WO1987007431A1 - Tunneling emitter bipolar transistor - Google Patents

Tunneling emitter bipolar transistor Download PDF

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WO1987007431A1
WO1987007431A1 PCT/US1987/001174 US8701174W WO8707431A1 WO 1987007431 A1 WO1987007431 A1 WO 1987007431A1 US 8701174 W US8701174 W US 8701174W WO 8707431 A1 WO8707431 A1 WO 8707431A1
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bipolar transistor
barrier layer
emitter
base
semiconductor material
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PCT/US1987/001174
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French (fr)
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Jingming Xu
Michael Shur
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Regents Of The University Of Minnesota
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • 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
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/20—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L29/201—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds including two or more compounds, e.g. alloys
    • H01L29/205—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds including two or more compounds, e.g. alloys in different semiconductor regions, e.g. heterojunctions
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • 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
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/08—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions with semiconductor regions connected to an electrode carrying current to be rectified, amplified or switched and such electrode being part of a semiconductor device which comprises three or more electrodes
    • H01L29/0895—Tunnel injectors

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Abstract

A bipolar transistor (10) has a barrier layer (24) interposed between its base (14) and its emitter (16). The barrier layer (24) is formed of a different, wider band gap, semiconducteur material than the base (14) and the emitter (16) and has the same conductivity type as the emitter (16). The barrier layer (24) exhibits a large difference in the effective electron mass and the effective whole mass, and presents a small barrier to majority carriers. The tunneling emitter bipolar transistor (10) exhibits a comparable current gain while having better temperature stability, less light sensitivity, and a much lower emitter resistance (leading to a much higher cut-off frequency) than conventional heterojunction bipolar transistors.

Description

TUNNELING EMITTER BIPOLAR TRANSISTOR
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates to bipolar transistors. In particular, the present invention is an improved bipolar transistor which includes a thin barrier layer of a wider band gap semiconductor material between the base and emitter.
2. Description of the Prior Art. The desire for higher speed bipolar transistors has led to the investigation of various transistor structures, and to the development of bipolar transistors using semiconductor materials other than silicon. Particular emphasis has been placed on devices using gallium arsenide (GaAs). Among the advantages offered by gallium arsenide over silicon transistors are the higher electron mobility of gallium arsenide, the availability of semi-insulating substrates, and expected superior radiation hardness and high temperature performance.
Heterojunction bipolar transistors (HBT), which typically use a wide band gap aluminum gallium arsenide (AlGaAs or Al Ga. As) emitter, offer several potential advantages over homojunction GaAs devices for high speed applications. In a AlGaAs/GaAs heterojunction bipolar transistor, the wide band gap AlGaAs emitter introduces an extra barrier for minority carrier injection from base to emitter. As a result, the emitter efficiency can be very high and nearly independent of the doping density of the base. As a result, the base can be doped heavily to reduce base resistance, without sacrificing emitter injection efficiency. Description of heterojunction bipolar transistors can be found, for example, in the following papers: H. Kroemer, "Heterostructure Bipolar Transistors: What Should We Build?", J. Vac Sci. Technol., B1(2), pp. 126-130, April-June 1983; N. Chand and H. Morkoc, "Doping Effects and Compositional Grading in AlxGa1 -xAs/GaAs Heterojunction Bipolar Transistors", IEEE Transactions on Electron Devices, Vol. Ed-32, No. 6, pp 1064-1068, June 1985; A. Grindberg, M. Shur, R. Fischer and H. Morkoc, "An Investigation of the Effect of Graded Layers and Tunneling on the Performance of AlGaAs/GaAs Heterojunction Bipolar Transistors", IEEE Transactions on Electron Devices, Vol. Ed-31, No. 12, pp 1758-1764, December 1984; P. Asbeck, D. Miller, R. Milano, J. Harris, Jr., G. Kaelin and R. Zucca, ''(Ga,Al)As/GaAs Bipolar Transistors For Digital Integrated Circuits", IEDM 81, pp 629-632, 1981.
Despite the advantages, AlGaAs/GaAs heterojunction bipolar transistors also have significant shortcomings. In particular, the AlGaAs emitter has several disadvantages related to the traps associated with the dopants, high contact resistance (typically much larger than for comparably doped GaAs), and higher series resistance because of the low mobility and electron velocity in AlGaAs. SUMMARY OF THE INVENTION The present invention is a new bipolar transistor which we call a tunneling emitter bipolar transistor (TEBT). This device uses a thin barrier layer of a wider band gap semiconductor material interposed between the base and emitter of a bipolar transistor. The barrier layer exhibits a large difference in effective electron mass and effective hole mass. The barrier layer, which has the same conductivity type as the emitter, preferably has a graded compositional profile so that the energy gap increases with distance from the base. The compositional profile, along with high doping levels of the emitter and the barrier layer, provides a barrier which is smaller and thinner for majority carriers than for the minority carriers in the emitter.
The TEBT provides improved emitter efficiency because of the mass filtering and the effect of the large barrier to minority carrier injection, and low emitter series resistance, resulting in high current gain and high cutoff frequency, while considerably reducing the undesirable effects of the emitter of a conventional HBT. Hence the TEBT has much better temperature stability and less light sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1A is a diagram showing a preferred embodiment of the tunneling emitter bipolar transistor of the present invention.
Figure 1B is a diagram showing the energy gap as a function of position along the TEBT of Figure 1A.
Figure 2A is a diagram showing the band structure of the TEBT at thermal equilibrium.
Figure 2B shows a portion of the band structure of Figure 2A, in expanded scale, around the emitter base junction.
Figure 3 is a diagram showing terminal voltage and current conventions of the tunnelling emitter bipolar transistor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1A is a schematic cross-section of tunneling emitter bipolar transistor 10 of the present invention. TEBT 10, in this embodiment, is an NPN transistor having an N type collector 12, a heavily doped P type (P+) base 14 and a heavily doped N type (N+) emitter 16. Collector contact 18, base contact 20, and emitter contact 22 make ohmic contact to collector 12, base 14 and emitter 16, respectively. TEBT 10 also includes a thin barrier layer
24 which is interposed between base 14 and emitter 16. Barrier layer 24, which is a heavily doped N type layer, has a different composition than base 14 and emitter 16. In one preferred embodiment, collector 12, base 14 and emitter 16 are all gallium arsenide, while barrier layer 24 is a thin AlxGa1 -xAs alloy semiconductor layer. The thickness of barrier layer 24 varies from about 10 Angstroms to about 200 Angstroms, depending on the value of x (the mole fraction of AlAs in the alloy semicondutor), the composition profile of barrier layer 24, and the doping levels of base 14, emitter 16 and barrier layer 24. In preferred embodiments, barrier layer 24 is between about 30 Angstroms and about 60 Angstroms. Barrier layer 24 is preferably graded to make the barrier smaller for injected electrons (majority carriers), while the barrier for holes (minority carriers ) remains basically unchanged when emitter 16 is doped heavily. As shown in Figure IB, the energy gap E increases with distance from junction 26. This is achieved by grading the composition of the barrier layer so that x increases with distance from junction 26. The graded composition also eliminates a sharp notch which otherwise exists at the emitter base junction.
TEBT 10 achieves enhanced emitter injection efficiency by taking advantage of a very large difference in the tunneling probabilities for electrons and holes in barrier layer 24. The improvement results from both the effect of "mass filtering" (because there is a large difference in the effective electron mass and the effective hole mass in barrier layer 24), and the effect of a large barrier to minority carrier injection and a small barrier to majority carrier injection.
Table 1 shows the characteristics and parameters of a TEBT device like the one shown in Figure 1A. In this device, collector 12, base 14, and emitter 16 are all GaAs, while barrier layer 24 is an AlxGa1 -xAs layer having a compositional profile in which x changes from zero at junction 26 to about 0.332 at interface 28 between barrier layer 24 and emitter 16.
Table 1 At Temperature: T=300K
Doping densities: Nde=2x1018; Na=0.8x1018;
Ndc=5x1016 (1/cm3) AlχGa1_xAs layer thickness: Lbar=200 Angstroms
Composition profile x: x changes lineary from 0.332 at interface 28 to 0 at junction 26
Lengths: We=0.45μm; Wb=0 .05μm; Wc=0.5μm Mobili ties : μp=136cm2/v . s , μn=2470cm2/v . s
Li fe time : τ b e=5x10- 7s Intrinsic concentration: ni=2x106/cm3
Width of depletion region in e-b junction:
Wbe=0.04um
Figure 2A shows a numerically calculated band diagram of TEBT device 10 using the device parameters given in Table 1. The conduction band discontinuity was assumed to be Δ Ec= . 51ΔEg where
Δ Εg is the band gap difference between AlxGa1 -xAs and GaAs. An expanded view of the region around the emitter-base junction 26 is shown in Figure 2B. The effective barrier for the conduction electrons is only a fraction of Δ Ec(x); but the effective barrier for holes is even slightly larger than Δ Ev(x). Also, the electron effective mass is smaller than the bole effective mass. Hence, the conduction electrons can tunnel through the AlxGa1 -xAs barrier layer 24 much easier than holes. Indeed, when the tunneling probability T is much smaller than 1, the following expression
T=exp(-2 [2m* (V(x)-E)/*
Figure imgf000008_0002
Figure imgf000008_0001
2]1/2dx) Eq 1
can be used as a good approximation. For a uniform
Al0 .3Ga0 . 7As material, the electron effective mass is about 0.092 me, heavy hole (=
Figure imgf000008_0003
93%) effective mass about 0.66 me, and light hole (=
Figure imgf000008_0004
7%) effective mass about 0.11 me.
An accurate quantitative calculation of the tunneling probabilities of electrons and holes requires very accurate knowledge of the shape of the barriers and the values of the effective masses. In addition, the grading of the composition in the AlxGa1 -xAs barrier layer 24 further complicates the evaluation of the effective masses.
Using approximated trapezoid barrier shapes, the average tunneling probability for holes Tp can be estimated as
TP<10 -4 Eq 2
and the estimated Tn for electrons is about
Tn =
Figure imgf000009_0001
0.1 Eq 3
which can be further increased by varying the composition and doping profiles.
Taking the thermionic emission effects into account, the transport rates through barrier layer 24 may be written as
Rp=Tp+(1-Tp)exp(- EvkT)
Figure imgf000009_0002
=exp(-ΔEv/kT) Eq 4
Rn=Tn+(1-Tn)exp(-ΔE'c/kT) Eq 5
for holes and electrons respectively, where ΔEv
Figure imgf000009_0003
=0.11eV is the valence band discontinuity, and Δ E'C=
Figure imgf000009_0004
0.077eV is the effective barrier height for electrons.
Figure 3 shows a diagrammatic representation of TEBT 10, with terminal voltage and current conventions illustrated. TEBT 10 is shown symbolically in a fashion similar to a conventional bipolar transistor, except that a small line crosses the emitter arrow to dictate the presence of barrier layer 24.
Following a conventional procedure described in H. T. Yuan, W.V. McLevige, and H.D. Shih, "GaAs Bipolar Digital Integrated Circuits", VLSI
Electronics, Vol. 11, ed. by N. Einspruch and W.
Wisseman, Academic Press, Inc., 1985, and taking Rp and Rn into consideration, the first order I-V characteristic of TEBT 10 for the n-p-n structure with terminal voltages and currents shown in Figure 3 may be described by the Ebers-Moll model:
Ie=-Ies[exp(qvbe/kT)-1)]+αrιcs[exp(qVbc/kT)-1]-Ier
Eq 6A
Ic=-αfIes[exp(qVbe/kT)-1)]-Ics[exp(qVbc/kT)-1]-Icr,
Eq 6B
where the recombination currents
Ier=Ae(qniWbe/τbe)[exp(qVbe/nkT)-1] Eq 7A
Icr=Ac(qniWbc/τbc)[exp(qVbc/nkT)-1] Eq 7B
are included.
To reduce Ier, tne compositional profile of barrier layer 24 may be graded symmetrically rather than linearly as shown in Figure 1B.
The parameters Ies and Ics are the emitter-base and collector-base junction reverse bias saturation currents contributed by both the electron component and hole component, that is
Ies=Ies(p)+Ies(n) Eq 8A
Ics-1cs(P)+Ies(n) Eq 8B
For the particular TEBT specified in Table 1, the emitter length We, base thickness Wb and collector length W are all small in comparison with their respective carrier diffusion lengths.
Consequently, Ies and Ics can be expressed in terms of the doping concentrations Na, Nde and Ndc, the hole and electron diffusion constants Dp and Dn and the hole and electron tunneling probabilities Tp and Tn, by
Ies=Aeqni 2[DpRp/WeNde+DnRn/WbNa]
Figure imgf000011_0001
Aeqni 2[(Dp/WeNde)exp(- Ev/kT)+DnRn/WbNa]
Eq 9A
Ics=Acqni 2[Dp/WcNdc+Dn/WbNa]
Eq 9B
where ni is the intrinsic carrier concentration of GaAs and Eq 4 and Eq 5 are used.
Assuming that the depletion-layer recombination currents are much larger than the neutral-base recombination currents, the common base forward and reverse current gain factors αf and αr can be writen as αf=Ies(n)/[les(n)+Ies(p)+(Ier+AeqniWbe/τbe)exp(-qVbe/kT)]
Eq 10A αr=Ics(n)/[ Ics(n)+Ics(p)+(Icr+AcqniWbc/τbc)exp(-qVbc/kT)]
Eq 10 B
Consequently the common emitter current gain may be found as
β=αf/[1-αf]
=Ies(n)/[Ies(p)+(Ier+(AeqniWbe/τbe) exp(-qVbe/kT)]
Eq 11
If the recombination current Ier is negligible in comparison with Ies(p), such as in the case of large Vbe, we may estimate β according to
β
Figure imgf000012_0001
=Ies(n)/les(p)
Figure imgf000012_0002
=(DnWeNde/DpWbNa)Rn/Rp
Figure imgf000012_0003
400x0.1x exp(ΔEv/kT)
Figure imgf000012_0004
2500 Eq 12
where the parameters in Table 1 are used.
This current gain may be further enhanced by the ballistic transport of hot electrons across the base as was originally proposed by H. Kroemer, "Heterostructure bipolar transistors: What should we build?", J. Vac. Sci. Technol., Vol. B1 , No. 2, pp 126-130, April-June 1983. Just as in a conventional HBT, the base doping level in TEBT 10 may be quite high, leading to a low base spreading resistance, low emitter-base capacitance and other factors favorable for a high frequency performance.
The TEBT 10 of the present invention offers a significantly higher cut-off frequency than is possible with state-of-the-art HBT devices. The reason is the high contact resistance to N type Al Ga. As emitters. As stated by H. Yuan, W. McLevige and H. D. Shih, VLSI Electronics, Vol. 11, ed. by Einspruch and W. Wisseman, Academic Press, Inc. 1985: "Taking the state of the art heterojunction bipolar transistor made from AlGaAs-GaAs as an example, it has a measured fT of 25 GHz, although the calculated value indicates that the fT should be as high as 65 GHz. This disparity is identified, because of the difficulty of making low-contact resistance to the AlGaAs emitter. Therefore, to achieve ultra-high switching speed...the emitter resistance must also be reduced proportionally."
By contrast, in TEBT 10, the emitter resistance is substantially reduced because of the use of highly doped GaAs as emitter 16. The emitter contact resistance Recon is much lower for TEBT 10 in comparison to the conventional HBT, and as a result a much higher cut-off frequency is achieved.
For high base currents (i.e. large base emitter voltages vbe), the cut-off frequency is given by:
fT=1/2 π[Recon(Cje+Cparastic)+Wb 2/2De]
Eq 13 where Recon is the emitter contact resistance,
Cje the emitter-base junction capacitance, and
Cparastic is the total parasitic capacitance that includes the collector-base junction capacitance, device isolation capacitance, and interconnect capacitance. The intrinsic base transit time is given by the second term in the bracket. As can be seen from Eq 13, TEBT 10 will have higher cut-off frequency fT due to the much lower Recon in TEBT 10, if everything else is kept the same as in a conventional heterojunction bipolar transistor. Taking the parameters given in Table 1, we have:
Recon=Rcon+Re
Figure imgf000014_0001
5x10-7+(We/qNdeμe)
Figure imgf000014_0002
5x10-7+5.7x10-8
Figure imgf000014_0003
=6x10-7 (Ω-cm2) Eq 14
where the contact resistance 5x10-7 Ω -cm2 is assumed for the ohmic contacts to GaAs.
Wb 2/De
Figure imgf000014_0004
=2x10-13 second Eq 15
The capacitances are approximately
Figure imgf000014_0005
C=Cie+Cparasitic 3.10-6F/cm2 Eq 16 and hence ,
fT
Figure imgf000015_0001
=80 GHz . Eq 17
For a conventional heterojunction bipolar transistor with AlGaAs emitter , Recon is higher (perhaps , not lower than 2.x10-6 Ω -cm2). Hence, for the devices with similar parameters,
Figure imgf000015_0002
fT(AlGaAs)=26GHz in approximate agreement with H. Yuan et al.
Although TEBT 10 has been described in terms of a GaAs device with an AlGaAs barrier layer 24, other combinations of semicondutor materials can be used to achieve similar device properties. TEBT 10 requires a large difference in the effective electron mass and the effective hole mass, a small barrier to the majority carriers, and good lattice matching between barrier layer 24 and the materials of base 14 and emitter 16.
In another embodiment, AlGaAs is the material for barrier layer 24, and InGaAs is the material used for collector 12, base 14, and emitter
16. Similar conduction band discontinuities to
AlGaAs/GaAs can be realized with a low mole fraction in the AlGaAs. There is a high electron mobility in
InGaAs, and the mass difference between electrons and holes is large. A thin AlGaAs layer can resolve the lattice mismatching due to the fact that the lattice strain is taken up coherently by epilayers resulting in a dislocation-free pseudomorphic material.
Still another material system is InAlAs/InGaAs, where InAlAs is the material used for barrier layer 24. The characteristics are similar to those of AlGaAs/lnGaAs.
Another material system uses InGaP as the barrier layer 24 and GaAs as the material for collector 12, base 14, and emitter 16. The conduction band discontinuity at interface 28 is approximately equal to the valence band discontinuity.
Still another material system is
AlGaAs/GaAs/GaBeAs. In this embodiment, AlGaAs is the material of barrier layer 24, GaAs is the material used for collector 12 and emitter 16. GaBeAs is used as the material for base 14, which allows ultra high doping in base 14.
In conclusion, the tunnelling emitter bipolar transistor (TEBT) of the present invention offers high emitter efficiency, low parasitic resistance, and significantly higher frequency performance than is possible with state-of-the-art heterojunction bipolar transistors, In addition, since the emitter of the TEBT is a material such as GaAs, the temperature instability, light sensitivity, and other undesirable effects associated with the heavily doped AlGaAs emitter of a HBT are significantly reduced. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:
1. A bipolar transistor comprising: a collector having a first conductivity type; a base having a second conductivity type; an emitter having the first conductivity type; and a barrier layer interposed between the base and the emitter, the barrier layer having the first conductivity type, having a wider band gap than the emitter, and exhibiting a difference in effective electron mass and effective hole mass, the barrier layer having a thickness such that the barrier layer is substantially transparent to first conductivity type carriers and less transparent to second conductivity type carriers.
2. The bipolar transistor of claim 1 wherein the barrier layer is formed by an alloy semiconductor which has a band gap which is a function of composition.
3. The bipolar transistor of claim 2 wherein the barrier layer has a compositional gradient with band gap varying as a function of distance from a junction between the base and the barrier layer.
4. The bipolar transistor of claim 3 wherein the band gap of the barrier layer increases with the distance from the junction.
5. The bipolar transistor of claim 4 wherein band gaps of the barrier layer and the base are essentially equal at the junction.
6. The bipolar transistor of claim 1 wherein the emitter and the barrier layer are heavily doped, first conductivity type and the base is heavily doped, second conductivity type.
7. The bipolar transistor of claim 1 wherein the collector and the emitter are formed by a first semiconductor material and the barrier layer is formed by a second alloy semiconductor material.
8. The bipolar transistor of claim 7 wherein the base is formed by the first semiconductor material.
9. The bipolar transistor of claim 7 wherein the base is formed by a third semicondcutor material.
10. The bipolar transistor of claim 9 wherein the first semiconductor material is GaAs, the second semicondutor material is AlGaAs, and the third semiconductor material is GaBeAs.
11. The bipolar transistor of claim 7 wherein the first semiconductor material is GaAs.
12. The bipolar transistor of claim 11 wherein the second semiconductor material is AlGaAs.
12. The bipolar transistor of claim 11 wherein the second semiconductor material is InGaP.
13. The bipolar transistor of claim 7 wherein the first semiconductor material is InGaAs.
14. The bipolar transistor of claim 13 wherein the second semiconductor material is AlGaAs.
15. The bipolar transistor of claim 13 wherein the second semiconductor material is InAlAs.
16. The bipolar transistor of claim 1 wherein the barrier layer has a thickness of between about 10 Angstroms and about 200 Angstroms.
17. The bipolar transistor of claim 1 wherein the barrier layer has a thickness of between about 30 Angstroms and about 60 Angstroms.
18. The bipolar transistor of claim 1 wherein the first conductivity type is n type and the second conductivity type is p type.
19. The bipolar transistor of claim 1 wherein the barrier layer exhibits a larger barrier to minority carrier injection than to majority carrier injection.
PCT/US1987/001174 1986-05-29 1987-05-18 Tunneling emitter bipolar transistor WO1987007431A1 (en)

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Publication number Priority date Publication date Assignee Title
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US4396931A (en) * 1981-06-12 1983-08-02 International Business Machines Corporation Tunnel emitter upper valley transistor
US4672404A (en) * 1982-09-17 1987-06-09 Cornell Research Foundation, Inc. Ballistic heterojunction bipolar transistor
US4672413A (en) * 1984-04-16 1987-06-09 Trw Inc. Barrier emitter transistor
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Title
ANATOLY, A. GRINBERG, MICHAEL S. SHUR, RUSSELL J. FISCHER, and HADIS MORKOC, "An Investigation of the Effect of Graded Layers and Tunneling on the Performance of AlGaAs/GaAs Hetero-Junction Bipolar Transistors", IEEE Transactions on Electron Devices, Vol. ED-31, No. 12 (December 1984), pp. 1758-1784, see entire document, especially pages 1762 and 1764. *
G.W. TAYLOR, R.S. MAND, J.G. SIMMONS, and A.Y. CHO, "Ledistor - a Three-Terminal Double Hetero-Structure Optoelectronic Switch", Appl. Phys. Lett., Vol. 50, No. 6, (09 February 1986), pp. 338-340, see entire document, especially Figure 6. *
NARESH CHAND and HADIS MORKOC, "Doping Effects and Compositional Grading in AlxGa1-xAs/GaAs Hetero-Junction Bipolar Transistors", IEEE Transactions on Electron Devices, Vol. ED-32, No. 6, (June 1985), pp. 1064-1068, see entire document, especially pages 1066 and 1067. *
S. LURYI, "An Induced Base Hot-Electron Transistor", IEEE Electron Device Letters, Vol. EDL-6, No. 4, (April 1985), pp. 178-180, see Figures 1, 2, and the Sentence before ACKNOWLEDGEMENT on page 180. *

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