Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
  • H01S5/06203—Transistor-type lasers

Definitions

• This invention relates to high speed semiconductor devices and methods, and also to semiconductor devices and methods for producing controlled light emission, and which are also simultaneously capable of electrical signal amplification.
• a part of the background hereof lies in the development of light emitters based on direct bandgap semiconductors such as lll-V semiconductors. Such devices, including light emitting diodes and laser diodes, are in widespread commercial use.
• Another part of the background hereof lies in the development of wide bandgap semiconductors to achieve high minority carrier injection efficiency in a device known as a heterojunction bipolar transistor (HBT), which was first proposed in 1948 (see e.g. U.S. Patent 2,569,376; see also H. Kroemer, "Theory Of A Wide-Gap Emitter For Transistors" Proceedings Of The IRE, 45, 1535-1544 (1957)). These transistor devices are capable of operation at extremely high speeds.
• HBT heterojunction bipolar transistor
• An InP HBT has recently been demonstrated to exhibit operation at a speed above 500 GHz. It is among the objects of the present invention to provide bipolar transistor devices and methods which can operate at extremely high speeds, even potentially exceeding those already achieved. It is also among the objects of the present invention to provide devices and methods for producing controlled light emission, and to also provide devices capable of simultaneous control of optical and electrical outputs.
• An aspect of the present invention involves a direct bandgap heterojunction transistor that exhibits light emission from the base layer. Modulation of the base current produces modulated light emission.
• light means optical radiation that can be within or outside the visible range.
• a further aspect of the invention involves three port operation of a light emitting HBT. Both spontaneous light emission and electrical signal output are modulated by a signal applied to the base of the HBT.
• Another aspect of the invention involves employing stimulated emission to advantage in the base layer of a bipolar transistor (e.g. a bipolar junction transistor (BJT) or a heterojunction bipolar transistor (HBT), in order to enhance the speed of the transistor. Spontaneous emission recombination lifetime is a fundamental limitation of bipolar transistor speed.
• BJT bipolar junction transistor
• HBT heterojunction bipolar transistor
• the base layer of a bipolar transistor is adapted to enhance stimulated emission (or stimulated recombination) to the detriment of spontaneous emission, thereby reducing recombination lifetime and increasing transistor speed.
• at least one layer exhibiting quantum size effects preferably a quantum well or a layer of quantum dots, preferably undoped or lightly doped, is provided in the base layer of a bipolar transistor.
• at least a portion of the base layer containing the at least one layer exhibiting quantum size effects is highly doped, and of a wider bandgap material than said at least one layer.
• the at least one quantum well, or layer of quantum dots, within the higher gap highly doped material enhances stimulated recombination and reduces radiative recombination lifetime.
• a two-dimensional electron gas (“2-DEG”) enhances carrier concentration in the quantum well or quantum dot layer, thereby improving mobility in the base region. Improvement in base resistance permits reduction in base thickness, with attendant reduction of base transport time.
• quantum well layer(s) and/or quantum dot layers exhibiting quantum size effects preferably have a thickness that is not greater than about 100 Angstroms.
• a method is set forth for producing controllable light emission from a semiconductor device, including the following steps: providing a heterojunction bipolar transistor device that includes collector, base, and emitter regions; and applying electrical signals across terminals coupled with the collector, base, and emitter regions to cause light emission by radiative recombination in the base region.
• the step of applying electrical signals includes applying a collector- to-emitter voltage and modulating light output by applying a modulating base current.
• a device having an input port for receiving an electrical input signal, an electrical output port for outputting an electrical signal modulated by the input signal, and an optical output port for outputting an optical signal modulated by the input signal, the device comprising a heterojunction bipolar transistor device that includes collector, base, and emitter regions, the input port comprising an electrode coupled with the base region, the electrical output port comprising electrodes coupled with the collector and emitter regions, and the optical output port comprising an optical coupling with the base region.
• a semiconductor laser including: a heterojunction bipolar transistor structure comprising collector, base, and emitter of direct bandgap semiconductor materials; an optical resonant cavity enclosing at least a portion of the transistor structure; and means for coupling electrical signals with the collector, base, and emitter regions to cause laser emission from the transistor structure.
• a method for increasing the speed of a bipolar transistor, including the following steps: providing a bipolar transistor having emitter, base, and collector regions; providing electrodes for coupling electrical signals with the emitter, base, and collector regions; and adapting the base region to enhance stimulated emission to the detriment of spontaneous emission, so as to reduce carrier recombination lifetime in the base region.
• the step of adapting the base region to enhance stimulated emission to the deteriment of spontaneous emission comprises providing, in the base region, at least one layer exhibiting quantum size effects, preferably a quantum well(s) and/or a layer(s) of quantum dots, preferably undoped or lightly doped.
• a plurality of spaced apart quantum size regions e.g. quantum wells and/or quantum dots
• the base region can be provided with several spaced apart quantum size regions of different thicknesses, with the thicknesses of the quantum size regions being graded from thickest near the collector to thinnest near the emitter.
• An injected electron is captured in a smaller well, tunnels into the next bigger well, and then the next bigger well, and so forth, until, at the biggest well closest to the collector, it tunnels to and relaxes to the lowest state of the biggest well and recombines.
• the arrangement of wells encourages carrier transport unidirectionally from emitter toward collector. Maximum recombination and light are derived from the biggest well as near as possible to the collector, which is an advantageous position, such as for optical cavity reasons. Carriers diffuse "downhill" in energy; i.e., toward the thicker wells. The asymmetry in well size provides improved directionality and speed of carrier transport. In embodiments as a light emitting HBT, light emission and device speed are both enhanced. Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.
• Figure 1 is a simplified cross-sectional diagram, not to scale, of a device in accordance with an embodiment of the invention, and which can be used in practicing an embodiment of the method of the invention.
• Figure 2 is a top view of the Figure 1 device layout for an embodiment of the invention.
• Figure 3 is CCD microscopic view of a test device in accordance with an embodiment of the invention.
• Figure 4 is a simplified schematic diagram of a three port device in accordance with an embodiment of the invention.
• Figure 5 is a graph of the common emitter output characteristics of the test device, also showing the observed light emission.
• Figure 7 is a graph showing light output as a function of base current for the test device.
• Figure 8 is a diagram and band diagram of a Type II InP/GaAsSb/lnP double heterojunction bipolar transistor (DHBT), a light emitting transistor (LET), with a 30 nm p-type GaAsSb base generating a recombination-radiation signal in normal transistor operation (collector in reverse bias).
• Figure 9 shows the common emitter output characteristics, collector current vs. collector-to-emitter voltage (l-V curve), of the Type-ll transistor of Figure 8 (emitter area 120 x 120 ⁇ m 2 ).
• Figure 10 is a graph of optical emission intensity (recombination radiation) of the Type-ll DHBT of Figure 8 as a function of base current, with an L-l characteristic demonstrating nearly linear behavior.
• the inset shows the wavelength of the recombination radiation from the p-type GaAsSb base at various base currents.
• Figure 11 illustrates three-port operation of the Type-ll DHBT of Figure 8 biased in the common emitter configuration: (a) a 10 kHz input signal (upper trace, port 1), (b) amplified output signal (middle trace, port 2), and (c) optical output modulated at 10 kHz (lowest trace, port 3).
• Figure 12 illustrates an embodiment of the invention that includes a light reflector.
• Figure 13 illustrates a laser device in accordance with an embodiment of the invention.
• Figure 14A shows a portion of a device in accordance with an embodiment of the invention, employing one or more quantum wells.
• Figure 14B shows a portion of a device in accordance with an embodiment of the invention, employing one or more regions of quantum dots.
• Figure 15 is a simplified cross-sectional diagram, not to scale, of a device in accordance with an embodiment of the invention, and which can be used in practicing an embodiment of the method of the invention.
• Figure 16 is an energy band diagram for an example of the device of Figure 15.
• Figure 17 is an energy band diagram for an example of another device in accordance with an embodiment of the invention.
• Figure 18 is an energy band diagram for an example of a further device in accordance with an embodiment of the invention.
• Figures 19-21 are energy band and structure diagrams for further devices with pluralities of quantum wells, in accordance with embodiments of the invention.
• Figure 22 is a diagram and band diagram of a quantum well (QW) InGaP/GaAs heterojunction bipolar transistor (QW HBT), a light emitting transistor (LET), with two 50 A InGaAs QWs imbedded in the p-type GaAs base to aid electron capture and enhance the recombination radiation.
• QW quantum well
• QW HBT quantum well
• LET light emitting transistor
• Figure 24 shows the optical output intensity (power) of the QW HBT of Figure 22 as a function of base current, demonstrating a nearly linear increase with current.
• Figure 25 shows graphs, for l B equal to 1 , 2, and 3 mA, of emission wavelength due to band-to-band recombination in the p-type GaAs base and the base InGaAs quantum wells of the QW HBT of Figures 22 and 23.
• Figure 26 illustrates three-port operation of the QW HBT of Figures 22 and 23 biased in the common emitter configuration: (a) a 1 GHz input signal (upper trace, port 1), (b) amplified output signal (middle trace, port 2), and (c) optical output modulated at 1 GHz (lowest trace, port 3).
• Figure 27 is a simplified cross-sectional diagram, not to scale, of a vertical cavity surface emitting laser in accordance with an embodiment of the invention.
• Figure 28 is a simplified cross-sectional diagram, not to scale, of a vertical cavity surface emitting laser in accordance with a further embodiment of the invention.
• Figure 29 is a simplified diagram of an array in accordance with an embodiment of the invention.
• Figure 30A is a cross-sectional broken-away view, not to scale, of a device in accordance with an embodiment of the invention and which can be used in practicing an embodiment of the method of the invention.
• Figure 30B is a cross-sectional broken-away view, not to scale, of another device in accordance with an embodiment of the invention and which can be used in practicing an embodiment of the method of the invention.
• Figure 1 illustrates a device in accordance with an embodiment of the invention and which can be used in practicing an embodiment of the method of the invention.
• a substrate 105 is provided, and the following layers are disposed thereon: subcollector 110, collector 130, base 140, emitter 150, and cap layer 160. Also shown are collector metallization (or electrode) 115, base metallization 145, and emitter metallization 165. Collector lead 117, base lead 147, and emitter lead 167 are also shown.
• This embodiment employs a fabrication process sequence which includes e-beam defined Ti/Pt/Au emitter contacts (165), a self-aligned emitter etch, a self- aligned Ti/Pt/Au base metal deposition, a base-collector etch, and collector metal deposition.
• a bisbenzocyclobutene (BCB) based etch-back process is employed for "backend" fabrication (i.e., to render the electrode and contact formation on the top of the transistor).
• BCB bisbenzocyclobutene
• the recombination process is based on both an electron injected from the n-side and a hole injected from the p-side, which in a bimolecular recombination process can be limited in speed.
• the base current can be classified into seven components, namely: (1) hole injection into the emitter region (/ ⁇ p ); (2) surface recombination current in the exposed extrinsic base region (i ⁇ surf)', (3) base ohmic contact recombination current (i ⁇ con t Y, (4) space charge recombination current (i ⁇ scr)', (5) bulk base non-radiative recombination current due to the Hall-Shockley-Reed process (HSR) (IBHSR)', (6) bulk base Auger recombination current (i ⁇ Aug)', and (7) bulk base radiative recombination current ( i ⁇ rad)-
• HSR Hall-Shockley-Reed process
• the base current and recombination lifetime can be approximated as primarily bulk HSR recombination, the Auger process, and radiative recombination.
• the light emission intensity ⁇ l in the base is proportional to i Brad and is related to the minority carrier electron with the majority hole over the intrinsic carrier concentration, (np-n, 2 ), in the neutral base region and the rate of radiative recombination process, B, set forth in Equation (3) below, where the hole concentration can be approximated as equal to base dopant concentration, N B .
• the optical recombination process in the base should be at least two times faster than the speed of the HBT. In other words, HBT speed, which can be extremely fast, is limiting.
• Figure 2 shows the top view of the device layout and Figure 3 shows a silicon CCD microscopic view of a fabricated 1 x 16 ⁇ m 2 HBT test device with light emission (white spots) from the base layer under normal operation of the transistor.
• one of the three terminals of a transistor is common to both the input and output circuits. This leads to familiar configurations known as common emitter (CE), common base (CB), and common collector (CC).
• CE common emitter
• CB common base
• CC common collector
• the common terminal (often ground reference) can be paired with one or the other of the two remaining terminals. Each pair is called a port, and two pairs for any configurations are called a two-port network.
• the two ports are usually identified as an input port and as an output port.
• a third port namely an optical output port, is provided, and is based on (recombination-radiation) emission from the base layer of the HBT light emitter in accordance with an embodiment of the invention.
• a common emitter configuration see Figure 4 when an electrical signal is applied to the input port (Port 1), there results simultaneously an electrical output with signal amplification at Port 2 and optical output with signal modulation of light emission at Port 3.
• the common emitter output characteristics of the test version of the Figure 1, 2 device are shown in Figure 5.
• a bias tee combines this AC signal with a DC bias voltage of 1.1V from a DC supply.
• the HBT transistor's emission area (open space of the base region) is less than 1- ⁇ m x 2- ⁇ m.
• the light from the small aperture is coupled into a multimode fiber probe with a core diameter of 25 ⁇ m.
• the light is fed into a Si APD detector with a 20-dB linear amplifier.
• a sampling oscilloscope displays both the input modulation signal and the output light signal.
• the optical emission wavelength is around 885nm due to the compositionally graded InGaAs base (1.4% In).
• Figure 6 shows the input (lower trace) reference and output (upper trace) light waveforms when the HBT is modulated at 1MHz (Fig. 6A) and also at 100KHz (Fig. 6B).
• the output signal has a peak-to-peak amplitude of 375 ⁇ V at 1MHz and 400 ⁇ V at 100KHz.
• GaAsSb has been proposed as an alternative to InGaAs for the base of InP HBTs due to a more favorable Type-ll bandgap line-up (hole confined, electron not) at the base-collector (or emitter) junction (as can be seen in Figure 8 below). Since Type-ll InP-based HBTs have a larger valence band discontinuity (as can be seen in Figure 8 below), there is superior hole blocking at the emitter (see R. Bhat, W.-P. Hong, C. Caneau, M. A. Koza, C.-K. Nguyen, and S. Goswami, Appl. Phys. Lett. 68, 985 (1995); . T. McDermott, E. R.
• Type-ll InP/GaAsSb DHBTs double HBTs
• InP/GaAsSb DHBTs have been reported to achieve cutoff frequencies above 300 GHz (see Dvorak, C. R. Bolognesi, O. J. Pitts, and S. P. Watkins, IEEE Elec. Dev. Lett. 22, 361 (2001).
• Figure 8 shows a band digaram of a Type II InP/GaAsSb/lnP double heterojunction bipolar transistor (DHBT), the physical structure and operation of which can be similar to that of Figures 1-4 above.
• the light-emitting transistor has a 30 nm p- type GaAsSb base generating a recombination-radiation signal in normal transistor operation (collector in reverse bias).
• the layer structure for the present example is grown by MOCVD on a semi-insulating, Fe-doped InP substrate.
• the HBLET includes: a 150 nm InP collector, Si-doped to 3xe16 cm "3 ; a 30 nm GaAs 0 . 5 iSb 0 .
• the DHBT device is fabricated using a standard mesa process.
• collector emitter voltage (l-V) curve of the described Type-ll transistor with a 120 x 120 ⁇ m 2 emitter area is shown in Figure 9.
• the ideality factor for lower base currents is around 1.9 indicating a considerable number of traps in the base- emitter space charge region (SCR).
• the ideality factor for higher base currents is 1.3, indicating more surface recombination once the SCR traps are filled.
• the current gain cutoff frequency, f t is measured at 800 MHz.
• the power gain cutoff frequency, f max is 300 MHz.
• the base current owing to surface recombination is relatively small for a Type II DHBT of such a large emitter area as 120 x 120 ⁇ m 2 .
• the base current can be approximated as primarily non-radiative Hall-Shockley-Read (HSR) recombination in the emitter — base space charge region and radiative recombination in the base neutral region.
• HSR Hall-Shockley-Read
• the light emission intensity ⁇ l is proportional to the component of base current supplying radiative recombination, i Br ad, which is proportional to the excess (injected) minority carriers, ⁇ n, in the neutral base region, the charge, q, the emitter area, A E , and inversely proportional to the radiative recombination lifetime, ⁇ rad .
• the base hole concentration is so high that an electron injected into the base, recombines rapidly (bi-molecularly).
• the base current merely re-supplies holes via relaxation to neutralize charge imbalance.
• the broadened light emission extends from 1450 to 1750 nm owing mainly to the alloy scattering of the GaAsSb base layer.
• a pattern generator producing an AC input signal at 10 kHz was used for a test of light output modulation.
• the 120 x 120 ⁇ m 2 DHBT recombination radiation was fed into a germanium PIN detector integrated with a JFET pre-amplifier.
• FIG. 11 shows traces from a four channel sampling oscilloscope and illustrates three port operation.
• the third trace (c) shows the output optical signal modulated at 10 kHz (at port 3).
• Figure 12 illustrates use of the three terminal light emitting HBT 810 in conjunction with a reflector cup 820 for enhancing light collection and directionality.
• Figure 13 illustrates the three terminal light emitting HBT, 910, in a lateral cavity, represented at 920, for operation as a lateral gain guided laser.
• the lateral cavity may be defined, for example, by cleaved edges on or near the light emitting region.
• An aspect of the invention involves employing stimulated emission to advantage in the base layer of a bipolar transistor (e.g. a bipolar junction transistor (BJT) or a heterojunction bipolar transistor (HBT), in order to enhance the speed of the transistor.
• BJT bipolar junction transistor
• HBT heterojunction bipolar transistor
• the base layer of a bipolar transistor is adapted to enhance stimulated emission (or stimulated recombination) to the detriment of spontaneous emission, thereby reducing recombination lifetime and increasing transistor speed.
• at least one layer exhibiting quantum size effects preferably a quantum well or a layer of quantum dots, preferably undoped or lightly doped, is provided in the base layer of a bipolar transistor.
• at least a portion of the base layer containing the at least one layer exhibiting quantum size effects is highly doped, and of a wider bandgap material than said at least one layer.
• the at least one quantum well, or layer of quantum dots, within the higher gap highly doped material enhances stimulated recombination and reduces radiative recombination lifetime.
• a two-dimensional electron gas (“2-DEG”) enhances carrier concentration in the quantum well or quantum dot layer, thereby improving mobility in the base region. Improvement in base resistance permits reduction in base thickness, with attendant reduction of base transport time.
• quantum wells are also utilized.
• Figure 14A shows the use of one or more quantum wells, 141 , 142, in the base region 140 of the Figure 1 device (or other embodiments), these quantum wells being operative to enhance the recombination process for improved device speed, modulation characteristics, and/or to tailor the spectral characteristics of the device.
• the quantum well(s) are of lower bandgap than the surrounding base layer (140) material and are undoped or lightly doped (e.g. below about 10 16 cm "3 ).
• the surrounding base layer (140) material is highly doped (e.g.
• the quantum well (or dot) layer(s) have a thickness not greater than about 100 Angstroms.
• a cavity with reflectors can be utilized laterally (e.g. Figure 13) or vertically (e.g. Figures 27 and 28) to obtain controlled laser operation of a light emitting HBT. As described above, enhancing stimulated emission can reduce recombination lifetime, to increase speed of operation. If desired, the reflectors (e.g.
• FIG. 14B shows use of one or more regions of quantum dots, 143, 144, in the base region 140 of the Figure 1 device (or other embodiments), these quantum dot regions being operative to enhance the recombination process for improved device speed, modulation characteristics, and/or to tailor the spectral characteristics of the device. Examples of structures and material systems with a lightly doped or undoped quantum well(s) in a highly doped p+ base are shown in Figures 15-22.
• DHBT Type I InP double heterojunction bipolar transistor
• 105 semi-insulating InP substrate
• 110 n+ InGaAs sub-collector
• 115 collector metallization
• 130 n- InP collector
• 140 p+ InP base
• 141 undoped InGaAs QW in base
• 147 base metallization
• 150 n InP emitter
• 160 n+ InGaAs emitter cap
• 165 emitter metallization.
• An example of a variation on this Type I structure includes the following elements: 130: n I nGaAsP collector; 140: p+ InGaAsP base; 141 : undoped InGaAs QW in base; 150: n InP emitter.
• An example of a Type II InP DHBT has the following elements: 105 semi-insulating InP substrate; 110 n+ InGaAs sub-collector; 115 collector metallization; 130 n- InP collector; 140 p+ InP base; 141 : undoped GaAsSb QW in base; 147: base metallization; 150: n InP emitter; 160: n+ InGaAs emitter cap; 165: emitter metallization.
• DHBT Type II structure
• SHBT Type II structure
• 130 n InP collector
• 140 p+ GaAsSb base
• 141 undoped InGaAs QW in base
• 150 n InP emitter.
• Type I GaAs SHBT or DHBT include the following elements: 130 n GaAs collector; 140 p+GaAs base; 141 undoped InGaAs QW in base; 150 InGaP emitter or 130: N GaAs collector; 140: p+ GaAs base; 141 : undoped InGaAs QW in base; 150: AIGaAs emitter.
• Other material systems for example devices based on GaN, can also be used.
• Figures 19, 20, and 21 show further band diagrams for HBTs quantum wells in the base.
• Figure 19 shows the structure and band diagram for an HBT with InP emitter and collector and an InGaAs subcollector.
• the base region comprises heavily doped (p+) GaAsSb that includes an undoped GaAsSb quantum well (which enchances recombination of holes) and a p+ InGaAs quantum well (which enhances recombination of electrons).
• Figure 20 shows the structure and band diagram for an HBT, again with InP emitter and collector and an InGaAs subcollector.
• the base region comprises heavily doped (p+) GaAsSb that includes two undoped GaAsSb quantum wells and two p+ InGaAs quantum wells.
• the base region includes three p+ InP quantum wells, with two intervening undoped InGaAs quantum wells.
• quantum wells can be employed to advantage in the base region of an HBT.
• QWs quantum wells
• the band diagram of a light emitting transistor, an InGaP (n) /GaAs (p+) /GaAs (n) single heterojunction bipolar transistor (SHBT), with two thin InGaAs quantum wells in the heavily doped base, is shown in Figure 22, and the layer structure will be evident from the diagram and the descriptions above and to follow.
• the base-emitter junction is forward biased and the base-collector junction is reversed biased in the common-emitter configuration in normal transistor mode operation. Light emission from the base at two different wavelengths is expected for band-to-band recombination transitions involving both the GaAs base and the InGaAs QWs.
• the light emission intensity ⁇ l is proportional to the component of base current supplying radiative recombination, ⁇ Brad , which is proportional to the excess minority carriers, ⁇ n, in the neutral base region, the charge, q, the emitter area, A E , and inversely proportional to the radiative recombination lifetime, ⁇ ra (M. Feng, N. Holonyak, Jr. and W. Hafez, Appl. Phys. Lett. 84, 151 , 5 Jan.
• Figure 23(a) shows the top view layout of the 45 ⁇ m-diameter HBT
• Figure 23(b) shows, via a microscope equipped with a charge-coupled device detector, a view of the same HBT with obvious spontaneous light emission (recombination radiation) from the base layer in the open area of the base to emitter region.
• Forward bias was used separately (data not shown) on the emitter-base (E-B) junction to merely reveal the light-emitting aperture of the HBT.
• E-B emitter-base
• C-B collector- base
• the light emission wavelength of the device operating as an HBT is centered near 910 nm for the band-to-band recombination transition of GaAs and 960 nm for the InGaAs QW transition.
• the light emission extends from 825 to 910 nm due to hot-electron injection from the InGaP emitter into the p-type GaAs base, and subsequent relaxation and recombination resulting in longer wavelength emission.
• a pattern generator produced an AC input signal at 1 GHz for a light output modulation test.
• the HBT light was coupled into a multimode fiber probe with a core diameter of 25 ⁇ m and captures only a small fraction of the light.
• the light was fed into a silicon avalanche photodetector (APD) equipped with a 20-dB linear amplifier.
• the 3 dB bandwidth of the APD with a linear amplifier was 700 MHz.
• FIG. 26 shows traces from a four channel sampling oscilloscope and illustrates three port operation.
• the third trace (c) shows the output optical signal modulated at 1 GHz (at port 3) with peak-to-peak amplitude of 1 mV.
• the optical output of an LET can be modulated faster than the power gain cutoff frequency, f max , and, in fact, can also be faster than the current gain cutoff frequency, f t , of the HBT since the base recombination process is much shorter than the delay time of the forward transit time of the HBT.
• This example with quantum-well enhancement of HBT base recombination, establishes high speed three-port operation of a quantum well(s) HBT in the common-emitter bias configuration.
• Figure 27 shows a vertical cavity surface emitting laser in accordance with an embodiment of the invention which employs light emission from the base region of an HBT.
• a substrate 1105 is provided, and the following layers are provided thereon.
• collector metallization 1115, base metallization 1145, and emitter metallization 1165 are also shown.
• the layers are grown by MOCVD
• the substrate 1105 is a semi-insulating InP substrate
• subcollector 1110 is n+ InGaAs
• collector 1130 is n- InP
• the base 1140 is a p+ InGaAs layer with a quantum well
• the emitter 1150 is n-type InP
• the emitter cap 1160 is n+ InGaAs.
• the transition layer is an n-type quaternary transition layer, for example InGaAsP.
• the reflector layers 1108 and 1168 are multiple layer DBR reflectors, which can be spaced apart by suitable distance, such as a half wavelength.
• modulation of the base current produces modulated light emission, in this case vertically emitted laser light represented by arrow 1190.
• modulated light emission in this case vertically emitted laser light represented by arrow 1190.
• the base layer 1140 can be provided with quantum well(s) or dot layer(s), as described elsewhere herein.
• Figure 28 shows a further embodiment of a vertical cavity surface emitting laser, which has a Bragg reflector as close as possible to the collector and with elimination of intervening lower gap absorbing layers between the DBRs.
• Figure 28 (which has like reference numerals to Figure 1 for corresponding elements), the lower DBR is shown at 111 , and an upper DBR is shown at 143.
• Arrow 190 represents the optical standing wave of the VCSEL.
• the DBR 141 can be a deposited Si-SiO 2 Bragg reflector. A further reflector can also be provided on the top of emitter 150.
• the base layer 140 can be provided with quantum well(s) or dot layer(s), as described elsewhere herein.
• Figure 29 shows a display 1310 using an array of light-emitting HBTs 1331 , 1332, 1341, etc. The light output intensities can be controlled, as previously described. Very high speed operation can be achieved, with or without useful light emission from some devices.
• the base region 140 of a bipolar transistor (for example the heterojunction biopolar light-emitting transistor of Figure 1) includes a first relatively thicker quantum well 3041 in relatively closer proximity to the collector region 130, and a second relatively thinner quantum well 3042 in relatively closer proximity to the emitter region 150.
• the grading of well geometry is used to promote carrier transport from the emitter toward the collector. (Regarding a graded energy gap for GaAs wells alternating with AIAs barriers in a laser diode, see N. Holonyak "Quantum-Well And Superlattice Lasers: Fundamental Effects" pp.
• the quantum well regions 3041 and/or 3042 could alternatively be quantum dot regions, or one could be a quantum well region and another could be a quantum dot region.
• the regions 3041 and 3042 can also, if desired, have different compositions.
• the spacings between quantum wells (and/or quantum dot regions) can be varied in thickness and/or in compositions. This form of the invention will be further understood from the embodiment shown in Figure 30B, in which the base region 140 includes spaced-apart quantum wells 3046, 3047, 3048 and 3049.
• the quantum well 3046 nearest the collector is 80 Angstroms thick
• the next quantum well (3047) is 40 Angstroms thick
• the next quantum well (3048) is 20 Angstroms thick
• the quantum well 3049 (which is closest to the emitter 150) is 10 Angstroms thick.
• the spacings or barriers between quantum wells are in the range between about 5 to 50 Angstroms, and are not necessarily all the same.
• An injected electron is captured in a smaller well, tunnels into the next bigger well, and then the next bigger well, and so forth, until, at the biggest well closest to the collector, it tunnels to and relaxes to the lowest state of the biggest well and recombines.
• the arrangement of wells encourages carrier transport unidirectionally from emitter toward collector.
• any or all of the quantum wells can be quantum dot regions, and/or can be of different compositions than the other quantum well (or dot) regions. Also, it will be understood that other numbers of wells can be used and that some of the quantum wells (or dot regions, as the case may be) in the base region can have the same thicknesses as other quantum wells in the base region.
• the principles hereof can also potentially have application to indirect bandgap materials (such as Ge and Si) in an HBT with a heavily doped base region, and with an optical port that is optically coupled with the base region.
• the light produced will generally be of less intensity than that produced by the direct bandgap HBT light emitters hereof.

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• 2004
  • 2004-08-20 EP EP04781659A patent/EP1656701A4/de not_active Withdrawn
  • 2004-08-20 KR KR1020067003675A patent/KR20060063947A/ko not_active Application Discontinuation
  • 2004-08-20 CA CA002536329A patent/CA2536329A1/en not_active Abandoned
  • 2004-08-20 JP JP2006524067A patent/JP2007503710A/ja active Pending
  • 2004-08-20 WO PCT/US2004/027019 patent/WO2005020287A2/en active Search and Examination

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