Abstract:
A semiconductor light-emitting transistor device, including: a bipolar pnp transistor structure having a p-type collector, an n-type base, and a p-type emitter; a first tunnel junction coupled with the collector, and a second tunnel junction coupled with the emitter; and a collector contact coupled with the first tunnel junction, an emitter contact coupled with the second tunnel junction, and a base contact coupled with the base; whereby, signals applied with respect to the collector, base, and emitter contacts causes light emission from the base by radiative recombination in the base.

Description:
This is a continuation of U.S. application Ser. No. 11/364,893 filed Feb. 27, 2006 now U.S. Pat. No. 7,535,034. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with Government support under Contract Number HR0011-04-1-0034 awarded by the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to semiconductor devices and methods, and also to PNP bipolar transistors, PNP bipolar light emitting transistors, and PNP bipolar transistor lasers. 
     BACKGROUND OF THE INVENTION 
     As described, for example, in PCT International Publication Number WO 2005/020287A2, and several publications [see, for example, M. Feng, N. Holonyak, Jr., and W. Hafez, “Light-Emitting Transistor: Light Emission From InGaP/GaAs Heterojunction Bipolar Transistors”, Appl. Phys. Lett. 84, 151 (2004); M. Feng, N. Holonyak, Jr., and R. Chan, “Quantum-Well-Base Heterojunction Bipolar Light-Emitting Transistor”, Appl. Phys. Lett. 84, 1952 (2004); M. Feng, N. Holonyak, Jr., B. Chu-Kung, G. Walter, and R. Chan, “Type-II GaAsSb/InP Heterojunction Bipolar Light-Emitting Transistor”, Appl. Phys. Lett. 84, 4792 (2004); G. Walter, N. Holonyak, Jr., M. Feng, and R. Chan, “Laser Operation Of A Heterojunction Bipolar Light-Emitting Transistor”, Appl. Phys. Lett. 85, 4768 (2004); R. Chan, M. Feng, N. Holonyak, Jr., and G. Walter, “Microwave Operation And Modulation Of A Transistor Laser”, Appl. Phys. Lett. 86, 131114 (2005); M. Feng, N. Holonyak, Jr., G. Walter, and R. Chan, “Room Temperature Continuous Wave Operation Of A Heterojunction Bipolar Transistor Laser”, Appl. Phys. Lett. 87, 131103 (2005)], there has been developed and demonstrated a direct bandgap heterojunction bipolar transistor (HBT) that exhibits light emission from the base layer. Modulation of the base current produces modulated light emission. [As used herein, “light” means optical radiation that can be within or outside the visible range.] Three port operation of a light emitting HBT has been demonstrated. Both spontaneous light emission and electrical signal output are modulated by a signal applied to the base of the HBT. 
     Another aspect disclosed in the referenced U.S. patent applications and/or publications 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. In a form of the disclosed structures, 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. Toward this end, and with other advantages, at least one layer exhibiting quantum size effects, such as a quantum well or a layer of quantum dots, can be provided in the base layer of the bipolar transistor. Preferably, 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. As disclosed in the referenced U.S. patent applications and/or publications, advantages in speed are applicable in high speed bipolar transistors in which light emission is utilized, and/or in high speed bipolar transistors in which light emission is not utilized. In light emitting bipolar transistor devices, for example heterojunction bipolar transistors of direct bandgap materials, the use of one or more layers exhibiting quantum size effects can also be advantageous in enhancing light emission and customizing the emission wavelength characteristics of the devices. By providing an optical resonant cavity enclosing at least a portion of the transistor base, a controllable high speed semiconductor laser is achieved. In this device, some gain β(β≡ΔI c /ΔI b ), is traded off for enhanced recombination (β spon &gt;β stim ). 
     The highly doped p-type base is essential to the operation of the n-p-n transistor lasers that have been constructed, but is also in some respects limiting. It poses conductive (resistive) loss because holes have relatively low mobility. Also, the high base doping (for example, of the order of 10 19  cm −3 ) leads to considerable free carrier absorption. 
     Although it has been understood that the above described types of light-emitting bipolar transistors and bipolar transistor lasers could theoretically be npn or pnp, to applicant&#39;s knowledge, the operational devices that have been made and demonstrated so far, have been npn devices. This is not surprising. In several respects, p-type material is recognized as being more difficult to work with than n-type material, and tends to be operationally inferior to corresponding n-type material with regard to carrier mobility and overall electrical efficiency. Accordingly, it is often considered desirable to favor the use of n-type semiconductor material in the fabrication of semiconductor devices such as III-V light emitting devices. However, even though the substrate and a fractional majority of the semiconductor volume in such devices may be n-type semiconductor or undoped semiconductor, a substantial amount of p-type material is generally considered necessary as a source of hole current in various semiconductor devices. 
     It is among the objects of the present invention to provide improved pnp bipolar transistors, and especially pnp light emitting bipolar transistors, including pnp bipolar transistor lasers. 
     SUMMARY OF THE INVENTION 
     For some applications, it may be desirable that a transistor laser be a pnp HBT rather than an npn HBT, assuming this leads to lower base region resistive loss (which is driven by lateral base currents) and, in addition, assuming lower free carrier absorption (N DONOR &lt;N ACCEPTOR ) since the base is located largely in the high field active region. In such applications we prefer to put heavily doped p-type crystal outside of the base region and to some extent outside of the high field active region of the transistor laser. Accordingly, one of the features of the invention is to devise an improved HBT laser, and to minimize the amount of acceptor-doped crystal required in the p-type emitter and in the high field p-type collector, by making these regions relatively thin and contacting them via tunnel junctions (i.e., p+ region contacted by n+ region to minimize the total thickness of p-type emitter and collector). [The “+” notation conventionally means “heavily doped”, and, for purposes hereof, is generally donor impurity concentration of at least about 10 18 /cm 3  for n+, and acceptor impurity concentration of at least about 10 19 /cm 3  for p+.] In accordance with a feature of the invention, a pnp HBT light emitter is made with just enough p-type crystal to render operative the emitter (hole injection) function and the carrier collector function. That is, the current of the device is carried, to the extent possible, in higher mobility n-type crystal and not lower mobility p-type crystal, thereby minimizing resistive loss. 
     Tunneling in GaAs, at an n+/p+junction, is well known (see, for example, N. Holonyak, Jr. and I. A. Lesk, Proc. IRE 48, 1405, 1960), and was once generally of interest for its negative resistance. Tunneling in GaAs can be enhanced with an InGaAs transition region (see, for example, T. A. Richard, E. I. Chen, A. R. Sugg. G. E. Hofler, and N. Holonyak, Jr., Appl. Phys. Lett. 63, 3613, 1993), and besides its negative resistance behavior, can be used in reverse bias as a form of “ohmic” contact. This allows, for example, the reversal of the doping sequence of an Al x Ga 1-x As—GaAs quantum well heterostructure laser (n→p to p→n) grown on an n-type GaAs substrate (see, for example, A. R. Sugg, E. I. Chen, T. A. Richard, S. A. Maranowski, and N. Holonyak, Jr., Appl. Phys. Lett. 62, 2510 (1993)). As described in the background portion of Holonyak et al. U.S. Pat. No. 5,936,266, a tunnel contact junction can be used in a light emitting semiconductor diode as a hole source and makes possible lateral bias currents (electron current) to drive a quantum well heterostructure (QWH) laser diode without the compromise of the low mobility and large resistive voltage drop of lateral conduction in thin p-type layers. This is particularly valuable in QWH laser diodes employing upper and/or lower native oxide confining layers (see, for example, M. Dallesasse, N. Holonyak Jr., A. R. Sugg, T. A. Richard, and N. E I Zein, Appl. Phys. Lett 57 2844, 1990; A. R. Sugg, E. I. Chen, T. A. Richard, N. Holonyak, Jr., and K. C. Hsieh, Appl. Phys. Lett. 62, 1259, 1993) that require lateral bias currents (see, for example, P. W. Evans, N. Holonyak, Jr., S. A. Maranowski, M. J. Ries, and E. I. Chen, Appl. Phys. Lett. 67, 3168, 1995), or in devices such as a vertical cavity surface emitting laser (VCSEL) where lateral hole currents have been employed (see, for example, D. L. Huffker, D. G. Deppe, and K. Kumar, Appl. Phys. Lett. 65, 97, 1994). The structure in the U.S. Pat. No. 5,936,266 involved lateral current flow in laser diodes with hole conduction along a layer introducing a large device series resistance, because of the low hole mobility in GaAs, with increased threshold voltages and device heating. The solution to this drawback in the &#39;266 patent involved a tunnel contact junction on the p side of an oxide confined QWH that was used to replace lateral hole excitation currents. The hole injection was supported by a lateral electron current, thus providing lower voltage drop and less series resistance. One of the objectives there, as here, was to minimize the amount of p-type material and, to the extent possible, employ only n-type layers (electron conduction) to carry the device current. However, the problems in the present situation have different aspects, since a bipolar transistor is involved. As will be seen, part of the solution involves use of a tunnel junction for conversion from electron current to hole current, and another part of the solution involves use of a tunnel junction, in opposing orientation, for conversion of hole current to electron current. 
     In accordance with an embodiment of the invention, there is provided a semiconductor light-emitting transistor device which comprises: a bipolar pnp transistor structure having a p-type collector, an n-type base, and a p-type emitter; a first tunnel junction coupled with said collector, and a second tunnel junction coupled with said emitter; and a collector contact coupled with said first tunnel junction, an emitter contact coupled with said second tunnel junction, and a base contact coupled with said base; whereby, signals applied with respect to said collector, base, and emitter contacts causes light emission from said base by radiative recombination in said base. In a preferred form of this embodiment of the invention, the first tunnel junction comprises a layered n+/p+ region with the n+ layer of said n+/p+ region being coupled with said collector contact and the p+ layer of said n+/p+ region being coupled with said collector. Also, the second tunnel junction comprises a layered n+/p+ region with the n+ layer of said n+/p+ region being coupled with said emitter contact and the p+ layer of said n+/p+ region being coupled with said emitter. 
     A form of the described embodiment is a semiconductor laser device comprising the above-defined semiconductor light-emitting transistor device, further including an optical resonant cavity enclosing at least a portion of the base of said device. In one version of this form of the invention, at least a portion of said device is in layered form, and the optical resonant cavity is a lateral cavity with respect to the layer plane of said at least a portion of said device. In another version of this form of the invention, the optical resonant cavity is a vertical cavity with respect to the layer plane of said at least a portion of said device. Also in a preferred embodiment, the base of said device comprises a heavily doped n+ region, and there is further provided a region in said base exhibiting quantum size effects, such as one or more quantum wells and/or quantum dot layers. 
     In accordance with another embodiment of the invention, a method is set forth for producing light modulated with an input electrical signal, including the following steps: providing a bipolar transistor device that includes a p-type collector, an n-type base, and a p-type emitter; providing a first tunnel junction coupled with said collector, and a second tunnel junction coupled with said emitter; providing a collector contact coupled with said first tunnel junction, and providing an emitter contact coupled with said second tunnel junction, and providing a base contact coupled with said base; applying electrical signals with respect to said collector, base, and emitter contacts to cause light emission by radiative recombination in the base region; and controlling the base current of said transistor device with said input electrical signal to modulate the light emission from said transistor device. 
     The pnp transistor laser can have a number of advantages as compared to the npn transistor laser, as follows: (1) Lower base doping, with resultant reduction in free carrier absorption, lower lasing threshold, and reduced self-heating in the base region, as well as improved QW recombination spectra. (2) Lower base sheet resistance due to superior electron mobility, with accordant improvement in upper base current injection limit, higher power operation, reduced resistive heating in the base region, and also improved base current distribution under the emitter, resulting in lower lasing threshold and reduced edge heating. (3) Lower contact resistance, with resulting reduction in heating effect. 
     On top of the listed advantages, the present invention, employing the described tunnel junctions in the pnp transistor laser, can have a number of further advantages, as follows: (1) Reduced contact resistance to emitter and collector contact layers, resulting in reduced heating effect and reduced capacitive effect. (2) Lower collector sheet resistance, resulting in reduced heating effect and higher upper power (collector current×V CE ) limit for collector current. (3) Lower series resistance through the emitter cladding layer, and accordant reduction of heating effect. (4) Lower free carrier absorption in the upper and lower cladding region (by minimization of highly doped P− region), resulting in reduced free carrier absorption, and accordant lower lasing threshold and reduced heating effect. It is recognized that the pnp HBT laser may not operate as at high a speed as a super high speed npn HBT, but it can still be an extremely high speed transistor laser with relatively lower current threshold and relatively higher collector voltage breakdown. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified diagram, not to scale, of the layer structure of a pnp HBT laser in accordance with an embodiment of the invention and which can be used in practicing an embodiment of the method of the invention. 
         FIG. 2  is a simplified diagram, not to scale, of the layer structure of a pnp HBT laser in accordance with another embodiment of the invention and which can be used in practicing an embodiment of the method of the invention. 
         FIG. 3  is a diagram, not to scale, of the detailed layer structure of the  FIG. 1  embodiment. 
         FIG. 4  is a diagram that illustrates the carrier flow pattern for an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified diagram of the layer structure for an embodiment of the invention. The substrate  110  can be undoped or doped, and has deposited thereon n-type cladding layer  115 , n-type collector contact layer  120 , a first tunnel junction  125 , p-type sub-collector  130  and collector  131  (which can be intrinsic or lightly doped p-type), n-type base  140  containing at least one quantum size layer  145  (e.g. quantum well and/or quantum dot layer(s)), p-type emitter  160 , second tunnel junction  170 , n-type upper cladding layer  180 , and n-type emitter contact layer  190 . When this  FIG. 1  embodiment is employed as an edge-emitting p-n-p heterojunction bipolar transistor (HBT) laser, the waveguide region (bracket  150 ) can be enclosed in an optical resonant cavity of width w equal to nλ/2, with n=1, 2, 3 . . . , and λ the characteristic emission wavelength. Note that the tunnel junctions  125  and  170  are preferably outside the active base region. In this embodiment, the first tunnel junction  125  includes a layered n+/p+ region with the n+ layer of the n+/p+ region being coupled with the collector contact layer  120 , and the p+ layer of the n+/p+ region being coupled with the collector  131 , via sub-collector  130 . Also in this embodiment, the second tunnel junction  170  includes a layered n+/p+ region with the n+ layer of the n+/p+ region being coupled with the emitter contact layer  190 , via upper cladding  180 , and the p+ layer of the n+/p+ region being coupled with the emitter  160 . 
     The embodiment of  FIG. 2  can be employed as a vertical cavity p-n-p heterojunction bipolar transistor (HBT) laser, by providing vertically oriented waveguide  250  within upper ( 295 ) and lower ( 205 ) DBRs, with waveguide dimension nλ/2, with n=1, 2, 3 . . . , and λ the characteristic emission wavelength. In the  FIG. 2  embodiment, the further layers include the following: Substrate  210  has deposited thereon the lower DBR  205 , n-type collector contact layer  220 , first tunnel junction  225 , p-type sub-collector  230  and collector  231  (which, as above, can be instrinsic or lightly doped p-type), n-type base  240  containing at least one quantum size layer  245  (again, e.g. quantum well and/or quantum dot layer(s)), p-type emitter  260 , second tunnel junction  270 , n-type emitter contact layer  290 , and the upper DBR  295 . In this embodiment, as before, the first tunnel junction  225  includes a layered n+/p+ region with the n+ layer of the n+/p+ region being coupled with the collector contact layer  220 , and the p+ layer of the n+/p+ region being coupled with the collector  231 , via sub-collector  230 . The second tunnel junction  270  of this embodiment includes a layered n+/p+ region with the n+ layer of the n+/p+ region being coupled with the emitter contact layer  290 , and the p+ layer of the n+/p+ region being coupled with the emitter  260 . 
       FIG. 3  illustrates in further detail an example of the embodiment of  FIG. 1 . The layered structure for this example is grown by MOCVD on a semi-insulating GaAs substrate  305 . Upward from the substrate, the epitaxial layers of the crystal include a 3000 Å n-type heavily doped GaAs buffer layer  308 , followed by a 634 Å n-type Al 0.35 Ga 0.65 As layer  316 , a 5000 Å n-type Al 0.95 Ga 0.05 As layer  317 , and a 200 Å n-type Al 0.35 Ga 0.65 As layer  318 , forming the lower cladding layers. These layers are followed by a 200 Å heavily doped n-type collector contact layer  320 , and then a 120 Å heavily doped n-type In 0.49 Ga 0.51 P etch stop layer  322 , and the tunnel junction  325 , which includes a 200 Å heavily Si-doped n-type Al 0.10 Ga 0.90 As layer  326  and a 120 Å heavily C-doped p-type Al 0.10 Ga 0.9 As layer  327 . Next are the sub-collector and collector layers which comprise a 200 Å lightly doped p-type Al 0.10 Ga 0.90 As layer  330  and a 400 Å lightly doped p-type GaAs layer  331 . In this example, there is a 1010 Å n-type GaAs base that includes eleven layers, three of which (represented collectively at 345) comprise a 190 Å InGaAs quantum well (QW) designed for emission at λ≈1000 nm. (These three layers comprise a 150 Å layer of In 0.2 Ga 0.8 As between 20 Å layers of In 0.1 Ga 0.9 As.) Starting after the last collector layer, the base layers are as follows: a 300 Å heavily Si doped n-type GaAs layer  341 , a 10 Å undoped GaAs layer  342 , followed by the previously described QW region  345 , and then a 10 Å undoped GaAs layer  346 , a 300 Å heavily Si doped n-type layer  347  and a 200 Å heavily doped n-type layer  348 . Then, a 100 Å heavily Si-doped n-type GaAs layer is grown as a base contact layer  355 . Subsequently, the following layers are grown: a heterostructure emitter comprised of a 150 Å p-type In 0.49 Ga 0.51 P layer  361  and a 200 Å p-type Al 0.35 Ga 0.65 As layer  362 . This is followed by the tunnel junction  370 , which includes a 150 Å heavily C doped p-type Al 0.35 Ga 0.65 As layer  371  and a 300 Å heavily Si doped n-type Al 0.35 Ga 0.65 As layer  372 . Then, the upper confining or cladding region comprises a 150 Å n-type Al 0.80 Ga 0.20 As oxidation buffer layer  381 , and a 4000 Å n-type Al 0.95 Ga 0.05 As oxidizable layer  382 , a 300 Å n-type Al 0.80 Ga 0.20 As oxidation buffer layer  383 , and a 500 Å n-type Al 0.35 Ga 0.65 As layer  384 . The layered structure is capped with a 1000 Å heavily Si doped n-type GaAs emitter contact layer  390 . 
     The process for fabricating the heterostructure bipolar pnp transistor laser continues by first patterning 4 μm protective SiN 4  stripes on the crystal with a photolithography step and reactive ion etching with Freon 14 (CF 4 ) gas. The top n-type GaAs contact layer  390  and Al 0.35 Ga 0.65 As transition layer  384  are then exposed by wet etching (1:8:80 H 2 O 2 :H 2 SO 4 :H 2 O) to form a ˜4 μm emitter mesa. Since 1:8:80 H 2 O 2 :H 2 SO 4 :H 2 O wet etching solution is not selective to an Al 0.95 Ga 0.05 As layer, a precise (˜20 s) time etching is used in this example to stop at the interface of Al 0.95 Ga 0.05 As layer  383 . Next, a wide 11 μm protective photoresist (PR) stripe is placed over the emitter mesa and the unprotected layers ( 362 ,  370 ,  381 , and  382 ) are removed with 1:8:80 H 2 O 2 :H 2 SO 4 :H 2 O selective wet etching solution, revealing the p-type In 0.49 Ga 0.51 P wide-gap emitter layer  361 . The protective photoresist (PR) stripe is then removed and the sample is oxidized for 7 min at 425° C. in a furnace supplied with N 2 +H 2 O, resulting in a ˜0.9 μm lateral oxidation which forms ˜2.2 μm oxide-defined apertures in the 4 μm emitter mesa. The samples are annealed (in N 2 ) at 425° C. for 7 minutes to reactivate p-dopants before the protective SiN 4  is removed by plasma (CF 4 ) etching. The emitter layer ( 361 ) In 0.49 Ga 0.51 P, is then removed using a wet etch (HCl), exposing the n-type GaAs base contact layer  355 . A 37 μm PR window, is then patterned to form the base mesa for the base contact. The layers from 326 to 355 are then removed using a selective etch (10:1 C 6 H 8 O 7 :H 2 O 2 ), and the In 0.49 Ga 0.51 P etch-stop layer  322  is removed by a wet etch (HCl), exposing the heavily doped n-type GaAs collector contact layer  320 . Subsequently, a 5 μm PR window is formed over the base mesa, a 7 μm PR window is formed over the emitter mesa and oxide layer, and a 20 μm PR window is formed over the collector material to deposit AuGe/Ni/Au (750/150/10000 Å) to form, simultaneously, n-type metal contacts to the emitter contact layer  390 , base contact layer  355  and collector contact layer  320 . After the metal lift-off step, the sample is then annealed at 350° C. to form ohmic contacts. Then, a layer of polyimide is applied and cured at 270° C. to reduce the surface leakage current of the device. An additional layer of silicon nitride is deposited on top of the polyimide using a plasma-enhanced chemical vapor deposition (PECVD) system. Via hole openings to create contacts to emitter, base, and collector metals are defined using another photolithography step. Using Freon 14 (CF 4 ) gas and PR as an etch mask, the dielectric via opening to the silicon nitride layer is performed with a reactive ion etching (RIE) system. The PR is then stripped with cleaning solvents. Oxygen (O 2 ) plasma is used to remove the polyimide layer, the silicon nitride layer acting as an etch mask. After the contact via fabrication step, another photolighography step is performed to deposit Ti/Au (150 Å/2.5 μm) to form contacts from the device to ground-signal-ground (GSG) high frequency probing pads. The GSG probe pads are designed, in this example, as 400 μm cells so that multiple integer resonator lengths of 400 μm can be cleaved for device fabrication. The sample is then lapped to a thickness of ˜50 μm. The HBTL samples are cleaved normal to the emitter stripes to form Fabry-Perot facets (at multiples of ˜400 μm), and the substrate side of the crystal is alloyed onto Cu heat sinks coated with In for device operation. 
       FIG. 4  shows the edge-emitting pnp HBT transistor laser with tunnel junction contacts on the p-type emitter and p-type collector. The device has the general layer structure of the  FIG. 1  embodiment, with metal contacts shown (collector contact  121 , base contact  155 , and emitter contact  191 ), and the electron and hole current paths also illustrated. As in  FIG. 1 , the layer structure for this example includes substrate  110 , n-type lower cladding  115 , n-type collector contact layer  120 , first tunnel junction  125 , p-type sub-collector  130 , p-type collector  131 , n-type base  140  (with QW), p-type emitter  160 , second tunnel junction  170 , n-type upper cladding  180 , and n-type emitter contact layer  190 . As represented in the diagram by the darkened arrows  490  and  420 , respectively, electron current is shown flowing, in n-type material, from the emitter contact to second tunnel junction  170 , and, in n-type material, from the first tunnel junction  125  to the collector contact. Also, the electron current flow in the n-type base is represented by darkened arrow  440 . As seen, the second tunnel junction  170  operates to convert electron current to hole current (lighter arrow  470 ), and the first tunnel junction  125  operates to convert hole current to electron current. In this manner, the relatively advantageous electron current in the n-type emitter contact layer and upper cladding, and also in the n-type collector contact layer, replaces what would otherwise be less efficient hole current in p-type material in a conventional pnp device.