIGS. 19A-19C illustrate a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
  • a silicon epitaxial layer 1930 is deposited or grown on a silicon substrate 1960 .
  • a trench is then formed.
  • a well 1920 is implanted, or grown as part of the epi layer 1930 .
  • an epitaxial Si 1-x Ge x layer 1934 is deposited, grown, or implanted along the sidewall of trench.
  • the Si 1-x Ge x layer's thickness ranges from 1 nm to 100 nm, depending on the germanium concentration.
  • the germanium concentration ranges from 10 at % through 80 at %. In other embodiments the germanium concentration may be different, for example it may be less than 10 percent by mole fraction, or more than 80 percent by mole fraction. In other embodiments, the range of germanium concentration may be less.
  • a silicon cap layer 1932 is grown as an epitaxial layer on the top of Si 1-x Ge x layer 1934 .
  • the thickness of the silicon cap layer 640 ranges from 1 nm to 10 nm, though in other embodiments it may be thinner or thicker than this range. Alternately, the silicon layer 1932 may remain after SiGe layer 1934 is implanted through it.
  • FIG. 20 is a flowchart of a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
  • a SiGe layer is grown in the channel region of a trench-gate power MOSFET device.
  • an epitaxial layer is formed.
  • a well layer is formed.
  • a trench is then etched in the epi layer in act 2030 .
  • a SiGe layer is grown in act 2040 , and in act 2050 a silicon cap is grown.
  • Gate and source regions are formed and contacted in acts 2060 and 2070 .
  • the present invention is shown and described in the context of a vertical MOSFET embodiment, but various embodiments of the present invention can be similarly implemented in other trenched gate structures such as trenched gate IGBTs, lateral trenched-gate MOSFETs, as well as vertical and lateral planar-gate MOSFETs and IGBTs. Also, the various transistor embodiments can be laid out using the well known open-cell or closed-cell configurations.

Abstract

Devices, methods, and processes that improve immunity to transient voltages and reduce parasitic impedances. Immunity to unclamped inductive switching events is improved. For example, a trench-gated power MOSFET device having a SiGe source is provided, where the SiGe source reduces parasitic npn transistor gain by reducing hole current in the body or well region, thereby decreasing the likelihood of a latch-up condition. A body tie on this device can also be eliminated to reduce transistor cell size. A trench-gated power MOSFET device having a SiGe body or well region is also provided. A SiGe body reduces hole current when the body diode is turned on, thereby reducing reverse recovery power losses. Device characteristics are also improved. For example, parasitic gate impedance is reduced through the use of a poly SiGe gate, and channel resistance is reduced through the use of a SiGe layer near the device's gate.

Description

  • The present application is a division of U.S. application Ser. No. 11/245,995, filed on Oct. 7, 2005, entitled “BANDGAP ENGINEERED MOS-GATED POWER TRANSISTORS,” and claims the benefit of U.S. provisional Application Nos. 60/617,167, 60/617,464, 60/617,463, and 60/617,168, each filed Oct. 7, 2004, each of which is incorporated by reference.
    • BACKGROUND
  • The present invention relates generally to power transistors, and more specifically to metal-oxide-semiconductor-gated (MOS-gated) power transistors with silicon-germanium (SiGe) sources, wells, channels, poly silicon-germanium gates, or a combination thereof.
  • Trench-gated power MOSFET devices are popular choices for use in several demanding applications, such as DC-to-DC converters. These applications can be very harsh, putting a tremendous strain on these transistors. For example, sourcing and sinking large currents into an inductive load can result in large voltage transients at one or more of the device's terminals. Specifically, a large voltage excursion seen by a trench-gated power MOSFET can activate a parasitic npn transistor, leading to a destructive failure. A less catastrophic, but still performance-degrading event, can occur when a large transient forward-biases a device's body diode, slowing the transistor's reverse recovery.
  • These inductive effects can limit the efficiency of a DC-to-DC converter, thereby wasting power. Other limits on efficiency include the physical limitations of the trench-gated power MOSFETs themselves. For example, parasitic impedances can cause power dissipation and heating of the devices. Among these parasitic impedances is the series resistance of the gates of the devices. While this series resistance can be mitigated using silicide, the efficacy of this procedure is limited due to the physical structure of these transistors. Channel resistance, or Ron, also limits device performance, thus limiting converter efficiency. Larger devices can reduce Ron, but this results in a more costly device.
  • Accordingly, what is needed are devices, methods, and processes that provide transistors having improved immunity to the effects of large transient voltages and that provide improved performance by reducing parasitic impedances.
    • SUMMARY
  • Accordingly, embodiments of the present invention provide devices, methods, and processes that improve immunity to transient voltages and reduce parasitic impedances.
  • Embodiments of the present invention provide transistors having improved immunity to unclamped inductive switching and other transient voltage events. For example, an exemplary embodiment of the present invention provides a trench-gated power MOSFET device having a SiGe source. A SiGe source reduces the gain of a parasitic npn transistor by reducing hole current in the body or well region, thereby decreasing the likelihood of a latch-up condition following an unclamped inductive switching event. The body tie on this device can also be eliminated in order to reduce transistor cell size.
  • Another exemplary embodiment of the present invention provides a trench-gated power MOSFET device having a SiGe body or well region. A SiGe body reduces hole current when the body diode is turned on, thereby reducing the power lost during its reverse recovery.
  • Embodiments of the present invention may also improve device characteristics. For example, an exemplary embodiment of the present invention reduces parasitic gate impedance by incorporating a poly SiGe gate. Another embodiment provides a channel having reduced channel resistance through the use of a SiGe layer near the device's gate.
  • Improvements that boost immunity to voltage transients and those that enhance device performance are not exclusive. For example, the use of a SiGe body decreases channel impedance, while a SiGe layer in the body region improves body diode reverse recovery. These embodiments may be used to improve n-channel or p-channel devices. Embodiments of the present invention may incorporate one of more of these or the other features described herein.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIG. 1 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention;
  • FIG. 2 illustrates a cross section of an n-channel trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention;
  • FIG. 3 illustrates the energy band structure through the source-to-well region of the device illustrated in FIG. 2;
  • FIG. 4 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention;
  • FIG. 5 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention;
  • FIG. 6 is a cross section of an n-channel trench-gate power MOSFET having a SiGe well according to an embodiment of the present invention;
  • FIG. 7 illustrates the net doping and germanium mole fraction of the MOSFET shown in FIG. 6;
  • FIG. 8 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe well region according to an embodiment of the present invention;
  • FIG. 9 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention;
  • FIG. 10 is a cross section of an n-channel trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention;
  • FIG. 11 illustrates material resistivity as a function of boron concentration for various materials;
  • FIG. 12 illustrates a DC-DC converter output including a p-channel high-side power-MOSFET device and an n-channel low-side power-MOSFET device;
  • FIG. 13 illustrates the increase in efficiency provided by the incorporation of an embodiment of the present invention;
  • FIG. 14 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention;
  • FIG. 15 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;
  • FIG. 16 shows the hole mobility measured of two different Si1-xGex films grown on relaxed Si1-xGex virtual substrate;
  • FIG. 17 illustrates a change in mobility as a function of germanium concentration in a channel region;
  • FIG. 18 illustrates a valence band offset for a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention;
  • FIGS. 19A-19C illustrate a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention; and
  • FIG. 20 is a flowchart of a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention.
    •  DESCRIPTION OF EXEMPLARY EMBODIMENTS
  • FIG. 1 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention. This device includes n-type source regions 110, a body formed by p-well 120, n-type drain region 130, substrate 160, gates 140, and metal contact 150.
  • A parasitic npn bipolar transistor is inherent in this structure. Specifically, the parasitic device's emitter is the source region 110, its base is the body or well region 120, while the parasitic device's collector corresponds to the epi region 130. Destructive failure can occur if this parasitic transistor is biased into its forward active mode of operation. This can occur, for example, during avalanche breakdown induced by an unclamped inductive switching (UIS) event. Holes generated by impact ionization can flow through the base formed by the P-Well 120 causing an ohmic voltage drop. If this voltage drop exceeds about 0.6V, the turn-on voltage for the well-to-source junction diode can become forward biased and inject electrons over the potential barrier, thus initiating forward active operation and potential failure.
  • Specifically, as shown in FIG. 1, an unclamped inductive switching event can pull the source low (1). This causes holes to flow into the body or well 120 (2). The well charges up, or increases in voltage compared to the source 110, turning on the p-well 120 to source 110 junction (3). The result is electron current to flow into the source 110 (4). This causes the parasitic npn to turn on (5), which can lead to device failure.
  • This propensity for this series of events to occur can be minimized by providing a low resistance path for the removal of holes from the parasitic base and by reducing the current gain of the parasitic npn 160. This may be achieved by contacting the body or p-well with the source connection 150.
  • But this body contact increases the size of n-channel MOSFET. Thus, there exists a trade-off between device size and immunity to this failure mechanism. A solution to this problem is to employ band-gap engineering techniques to narrow the energy gap of the source region with respect to the well region. This can be achieved by forming the source region using an alloy of Si1-xGex, where x is the mole fraction of germanium in the alloy, with typical values 0.1<x<0.3. In other embodiments, other concentrations can be used, for example the concentration of germanium can be less than 10 percent, or greater than 30 percent. The SiGe source region can be fabricated by standard techniques such as epitaxial growth or by ion implantation.
  • FIG. 2 illustrates a cross section of an n-channel trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention. This device includes an n-type source region 210 that is formed from silicon germanium, a body formed by p-well 220, n-type drain region 230, gates 240, metal contact 250, and n substrate 260. As seen in this figure, elimination of the p-well contact allows a significant reduction in cell pitch. This has the additional benefit of reducing on-state resistance and improving current conduction capability by increasing the size of the source region relative to the device of FIG. 1. This figure, as with the other included figures, is shown for illustrative purposes, and does not limit either the possible embodiments of the present invention or the claims. Also, while in this and the other figures an n-channel transistor is shown, embodiments of the present invention can similarly be used to improve p-channel devices. Features shown in other figures may also be included in this figure. For example, the p-well 220 may also be SiGe, while the gate may be poly SiGe.
  • The use of SiGe in the source 210 allows for holes to flow to the source, reducing the hole current in the body or p-well 220. Specifically, the presence of germanium in the alloy creates a narrowing of the energy gap (Eg) that is roughly proportional to the germanium mole fraction. Most of this band offset occurs in the valence band. This allows holes to easily flow to the source region, providing a path for hole removal reducing the npn gain. Now when an unclamped inductive event pulls the source low (1), the holes flow to the source 210, and the well does not charge (2).
  • FIG. 3 illustrates the energy band structure through the source-to-well region of the device illustrated in FIG. 2. FIG. 3 was calculated for a germanium mole fraction of 30%. The data show approximately a 0.21 eV reduction of the energy gap of the source with respect to the well, that is, 0.9 eV in the source compared with 1.1 eV in the well or body region. In theory, this leads to an increase in the hole current crossing the junction by a factor ˜exp(Eg/kT). In practice, the improvement is somewhat less than this, since not all the energy band offset occurs in the valence band.
  • In a specific embodiment of the present invention, the source 210 has a germanium content of 30 percent by mole fraction. A comparison between devices having a silicon source and this SiGe source shows an increase in hole current and reduction in the npn current gain for a device using a SiGe source region. Specifically, at a typical bias of 0.75V, the hole current is increased by about 100× and the parasitic current gain is reduced by a factor of about 500. This data should be considered worst case, since in practice the well region would not be left entirely floating, but would be remotely contacted in the third dimension (perpendicular to the illustrated cross-section of FIG. 2). This reduction in parasitic current gain makes it less likely that the parasitic transistor 160 would conduct current leading to a catastrophic failure during a UIS event.
  • FIG. 4 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe source according to an embodiment of the present invention. In this embodiment, a SiGe source is formed in order to achieve a greater immunity to failure caused by an unclamped inductive switching event.
  • Specifically, in act 410, an epitaxial layer is formed. A SiGe layer is formed in the epitaxial layer in act 420. A trench is cut in the epitaxial layer in act 430, while in act 440, a gate is formed in the trench. The gate is contacted with metal, and the SiGe layer is contacted to form a source region in acts 450 and 460.
  • These trench-gate power MOSFETs are particularly useful in low-voltage DC-to-DC converter applications due to their low conduction losses and fast switching time. But the source being pulled low is not the only inductive effect these devices are exposed to. During normal operation, the MOSFET source can become positively biased with respect to the drain, leading to turn-on of the parasitic body-to-drain diode (also commonly referred to as the body diode). It should also be noted that while this and the other embodiments of the present invention are well suited to DC-to-DC power converters, other types of systems may incorporate devices that are improved by the incorporation of embodiments of the present invention.
  • FIG. 5 illustrates this mechanism. This device includes n-type source regions 510, p-type body region 520, n-type drain region 530, gates 540, and metal contact 550. Specifically, the source can be pulled high (1). This causes the body or well diode to turn on (2). When the source returns low (3), holes continue to be injected into the epi 530 or drain. This hole current slows the reverse recovery (4). This leads to energy losses due to both the forward conduction of the diode and also the reverse recovery of the diode. The dissipated power can be roughly approximated as follows:
  • P cond = V f × I out × t deadtime × F sw Psw = Vin × trr × Irr 2 × Iout × Fsw
  • Where Vf is the body-diode forward voltage, Iout is the diode forward current, Fsw is the switching frequency, trr is the reverse recovery time, and Irr is the reverse recovery current. For a typical n-channel MOSFET, the reverse recovery characteristics are primarily governed by holes injected from the body region (p-type) into the drain region (n-type).
  • This analysis shows that diode energy losses can be decreased by reducing the diode forward voltage, reverse recovery current, and reverse recovery time simultaneously. Unfortunately, for conventional silicon technology, there exists an inverse relationship between the reverse recovery parameters and the forward voltage. Techniques that improve the forward voltage typically degrade the reverse recovery, while those that improve reverse recovery degrades the forward voltage. This makes their simultaneous reduction impractical using conventional silicon techniques.
  • Embodiments of the present invention employ band-gap engineering techniques to narrow the energy gap of the body region with respect to the drain epitaxial region. This can be achieved by forming the transistor body region to include an alloy of Si1-xGex where x is the mole fraction of germanium in the alloy, with typical values 0.1<x<0.3. In other embodiments, other concentrations can be used, for example the concentration of germanium can be less then 10 percent, or greater than 30 percent. The body region comprising Si1-xGex can be fabricated by standard techniques such as epitaxial growth or by ion implantation.
  • FIG. 6 is a cross section of an n-channel trench-gate power MOSFET having a SiGe well according to an embodiment of the present invention. This device includes n-type source regions 210 and p-type body region 220, which comprises silicon germanium, n-type drain region 230, and gates 240. It should be noted that the p-type body region 220 to metal 250 contact area can be omitted, as it was in FIG. 2 above. Also, the gate may be polysilicon germanium, or poly SiGe.
  • The presence of germanium in the alloy creates a narrowing of the energy gap (Eg) that is roughly proportional to the germanium mole fraction. The effect of the smaller bandgap in the body region is to increase electron injection at a given forward bias. This in turn implies reduced hole injection for a given current density.
  • FIG. 7 illustrates the net doping and germanium mole fraction of the MOSFET shown in FIG. 6. The net doping 710 and germanium mole fraction 720 are plotted along the Y-axis as a function of depth along the X-axis. The net doping is typical of that in the trench-gate MOSFET body and drain regions. The germanium profile has a peak mole fraction of 0.15 and width of 100 nm at its peak.
  • FIG. 8 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a SiGe well region according to an embodiment of the present invention. In this embodiment, a trench-gate power transistor having a SiGe body region is formed.
  • Specifically, in act 810, an epitaxial layer is formed. A SiGe well is formed in act 820. This layer can be grown on the epitaxial layer formed in act 810, or it may be implanted into the epitaxial layer formed in act 810. A source is formed in the well in act 830.
  • A trench is formed or cut in the epitaxial layer in act 840, and a gate is formed in the trench in act 850. The gate is contacted, and the source is contacted in acts 860 and 870.
  • In a specific embodiment of the present invention, the use of silicon germanium results in a significant decrease in hole current as well as a small reduction in the forward voltage. In this embodiment, at a typical bias of 0.75V, the hole current is decreased by a factor of approximately 5 as compared to a silicon well structure. This results in a significant improvement in the reverse recovery characteristics of the transistor. Further, unlike conventional techniques wherein improved reverse recovery is obtained at the expense of increased forward voltage, the improvement in reverse recovery is achieved along with a reduction in the forward voltage.
  • These trench-gate power MOSFETs can be used in DC-to-DC converter circuits to achieve voltage conversion with a high efficiency, that is, with a minimum loss of power. Some improvements that can be made to these devices to achieve this goal include lowering device on resistance (Rdson), input capacitance (Ciss), gate charge (Qg and Qgd), gate impedance (ESR or Rg), body diode reverse recovery (Trr), or a combination of the above.
  • Conventionally, gate impedance reduction is commonly achieved by saturating gate doping or by adding silicide. Unfortunately, due to the large thermal budget (high temperatures) in trench-gate MOSFET devices, large concentrations of dopant in the gate can enhance dopant penetration into the channel (or well or bulk), particularly when gate oxide is thin. This dopant penetration results in a shift in device threshold voltage (Vth). As can be seen in the next figure, the effectiveness of adding a silicide layer is also limited.
  • FIG. 9 is a cross section of an n-channel trench-gate power MOSFET that is improved by the incorporation of an embodiment of the present invention. This cross section includes a device having a gate 940, source 910, body or well region 920, and drain or epi region 930. The gate 940 includes a silicide layer 942. In a trench-gate power MOSFET device, a small gate dimension is desirable for low voltage, low Rdson, and low cost. But the narrow gate dimension limits the effectiveness of the silicide layer 942. That is, the portion of the gate 940 below the silicide layer 942 retains the characteristics of polysilicon: it does not benefit from the presence of the silicide layer 942.
  • Accordingly, embodiments of the present invention provide a lower gate resistance to enable faster switching speeds for MOS-gated power transistors without the above drawbacks and limitations. Specifically, embodiments of the present invention provide a lower gate resistance by using a polysilicon germanium (Si1-xGex) gate for MOS-gated power transistors such as power trench-gate MOSFET transistors. Incorporating a poly Si1-xGex gate offers improved performance as compared to devices utilizing a conventional polysilicon and silicided gate.
  • For example, the smaller bandgap of Si1-xGex results in a larger number of carriers at a given temperature and doping concentration. This in turn reduces gate depletion that shifts the threshold voltage Vth and also lowers gate impedance. Moreover, the carrier mobility in poly Si1-xGex is over 3 times higher than that of poly silicon. This further lowers gate impedance by 67% and allows the transistor to switch faster.
  • FIG. 10 is a cross section of an n-channel trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention. This cross section includes a device having a gate 1040, source 1010, body or well region 1020, and drain or epi region 1030. In this embodiment, the entirety of the gate 1040 benefits from the lower resistance of the poly SiGe, as opposed to the narrower improvement provided by the silicide layer 942 in FIG. 9. It should be noted that the p-type body region 1020 to metal 1050 contact area can be omitted, as it was in FIG. 2 above. Also, the source 1010, p-well 1020, or both, may be formed using SiGe.
  • Poly SiGe gates can also be used to improve electrostatic discharge devices. For example, a Zener device (n+p) built using a Si1-xGex poly gate is able to withstand higher electrostatic discharge power than that of a conventional poly silicon gate device. High carrier mobility of poly Si1-xGex reduced series resistance, particularly in the low doped p-type regions of the Zener, resulting in less ohmic loss and a cooler operation. As with the other embodiments shown, p-channel devices can also be improved. In p-channel devices, boron penetration from a Si1-xGex poly gate to body or well via the gate oxide is reduced. This allows the use of thin oxide without threshold voltage instability.
  • Gate series resistance is greatly reduced using a SiGe poly gate. By way of comparison, a silicided gate only lowers gate resistance by a factor of two in a conventional trenched gate device. This limited decrease is due to the small geometry available for silicide formation over the gate material in a narrow trench. Using poly Si1-xGex as gate material reduces the gate impedance by more than 67% under the same gate cross section, thus offering an even better performance without the extra expense and difficulty of using a silicide process. Moreover, poly SiGe remains stable at 1000° C. while the use of a silicided gate limits subsequence process temperature to less than 850° C. due to possible agglomeration. Among other advantages and feature, the use of the poly SiGe gate 1040 eliminates the need for a silicide layer 942 on the gate.
  • FIG. 11 illustrates material resistivity as a function of boron concentration for various materials. As can be seen, the resistivity of SiGe poly is much less than conventional polysilicon. In this example, the concentration of germanium is 35 percent by mole fraction at boron concentration of 2×10̂̂19, where ̂̂ is a symbol for “to the power of.” In other embodiments, the concentration may be higher than 35 percent, or less than 35 percent.
  • Overall, using poly Si1-xGex gate can reduce gate impedance without altering existing layout, and the process is compatible to present silicon technologies. The benefit of a 67% reduction in gate impedance becomes evident when considering the efficiency of a DC-DC converter having a p-channel high-side MOSFET and an n-channel one on the low side as shown in FIG. 12.
  • FIG. 13 illustrates the increase in efficiency provided by the incorporation of an embodiment of the present invention. The results in FIG. 13 indicate that the peak efficiency increases from 83% to 88%. This equates to a saving of 30% power that would otherwise be lost. Specifically, the peak efficiency for a conventional poly gate 1310 is lower than the peak efficiency for a SiGe gated device 1320. Data in FIG. 13 also suggests that this converter efficiency is approaching to that with n-channel MOSFETs in both the high- and low-side (curve 1330). This significantly reduces the power consumption of gate driver and will enable designers to simplify driver circuits but at the expense of 1-2% efficiency loss at high output current.
  • Table 1 lists electrical properties for various materials that could be used in forming a MOSFET gate. Again, SiGe poly has a lower resistivity than conventional polysilicon. Using this material as a gate reduces gate series resistance and increases circuit efficiency.
  • TABLE 1
    Electrical properties of various gate materials
    Gate Material Mobility Resistivity
    (concentration = 2 × 1018 cm−3) (cm2/V/sec) mΩ-cm
    p-type crystalline silicon 78 30
    p-type poly silicon NA 100 [8] 
    p-type Si0.9Ge0.1 275 [3] 12
    p-type poly silicon (2 × 1019) NA 7.5 [9]
    p-type Si0.65Ge0.35 (2 × 1019) NA 2.0 [9]
  • FIG. 14 is a flowchart of a method of manufacturing a trench-gate power MOSFET having a poly SiGe gate according to an embodiment of the present invention. In this embodiment, a poly SiGe gate is incorporated into a trench-gate power MOSFET device.
  • Specifically, in act 1410, an epitaxial layer is formed. In act 1420, a well layer is formed in the epitaxial layer. A source is formed in the well in act 1430.
  • In act 1430, a trench is formed in the epitaxial layer. A poly SiGe gate is formed in the trench in act 1440. The poly SiGe gate and source are contacted in acts 1450 and 1460.
  • Even with a poly SiGe gate, or a SiGe source, channel impedance limits the performance of trench-gated power MOSFET devices. That is, it is desirable to reduce the on-resistance (Rdson) of MOS-gated power transistors such as n-channel and p-channel power trench-gate MOSFET devices, particularly in low voltage applications. For example, in the application of synchronous DC-DC converter, the gate driver for a p-channel MOSFET consumes less power than a driver for an n-channel device. Thus, it is very desirable to use p-channel devices for the high side driver instead of n-channel devices, as shown in FIG. 12.
  • But the Rdson for a p-channel device is much higher than a similar n-channel MOSFET, and this limits its application to only small current region. In low voltage applications, channel resistance dominates the device Rdson. The channel resistance (Rch) is:
  • R ch = L Z μ p C ox ( V G - V T )
  • where L is the channel length, Z is the channel width, Cox is the gate oxide capacitance per unit area, VG is the gate voltage and VT is the threshold voltage. To reduce channel resistance, it is desired to shrink channel length, gate oxide thickness, and lower the threshold voltage. Due to the process limitation or sub-threshold issue, these approaches are limited.
  • In transistors such as the transistor of FIG. 1, the series on-resistance is dominated by the resistivity of the well region 120, particularly the resistivity of the portion of the well region 120 near the gates 140. If this resistivity is reduced, the series on-resistance of the device is reduced. One way to reduce this resistivity is to utilize a material having a higher mobility. Specifically, if the portion of the channel that begins to conduct first, that is the portion near the gates 140, is made of a higher mobility material, the series on-resistance can be reduced. Accordingly, embodiments of the present invention employ strained Si1-xGex in its channel area to reduce channel resistance.
  • FIG. 15 is a cross section of a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. This device includes p-type source regions 1510, an n-well or body 1520, p-type drain region 1530, gates 1540, a SiGe layer 1534. The SiGe layer 1534 is isolated from the gates 1540 by a silicon layer 1532. This silicon layer 1532 provides a proper interface to the polysilicon gate. It should be noted that the n-type body region 1520 to metal 1550 contact area can be omitted, as the p-type body region contact was removed in FIG. 2 above. Also, the source 1510 may be formed using SiGe, while the gate 1540 may be poly SiGe.
  • A layer of Si1-xGex layer 1534 is grown commensurately along the sidewall of a silicon trench. Because the lattice constant of Si1-xGex is larger than that of silicon, depending on the germanium mole fraction, the Si1-xGex layer is under biaxial compressive strain in [100]/[010] directions. A strained Si1-xGex planar layer has increased hole mobility compared to that of bulk silicon under either compressive or tensile biaxial strains. This increased mobility results in reduced resistivity in the SiGe layer 15341.
  • FIG. 16 shows the hole mobility measured of two different Si1-xGex films grown on relaxed Si1-xGex virtual substrate. The bulk silicon universal mobility curve 1630 is included as a basis of comparison. The hole mobility in Si1-xGex channel layer is greatly enhanced due to valence band splitting and shape change of valence band. This enhancement also depends on the germanium concentration relative to that in Si1-xGex virtual substrate; indicating a strain related phenomena. The improved hole mobility with 20 at % Ge 1640 and 84 at % Ge 1650 are shown.
  • FIG. 17 illustrates a change in mobility as a function of germanium concentration in a channel region. For example, a mobility enhancement of a factor of two is observed in Si1-xGex film with a germanium concentration of 50 at %. Thus, the device using Si50Ge50 as channel possesses a channel resistance only half of that of the same device using bulk silicon. This mobility enhancement also significantly improves sub-threshold characteristics, which is important for low voltage applications.
  • FIG. 18 illustrates a valence band offset for a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. This offset equals:

  • ΔE v≅0.65x−0.22x 2
  • where x is the germanium content in the film. This offset helps to confine the holes in the strained Si1-xGex channel. The parasitic channel resistance from silicon cap layer is then minimized.
  • FIGS. 19A-19C illustrate a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. In FIG. 19A, a silicon epitaxial layer 1930 is deposited or grown on a silicon substrate 1960. A trench is then formed. At some point, a well 1920 is implanted, or grown as part of the epi layer 1930.
  • In FIG. 19B, an epitaxial Si1-xGex layer 1934 is deposited, grown, or implanted along the sidewall of trench. The Si1-xGex layer's thickness ranges from 1 nm to 100 nm, depending on the germanium concentration. In one embodiment, the germanium concentration ranges from 10 at % through 80 at %. In other embodiments the germanium concentration may be different, for example it may be less than 10 percent by mole fraction, or more than 80 percent by mole fraction. In other embodiments, the range of germanium concentration may be less.
  • In FIG. 19C, a silicon cap layer 1932 is grown as an epitaxial layer on the top of Si1-xGex layer 1934. In one embodiment, the thickness of the silicon cap layer 640 ranges from 1 nm to 10 nm, though in other embodiments it may be thinner or thicker than this range. Alternately, the silicon layer 1932 may remain after SiGe layer 1934 is implanted through it.
  • FIG. 20 is a flowchart of a method of manufacturing a p-channel trench-gate power MOSFET having a SiGe channel region according to an embodiment of the present invention. In this embodiment, a SiGe layer is grown in the channel region of a trench-gate power MOSFET device.
  • Specifically, in act 2010 an epitaxial layer is formed. In act 2020, a well layer is formed. A trench is then etched in the epi layer in act 2030. A SiGe layer is grown in act 2040, and in act 2050 a silicon cap is grown. Gate and source regions are formed and contacted in acts 2060 and 2070.
  • It will be appreciated by one skilled in the art that this and the other included flowcharts are highly simplified and illustrate only a few fundamental acts in the manufacturing process. Variations of the processes that may be improved by incorporating an embodiment of the present invention are myriad and not required for an understanding of the present invention.
  • The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. For example, it is understood that the doping polarities of the structures shown and described could be reversed and/or the doping concentrations of the various elements could be altered without departing from the invention. As another example, the present invention is shown and described in the context of a vertical MOSFET embodiment, but various embodiments of the present invention can be similarly implemented in other trenched gate structures such as trenched gate IGBTs, lateral trenched-gate MOSFETs, as well as vertical and lateral planar-gate MOSFETs and IGBTs. Also, the various transistor embodiments can be laid out using the well known open-cell or closed-cell configurations.

Claims (20)

1. An MOS-gated transistor, comprising:
a trench;
a body region adjacent to the trench;
a source region adjacent to the trench and forming a first pn junction with the body region;
a drain region adjacent to the trench and forming a second pn junction with the body region;
a channel region vertically extending along a side wall of the trench and being adjacent to at least a portion of the body region, and
a gate region extending in the trench between the source region and the drain region, the gate region including a polycrystalline semiconductor material having a resistivity lower than that of a polycrystalline silicon and lower than that of a single crystalline silicon.
2. The transistor of claim 1, further comprising a source contact that contact the source region and does not contact the body region.
3. The transistor of claim 1, wherein the gate region comprises polysilicon germanium.
4. The transistor of claim 1, wherein the gate region comprises silicon-germanium (SiGe).
5. The transistor of claim 4, wherein the gate region comprises a silicon-germanium (SiGe) material without a metal silicide.
6. The transistor of claim 4, wherein the gate region comprises a silicon-germanium (SiGe) region in direct contact with a gate dielectric.
7. The transistor of claim 4, wherein the gate region comprises Si1-x Gex where 0.1<x<0.8.
8. The transistor of claim 4, wherein the germanium concentration is between 10 and 30 percent mole fraction.
9. The MOS-gated transistor of claim 1, wherein:
the MOS-gated transistor is a p-type trench MOS-gate transistor having a p-type SiGe trench gate;
a source terminal of the p-type MOS-gated transistor is coupled to an input of a driver circuit;
a drain terminal of the p-type MOS-gated transistor is coupled to an output of the driver circuit;
the drain terminal of the p-type MOS-gated transistor is coupled to a drain terminal of an n-type MOS-gated trench transistor; and
a source terminal of the n-type MOS-gated trench transistor is coupled to a ground terminal.
10. An MOS-gated transistor, comprising:
a first trench and a second trench separated by a mesa region;
a body region in the mesa region and adjacent to the first and the second trenches;
a first source region adjacent to the first trench and forming a first pn junction with the body region;
a second source region adjacent to the second trench and forming a second pn junction with the body region;
a drain region adjacent to the first and the second trenches and forming a third pn junction with the body region;
a first channel region vertically extending along a side wall of the first trench and being adjacent to at least a first portion of the body region;
a second channel region vertically extending along a side wall of the second trench and being adjacent to at least a second portion of the body region;
a first gate region extending in the first trench between the first source region and the drain region, the first gate region including a polycrystalline semiconductor material having a resistivity lower than that of a polycrystalline silicon and lower than that of a single crystalline silicon; and
a conductive material in contact with the first source region, the second source region, and a third portion of the body region between the first and the second source regions.
11. The transistor of claim 10, wherein the gate region comprises polysilicon germanium.
12. The transistor of claim 10, wherein the gate region comprises silicon-germanium (SiGe).
13. The transistor of claim 12, wherein the gate region comprises Si1-x Gex where 0.1<x<0.8.
14. The transistor of claim 10, wherein the gate region comprises a silicon-germanium (SiGe) region in direct contact with a gate dielectric.
15. A driver circuit, comprising:
a p-type trench MOS-gate transistor having a p-type SiGe trench gate;
a source terminal of the p-type trench MOS-gated transistor coupled to an input of the driver circuit;
a drain terminal of the p-type trench MOS-gated transistor coupled to an output of the driver circuit;
an n-type MOS-gated trench transistor, a drain terminal of the n-type MOS-gated transistor being coupled to the drain terminal of the p-type MOS-gated transistor; and
a source terminal of the n-type MOS-gated trench transistor coupled to a ground terminal.
16. The driver circuit of claim 15, wherein the p-type SiGe trench gate comprises Si1-x Gex where 0.1<x<0.8.
17. The driver circuit of claim 15, wherein the p-type SiGe trench gate comprises Si1-x Gex where 0.1<x<0.3.
18. The driver circuit of claim 15, wherein p-type SiGe trench gate is a polycrystalline semiconductor material having a resistivity lower than that of a polycrystalline silicon and lower than that of a single crystalline silicon.
19. The driver circuit of claim 15, wherein p-type SiGe trench gate comprises a silicon-germanium (SiGe) region in direct contact with a gate dielectric.
20. The driver circuit of claim 15, further comprising a control signal coupled to a gate terminal of the p-type trench MOS-gate transistor.
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