Tunneling Emitter Bipolar Transistor

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.sub.x Ga.sub.1-x 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., Bl(2), pp. 126-130, April-June 1983; N. Chand and H. Morkoc, "Doping Effects and Compositional Grading in Al.sub.x Ga.sub.1-x As/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.

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 larger 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.

FIG. 1A is a diagram showing a preferred embodiment of the tunneling emitter bipolar transistor of the present invention.

FIG. 1B is a diagram showing the energy gap as a function of position along the TEBT of FIG. 1A.

FIG. 2A is a diagram showing the band structure of the TEBT at thermal equilibrium.

FIG. 2B shows a portion of the band structure of FIG. 2A, in expanded scale, around the emitter base junction.

FIG. 3 is a diagram showing terminal voltage and current conventions of the tunnelling emitter bipolar transistor.

FIG. 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 Al.sub.x Ga.sub.1-x As 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 FIG. 1B, the energy gap E.sub.g 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 FIG. 1A. In this device, collector 12, base 14, and emitter 16 are all GaAs, while barrier layer 24 is an Al.sub.x Ga.sub.1-x As 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.

FIG. 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 .DELTA.E.sub.c =0.57.DELTA.E.sub.g where .DELTA.E.sub.g is the band gap difference between Al.sub.x Ga.sub.1-x As and GaAs.

An expanded view of the region around the emitter-base junction 26 is shown in FIG. 2B. The effective barrier for the conduction electrons is only a fraction of .DELTA.E.sub.c (x); but the effective barrier for holes is even slightly larger than .DELTA.E.sub.v (x). Also, the electron effective mass is smaller than the hole effective mass. Hence, the conduction electrons can tunnel through the Al.sub.x Ga.sub.1-x As barrier layer 24 much easier than holes. Indeed, when the tunneling probability T is much smaller than 1, the following expression

can be used as a good approximation. For a uniform Al.sub.0.3 Ga.sub.0.7 As material, the electron effective mass is about 0.092 m.sub.e, heavy hole (.apprxeq.93%) effective mass about 0.66 m.sub.e, and light hole (.apprxeq.7%) effective mass about 0.11 m.sub.e.

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 Al.sub.x Ga.sub.1-x As barrier layer 24 further complicates the evaluation of the effective masses.

Using approximated trapzoid barrier shapes, the average tunneling probability for holes T.sub.p can be estimated as

and the estimated T.sub.n for electrons is about

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

for holes and electrons respectively, where .DELTA.E.sub.v .apprxeq.0.11 eV is the valence band discontinuity, and .DELTA.E'.sub.c .apprxeq.0.77 eV is the effective barrier height for electrons.

FIG. 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 R.sub.p and R.sub.n into consideration, the first order I-V characteristic of TEBT 10 for the n-p-n structure with terminal voltages and currents shown in FIG. 3 may be described by the Ebers-Moll model:

where the recombination currents

are included.

To reduce I.sub.er, the compositional profile of barrier layer 24 may be graded symmetrically rather than linearly as shown in FIG. 1B.

The parameters I.sub.es and I.sub.cs are the emitter-base and collector-base junction reverse bias saturation currents contributed by both the electron component and hole component, that is

For the particular TEBT specified in Table 1, the emitter length W.sub.e, base thickness W.sub.b and collector length W.sub.c are all small in comparison with their respective carrier diffusion lengths. Consequently, I.sub.es and I.sub.cs can be expressed in terms of the doping concentrations N.sub.a, N.sub.de and N.sub.dc, the hole and electron diffusion constants D.sub.p and D.sub.n and the hole and electron tunneling probabilities T.sub.p and T.sub.n, by ##EQU1## where n.sub.i 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 .alpha..sub.f and .alpha..sub.r can be writen as

Consequently the common emitter current gain may be found as ##EQU2##

If the recombination current I.sub.er is negligible in comparison with I.sub.es (p), such as in the case of large V.sub.be, we may estimate .rho. according to ##EQU3## 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. Bl, 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.sub.x Ga.sub.1-x 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 f.sub.T of 25 GHz, although the calculated value indicates that the f.sub.T 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 R.sub.econ 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 V.sub.be), the cut-off frequency is given by:

where R.sub.econ is the emitter contact resistance, C.sub.je the emitter-base junction capacitance, and C.sub.parastic 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 f.sub.T due to the much lower R.sub.econ 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: ##EQU4## where the contact resistance 5.times.10.sup.-7 .OMEGA.-cm.sup.2 is assumed for the ohmic contacts to GaAs.

The capacitances are approximately

and hence,

For a conventional heterojunction bipolar transistor with AlGaAs emitter, R.sub.econ is higher (perhaps, not lower than 2..times.10.sup.-6 .OMEGA.-cm.sup.2). Hence, for the devices with similar parameters,

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/InGaAs.

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.