JPH04332172A - Semiconductor device equipped with conductivity modulation type mosfet - Google Patents

Abstract

PURPOSE:To control the thickness of a conductivity modulation layer to the minimum to realize a semiconductor device(IGBT) with little turn-OFF loss by having a circuit for protecting elements from overvoltage built in the title IGBT. CONSTITUTION:When avalanche layers 11a, 11b with small curvature are formed in the surface of a conductivity modulation layer 4 and overvoltage is applied thereto, a diode constituted by these layers 11a, 11b and modulation layer 4 enters avalanche, a gate potential becomes positive relative to a source potential and IGBT is placed in the ON state. As a result, a current flows and the IGBT is protected from a damage caused by the overvoltage.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a conductivity modulated MOSFE used as a switching element for driving multiple power sources.
The present invention relates to the configuration of a semiconductor device including T (IGBT).

0002

2. Description of the Related Art FIG. 2 shows the structure of a conventionally used conductivity modulated MOSFET (IGBT). This IGBT includes a P+ type drain layer 1 and an N+ type buffer layer 2 laminated on this drain layer 1.
On the semiconductor substrate 3 formed by the above, an N- type conductivity modulation layer 4 is formed by epitaxial growth. A gate electrode 6 made of polysilicon is formed on the surface of this conductivity modulation layer 4 with a gate oxide film 5 interposed therebetween. Furthermore, by a self-alignment method using this gate electrode 6 as a mask, impurities are introduced into the surface of the conductivity modulation layer 4, and P
A base layer 7 of the mold is formed. Similarly, using an aluminum source electrode 8 formed on the base layer 7, N
A type source layer 9 is introduced to form a cell (MOS section) 10.

In such an IGBT, when a positive potential with respect to the source electrode 8 is applied to the gate electrode 6, the surface of the base layer 7 under the gate electrode is inverted to form a channel. Electrons are injected from the source layer 9 into the conductivity modulation layer 4 through this channel. Correspondingly, holes are injected from the drain layer 1, so that the conductivity of the conductivity modulation layer 4 increases rapidly, resulting in a low resistance element. IGBTs exhibiting such characteristics are devices that are attracting attention as insulated gate bipolar devices.

FIG. 5 shows the voltage at turn-off when this IGBT is used as a voltage resonance type switching element.
The current waveform is shown. As can be seen in Figure 5, IGB
At T, the moment t0 when the gate voltage VG is cut off
, the main current iM decreases rapidly and the tail current iT
After this occurs, it becomes zero at time t1. During this time, voltage VM begins to be applied to the IGBT from time t0, and the product of this voltage VM and the current value from time t0 to t1 becomes turn-off loss Eoff.

[0005]

[Problem to be solved by the invention] This turn-off loss E
Off is desirably small when using the IGBT as a switching element. In order to reduce this loss Eoff, it is effective to reduce the tail current iT. This current iT is generated by the source-drain voltage VM after the carriers of electrons and holes injected to bring the conductivity modulation layer 4 into the conductivity modulation state disappear after the channel disappears and the injection is stopped. This occurs when holes are swept out to the source side and electrons are swept out to the drain side as . Therefore, it is impossible to reduce this current iT to zero. However, if the conductivity modulation layer 4 is made thinner, the hFE of the IGBT built-in transistor becomes thinner.
By improving this, the number of carriers involved in conductivity modulation decreases, making it possible to suppress the amount of carriers swept out. Therefore, it is possible to reduce the tail current iT.

However, when focusing on the breakdown voltage performance of this IGBT, the breakdown voltage performance is mainly determined by the thickness of the N- type conductivity modulation layer 4. In other words, high voltage IGB
In order to achieve T, it is necessary to maintain the thickness of the conductivity modulation layer 4, and in order to ensure voltage resistance performance, the tail current iT must be maintained.
could not be reduced.

[0007]Furthermore, in order to ensure a safe operation range for use, the element is required to have a voltage resistance several times higher than the voltage generated during normal operation. This requirement is for the purpose of using elements in the circuit configuration that have a withstand voltage higher than the withstand voltage generated during normal operation so that the elements will not be destroyed even when the circuit operates abnormally. For example, if the device is used in a circuit in which 1000V is applied to the device during normal operation, a device with withstand voltage performance of 1200V, which is 20% higher, is selected. Similarly, IGBTs are also required to have a several-fold higher withstand voltage performance, so it is difficult to reduce the thickness of the conductivity modulation layer 4 described above, and for this reason, the loss Eoff
was difficult to reduce.

In view of these problems, in the present invention, the thickness of the conductivity modulation layer is adjusted to correspond to the voltage resistance during normal operation by processing the extra voltage resistance required by the circuit in a built-in circuit. The aim is to realize an IGBT with less turn-off loss.

[0009]

[Means for Solving the Problems] In order to solve the above problems, the present invention first incorporates an avalanche diode between the gate and drain of an IGBT. That is, the IGBT of the present invention includes a MOS section including a conductivity modulation layer of the second conductivity type, a base layer of the first conductivity type and a source layer of the second conductivity type formed on the surface of the conductivity modulation layer. and a conductivity modulation type MOSFET having a first conductivity type drain layer formed on the front or back surface of the conductivity modulation layer facing the MOS portion, the conductivity modulation type MOSFET having A first conductivity type avalanche layer having a small curvature in at least a portion of its cross section is formed on the surface of the conductivity modulation layer, and this avalanche layer is characterized in that it is connected to the gate electrode of the MOS section.

Further, it is preferable that the avalanche layer is connected to the gate electrode through a Zener diode, and the gate electrode is connected to the source electrode of the MOS section through at least one Zener diode. This is effective.

[0011] Such a Zener diode may be formed of polysilicon and may be built into a semiconductor device.

0012

[Operation] In the IGBT as described above, the first conductivity type avalanche layer on the surface of the conductivity modulation layer has a small curvature and a portion where high electric field concentration is likely to occur. Therefore, if an overvoltage higher than normal is applied between the source and drain of the IGBT in the off state where the gate and source are at the same potential, the breakdown voltage will be exceeded at the small curvature part, and the first A diode formed by an avalanche layer of a conductivity type and a conductivity modulation layer of a second conductivity type enters the avalanche. Therefore, a current flows between the gate and the drain via the avalanche layer, and a higher potential than the source is applied to the gate. Therefore, the MO of this IGBT
The S portion is turned on, a current flows between the source and the drain, and the IGBT is protected from an overvoltage condition between the source and the drain.

[0013] As described above, in the IGBT of the present invention,
Since a protection circuit against overvoltage is built into the IGBT body, there is no need to maintain excess voltage resistance in anticipation of overvoltage. Therefore, since the thickness of the conductivity modulation layer is suppressed to the necessary minimum, the amount of carriers swept out when the device is turned off is suppressed, and turn-off loss is reduced.

[0014]

Embodiments Below, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows the structure of an IGBT according to an embodiment of the present invention. The drain layer 1, buffer layer 2, conductivity modulation layer 4, base layer 7, source layer 9,
The structures of the gate electrode 6 and the source electrode 8 are the same as those of the conventional IGBT described above, so the same numbers are given and the explanation is omitted. In the IGBT of this example, in addition to the above-mentioned layers, an avalanche layer 11a is provided on the surface of the conductivity modulation layer 4, which is P-type, shallower than the normal cell 10, and has a small well curvature.
, 11b are formed. Furthermore, Zener diodes 12, 13, and 14 made of polysilicon are formed on the oxide film on the surface of the conductivity modulation layer 4. The avalanche layers 11a and 11b are connected to each other via the Zener diode 12.
It is connected to a gate electrode 6, and this gate electrode 6 is connected to a source electrode 8 via Zener diodes 13 and 14.

FIG. 2 shows an equivalent circuit of the IGBT of this example. In this example, an avalanche diode 11 is connected between the gate G and drain D of the conventional IGBT shown in FIG.
and Zener diodes 12 are inserted so that the directions of the diodes face each other.

Further, Zener diodes 13 and 14 are inserted between the gate G and the source S so that the directions of the diodes face each other.

The on-state and off-state operations of the IGBT of this example are the same as those of the conventional IGBT described above, so a description thereof will be omitted. Focusing on the avalanche layer 11, which is a feature of the IGBT of this example, the base layer 7 and the avalanche layer 11 have a negative potential with respect to the drain layer 1 in the off state, that is, the voltage blocking state of the IGBT.
From there, a depletion region 30 extends toward the drain layer 1, and the depletion electric field is concentrated at the tip 20 of the avalanche layer, which has a small curvature. In such a state, if an overvoltage is applied between the source and drain of the IGBT due to abnormal circuit operation, the breakdown voltage will be exceeded at the tip 20 of the avalanche layer 11 where the depletion field is concentrated. Therefore, the diode formed by the avalanche layer 11 and the conductivity modulation layer 4 enters the avalanche state, and current flows through this diode. Then, the gate potential VG stops rising, and the potential VG becomes positive with respect to the source potential VS. Therefore, the MOS of the cell 10 is turned on, electrons are injected into the conductivity modulation layer 4, and this IGBT is turned on. Therefore, current flows from the drain to the source and the overvoltage condition between the source and drain is eliminated, so that in the IGBT of this example, destruction of the element due to overvoltage is prevented. When the overvoltage condition is eliminated, the avalanche of the diode composed of the avalanche layer 11 and the conductivity modulation layer 4 stops, and the gate potential VG becomes the source potential VS.
, and the IGBT is turned off.

Further, a Zener diode 12 is connected between the avalanche layer 11 and the gate electrode 6.
1, the gate electrode 6 is inserted with the direction of the gate electrode 6 as the forward direction.

Therefore, the on/off state of the gate potential VG during normal operation of the IGBT of this example is prevented from propagating to the drain 1 side. By making this Zener diode 12 made of polysilicon, the IGBT
can be easily formed on the surface of the gate electrode by the same process as the gate electrode.

On the other hand, a pair of Zener diodes 13 and 14 are inserted between the gate electrode 6 and the source electrode 8 to protect the gate oxide film 5. These Zener diodes 13 and 14 are arranged so that the forward directions of each diode face each other, and the surge voltage that occurs when turning the gate on and off due to overvoltage is absorbed.
Destruction of gate oxide film 5 is prevented. Furthermore, these Zener diodes 13 and 14 are also made of polysilicon, and are incorporated as a built-in circuit on the IGBT of this example.

As described above, the IGBT of this example is an IGBT having a built-in overvoltage protection circuit using an avalanche diode.
It also has a built-in gate protection circuit using a Zener diode. Therefore, it is possible to safely cope with overvoltage caused by abnormal operation of the circuit. Therefore, it is not necessary to use an IGBT with a high withstand voltage in the circuit in consideration of safety compared to the conventionally required withstand voltage, and it is possible to maintain the thickness of the conductivity modulation layer to the necessary minimum. In the IGBT of this example, since the thickness of the conductivity modulation layer can be suppressed, it is possible to suppress the tail current accompanying carrier sweep-out during turn-off, and it is possible to provide an element with small turn-off loss. .

Furthermore, in the IGBT of this example,
Since an overvoltage protection circuit and a gate protection circuit are built into the IGBT, it is a three-terminal IGBT that is no different from conventional IGBTs. Therefore, when used in circuits, it can be used in the same manner as in the past. Furthermore, since the diodes used in the above circuit are made of polysilicon, they can be manufactured using conventional IGBT manufacturing processes.
It is easy to manufacture and does not cause an increase in costs. Furthermore, the area occupied by the above protection circuit on the IGBT is 1
%, it is possible to incorporate a protection circuit with almost no effect on the characteristics of the element.

[0024] In this example, the explanation is based on a vertical type in which the drain layer is formed on the back surface of the IGBT.
A horizontal IGBT in which the drain layer is formed on the surface of the IGBT in the same way as the source layer can also incorporate the above-mentioned protection circuit.

[0025]

[Effects of the Invention] As explained above, the semiconductor device (IGBT) equipped with the conductivity modulated MOSFET of the present invention has the following features:
A protection circuit against overvoltage is configured by an avalanche diode and a Zener diode built into the IGBT. Therefore, damage to the element due to overvoltage caused by abnormal circuit operation can be dealt with by the built-in protection circuit. Therefore, since the thickness of the conductive protective layer can be kept to a thickness that corresponds to the withstand voltage required for normal operation, the tail current at turn-off can be suppressed, making it possible to realize an IGBT element with low turn-off loss. Become.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the structure of an IGBT according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an equivalent circuit of the IGBT shown in FIG. 1;

FIG. 3 is a cross-sectional view showing the structure of a conventional IGBT.

FIG. 4 is a circuit symbol of the IGBT shown in FIG. 3;

FIG. 5 is a graph diagram showing a turn-off waveform in a voltage resonance type circuit using an IGBT.

[Explanation of symbols]

1... P+ type drain layer 2...
N+ type buffer layer 3... Semiconductor substrate 4... N- type conductivity modulation layer 5...
Gate oxide film 6...Gate electrode 7...P-type base layer 8...Source electrode 9...N-type source layer 10...Cell (MOS section) 11, 11a, 11b... P-type avalanche layers 12, 13, 14... Zener diode 2
0... Tip portion 30 of the avalanche layer... Depletion electric field VG... Gate potential VS... Source potential VM... Blocking voltage iM... Main current iT... Tail current

Claims (4)

[Claims] 1. A MOS section comprising: a conductivity modulation layer of a second conductivity type; a base layer of a first conductivity type and a source layer of a second conductivity type formed on the surface of the conductivity modulation layer; M.O.S.
In a semiconductor device comprising a conductivity modulation type MOSFET, the conductivity modulation layer has a first conductivity type drain layer formed on a front surface or a back surface of the conductivity modulation layer facing the conductivity modulation layer. A first conductivity type avalanche layer having a small curvature at least in part is formed on the surface of the conductivity modulated MOSFET, the avalanche layer being connected to the gate electrode of the MOS section. semiconductor device. 2. The conductivity modulated MOSFE according to claim 1, wherein the avalanche layer is connected to the gate electrode via a Zener diode.
A semiconductor device equipped with a T. 3. The semiconductor according to claim 1, comprising a conductivity modulated MOSFET, wherein the gate electrode is connected to a source electrode of the MOS section via at least one Zener diode. Device. 4. The semiconductor device according to claim 2, wherein at least one of the Zener diodes is made of polysilicon and is built in the semiconductor device.