|United States Patent
,   et al.
April 19, 1994
Article comprising a "ballistic" heterojunction bipolar transistor
The disclosed heterojunction bipolar transistor, to be referred to as the
"coherent" transistor (CT), is capable of providing gain above the
conventionally defined cut-off frequencies f.sub.T and f.sub.max.
Substantially, mono-energetic (average energy .DELTA.) carriers are
injected in beam-like fashion into the base, with kT<.DELTA.<hv.sub.opt,
where k, T and h have their conventional meaning, and v.sub.opt is the
frequency of the lowest relevant optical phonon in the base of width
W.sub.B. Exemplarily, W.sub.B is about 100 nm, .DELTA. is about 20 meV,
the CT comprises Si.sub.1-x Ge.sub.x or III/V material, with the base
being doped n-type. The CT utilizes substantially collisionless minority
carrier transport through the base, and is designed such that, at an
operating temperature which typically is .ltorsim.77K, the variance of the
average base transit time (.DELTA..tau..sub.B) is much less than the base
transit time .tau..sub.B, typically less than 0.5 .tau..sub.B, preferably
about .tau..sub.B /5 or less. Transistors according to the invention
typically will have an operating frequency in the range 100 GHz-1THz, and
can be advantageously used in many areas of technology, e.g., high speed
computing or communications.
Grinberg; Anatoly A. (Plainfield, NJ);
Luryi; Serge (Bridgewater, NJ)
AT&T Bell Laboratories (Murray Hill, NJ)
November 25, 1992|
|Current U.S. Class:
||257/26; 257/27; 257/29; 257/197; 257/198 |
|Field of Search:
References Cited [Referenced By]
U.S. Patent Documents
Chen et al., "Subpicosecond InP/InGaAs Heterostructure Bipolar
Transistors," IEEE Electron Device Letters, vol. 10, No. 6, Jun. '89, pp.
Tiwari, "Frequency Dependence of the Unilateral Gain in Bipolar
Transistors," IEEE Electron Device Letters, vol. 10, No. 12, Dec. 1989,
Dagli, "A Unipolar Transistor with Negative Output Resistance," Solid-State
Electronics, vol. 33, No. 7, 1990, pp. 831-836.
Wright, "Small-Signal Theory of the Transistor Transit-Time Oscillator
(Translator)," Solid-State Electronics, vol.22, 1979, pp. 399-403.
Burghartz et al., "Self-Aligned SiGe-Base Heterojunction Bipolar Transistor
by Selective Epitaxy Emitter Window (SEEW) Technology", IEEE Electron
Device Letters, vol. 11, No. 7, Jul. 1990, pp. 288-289.
S. M. Sze, "Physics of Semiconductor Devices", 2nd Ed., John Wiley & Sons,
1981, Chapter 3.
G. T. Wright, "Small-Signal Theory of the Transistor Transistor
Transit-Time Oscillator (Translator)", Solid State Electronics, vol. 22,
pp. 399-403, 1979.
S. Tiwari, "Frequency Dependence of the Unilateral Gain in Bipolar
Transistors", IEEE Electron Device Letters, vol. 10, p. 574, 1989.
Y. K. Chen, et al., "Subpicosend InP/InGaAs Heterostructure Bipolar
Transistors", IEEE Electron Device Letters, EDL-10, No. 6, p. 267, 1989.
N. Dagli, "A Unipolar Transistor With Negative Output Resistance", Solid
State Electronics, vol. 33, No. 7, pp. 831-836, 1990.
A. A. Grinberg, et al., "Ballistic Versus Diffusive Base Transport in the
High-Frequency Characteristics of Bipolar Transistors", Applied Physics
Letters, vol. 60, pp. 2270-2272, 1992.
Primary Examiner: Mintel; William
Attorney, Agent or Firm: Pacher; Eugen E.
1. An article comprising a heterojunction bipolar transistor comprising
first, second and third semiconductor regions, to be referred to as
emitter, base and collector, respectively, and further comprising means
for electrically contacting said emitter, base and collector,
respectively, the base being intermediate the emitter and collector and
having a width W.sub.B, the emitter and collector each comprising
semiconductor material of a first conductivity type, and the base
comprising material of a second conductivity type that differs from the
first conductivity type, associated with the transistor being a unilateral
power gain (U), a common emitter current gain (.beta.), conventional
cut-off frequencies f.sub.max and f.sub.T, a minority carrier average
injection energy .DELTA. and a minority carrier average ballistic base
transit time .tau..sub.B ; and associated with the material of the base is
an optical phonon frequency v.sub.opt ;
CHARACTERIZED IN THAT
a) kT<.DELTA.<hv.sub.opt, where T is the transistor absolute operating
temperature, k is Boltzmann's constant, and h is Planck's constant;
b) W.sub.B is at least 100 nm;
c) the transistor is an abrupt junction transistor selected such that, at
temperature T, .DELTA..tau..sub.B is less than about 0.5 .tau..sub.B ;
where .DELTA..tau..sub.B is the variance of .tau..sub.B ; and further is
selected such that
d) at temperature T, the absolute value of U is greater than unity at least
at one frequency above f.sub.T.
2. An article according to claim 1, wherein .DELTA. is at least 3 kT, and
wherein the first conductivity type is p-type conductivity.
3. An article according to claim 1, wherein the base comprises material
selected from the group consisting of Si.sub.x Ge.sub.1-x (x<1) and III/V
4. An article according to claim 1, wherein the transistor furthermore is
selected such that .beta. is greater than unity at least at one frequency
5. An article according to claim 1, wherein said frequency is in the range
100 GHz-1 THz.
6. An article according to claim 1, comprising means for cooling the
transistor to a temperature that is less than or equal to 77K.
7. An article according to claim 6, comprising means for cooling the
transistor to a temperature that is less than or equal to about 15K.
FIELD OF THE INVENTION
This application pertains to heterojunction bipolar transistors (HBTs).
BACKGROUND OF THE INVENTION
Since the invention of the transistor in 1947, much effort has been
directed towards extension of the device operating range towards higher
and higher frequencies.
Conventionally, the cut-off frequency f.sub.T (defined as the frequency at
which the current gain .beta., i.e., the absolute value of the parameter
h.sub.fe .tbd..differential.J.sub.c /.differential.J.sub.B, is unity) is
used as a figure of merit that is indicative of the high frequency
capability of a transistor. See for instance, S. M. Sze, "Physics of
Semiconductor Devices", 2nd Edition, John Wiley & Sons, 1981, Chapter 3,
incorporated herein by reference. It is well known that .beta. at high
frequencies decreases at a value of 10 dB/decade.
Another parameter that can be used to characterize the high frequency
capabilities of a (typically microwave) transistor is the unilateral
(power) gain U. See S. M. Sze, op. cit., pp. 160-165. The frequency at
which the unilateral gain is unity is the maximum oscillating frequency
f.sub.max, which can, but need not, be larger than f.sub.T. Both f.sub.T
and f.sub.max are conventionally determined by extrapolation of the
measured roll-off in h.sub.fe and U, respectively.
G. T. Wright, (see, for instance, Solid State Electronics, Vol. 22, p. 399,
1979) proposed extension of active transistor operation of frequencies
beyond the conventional cutoff frequency f.sub.T. The proposal involved
the utilization of transit time resonances that arise from carrier drift
in the collector space charge region. The proposed model suggested for an
ideal transistor (i.e., a transistor without any parasitic impedances) the
possibility that .vertline.U.vertline. could exceed unity at frequencies
above f.sub.max. However, it has now been shown (S. Tiwari, IEEE Electron
Device Letter, Vol. 10, No. 12, p. 574, 1989) that the proposed
utilization of transit time resonances in a conventional GaAs/AlGaAs HBT
would require reductions of each of the base and collector resistances and
of the collector capacitance by at least an order of magnitude from state
of the art values. Clearly, the proposed mechanism is, at least for the
foreseeable future, not likely to be embodied in a practical device. To
the best of our knowledge, transit time resonances of the prior art type
were not considered with regard to hot electron HBTs. N. Dagli, (Solid
State Electronics, Vol. 33 (7), p. 831) proposed a hot electron unipolar
transit time transistor.
HBTs with substantially collisionless base transport are known. See, for
instance, U.S. Pat. No. 4,829,343. Herein free carrier (not necessarily
electron) base transport is considered to be "ballistic" if the mean free
path (.LAMBDA.) of the carriers in the base material is 3/8W.sub.B, the
base width. As those skilled in the art know, the mean free path can, at
least in principle, be determined by transport measurements in a magnetic
The cut-off frequency of a prior art ballistic HBT cannot be less than
(2.pi..tau..sub.B).sup.-1, where .tau..sub.B is the average base transit
time of the minority carriers. Therefore, prior art ballistic HBTs are
typically designed to minimize .tau..sub.B. This generally involves
maximizing carrier velocity through choice of low effective mass minority
carriers (almost invariably resulting in the choice of n-p-n III/V
transistors), and through choice of a design that exhibits a relatively
large value of the parameter .DELTA., the injection energy. It also
typically involves minimization of the base width W.sub.B.
Although HBTs having f.sub.T substantially above 100 GHz have recently been
reported (see, for instance, Y. K. Chen, et al. IEEE Electron Dev. Lett.,
Vol. 10, No. 6, p. 267, 1989), it would clearly be highly desirable to
have available transistors that can operate at even higher frequencies.
This application discloses such a transistor. The novel device, to be
referred to as the coherent transistor (CT), has utility in many fields,
e.g., high speed computation or communications.
SUMMARY OF THE INVENTION
Broadly speaking, the invention is a novel HBT that can exhibit power gain
(preferably also current gain) at frequencies above the conventionally
defined f.sub.T and f.sub.max.
More specifically, the invention typically is embodied in an article that
comprises a HBT that compromises first, second and third semiconductor
regions, to be referred to as emitter, base and collector, respectively.
The article also comprises means for electrically contacting the emitter,
base and collector, respectively. The base is intermediate the emitter and
collector and has a width W.sub.B. The emitter and collector each
comprises material of a first conductivity type, and the base comprises
material of a second conductivity type. Associated with the transistor is
a current gain .beta., a unilateral power gain U, and conventional cut-off
frequencies f.sub.T and f.sub.max. Significantly, the transistor is
selected such that .DELTA..tau..sub.B is less than about 0.5 .tau..sub.B,
where .DELTA..tau..sub.B is the variance of .tau..sub.B, and such that the
absolute value of U is greater than unity at least at one frequency above
f.sub.max and f.sub.T.
Typically, in a transistor according to the invention, the minority
carriers are injected into the base over a (typically relatively abrupt)
barrier, with the average injection energy .DELTA. of the carriers being
selected such that kT<.DELTA.<h.nu..sub.opt, where k is the Boltzmann
constant, T is the absolute temperature of the transistor during
operation, h is Planck's constant, and .nu..sub.opt is the frequency of
the lowest optical phonon in the base material. In preferred embodiments,
.DELTA.3/83 kT. Since h.nu..sub.opt is, exemplarily, about 59 meV in Si
and about 38 meV in GaAs, it can be readily seen that typically
A significant aspect of the invention is substantially collimated (in the
forward direction) injection of substantially mono-energetic minority
carriers into the base, and substantially ballistic transport of these
carriers through the base to the base/collector junction. This is
expressed by the requirement that .DELTA..tau..sub.B is much less than
.tau..sub.B (preferably, .DELTA..tau..sub.B 3/8.tau..sub.B /5) where
.tau..sub.B is the average base transit time for the carriers, and
.DELTA..tau..sub.B is the variance of .tau..sub.B. Operation of the
transistor at cryogenic temperatures, together with the choice of
injection energy less than the energy of any relevant optical phonon can
result in a ballistic scattering length (herein equivalent to .LAMBDA.) of
about 100 nm or even more. Furthermore, .DELTA..tau..sub.B typically is
proportional to T, as those skilled in the art know. Thus, the condition
that .DELTA..tau..sub.B is substantially less than .tau..sub.B can, in
general, be readily met by appropriate choice of operating temperature.
Since the injected carriers have average energy .DELTA., .tau..sub.B
=W.sub.B (2.DELTA./m).sup.-1/2, where m is the effective minority carrier
mass in the base and .DELTA..tau..sub.B =(<.tau..sub.B.sup.2
>-<.tau..sub.B >.sup.2).sup.1/2, where the brackets signify the ensemble
average of the variable within the brackets. The quantities .tau..sub.B
and .DELTA..tau..sub.B thus are well defined and also determinable for any
particular transistor according to the invention. For instance, for the
typical case of a thermal distribution of carriers on the top of the
barrier, it is known that (.DELTA..tau..sub.B /.tau..sub.B) is
approximately equal to (kT/2.DELTA.).
A HTB that meets the fundamental requirement .DELTA..tau..sub.B
<<.tau..sub.B will herein be referred to as a "coherent" transistor (CT)
since, in such a device, a minority carrier pulse experiences relatively
little dispersion during its propagation through the base. We have
discovered that a CT can exhibit (current and/or power) gain at
frequencies above f.sub.T and f.sub.max, thus making possible operation at
previously unattainable frequencies.
As will be shown in more detail below, an ideal CT (neglecting extrinsic
impedances and also neglecting a transit delay .theta. in the
base/collector junction) has current gain .beta.>1 in a set of resonant
bands of frequencies centered at f.sub.n =2.pi.nf.sub.T where n=1,2, . . .
, and f.sub.T =(2.pi..tau..sub.B).sup.-1 is approximately equal to the
usual cut-off frequency. The magnitude of the resonance peaks decreases
with frequency as 2(2.pi.f.sub.n .DELTA..tau..sub.B).sup.-2. Thus, it is
the dispersion of the minority carries during base transit rather than the
average time of flight that determines the extended current gain. Taking
into account extrinsic impedances and other effects that are unavoidably
present in an actual CT, the above described properties are modified to
some extent. For example, the positions of the resonant peaks in the
current gain are no longer simple multiples of f.sub.T. However, the basic
advantage of the CT, namely, the possibility of providing gain at a
frequency above f.sub.T and f.sub.max, is preserved.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 schematically shows relevant aspects of the band structure of an
FIG. 2 shows the square of the intrinsic current gain of an exemplary CT as
a function of frequency;
FIG. 3 shows current gain vs. frequency for three different transistors,
including an exemplary CT;
FIG. 4 is an equivalent circuit for an abrupt junction HBT;
FIG. 5 schematically depicts an exemplary CT; and
FIG. 6 shows .beta..sup.2 and .vertline.U.vertline. of an exemplary CT as a
function of frequency.
DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS
FIG. 1 schematically depicts the band diagram of an abrupt-junction HBT
that can, assuming an appropriate choice of parameters, advantageously
embody the invention. By an "abrupt-junction" HBT, we mean herein a HBT in
which the width of the emitter/base junction "transition" region is small,
typically no more than 0.1 W.sub.B, frequently only a few crystal layers.
The "transition" region is the region in which the relevant band edge
drops from the peak of the emitter/base energy barrier to the constant
base value. As those skilled in the art will recognize, the exemplary band
diagram corresponds to a conventionally biased n-p-n HBT. Numerals 11-13
designate emitter, base and collector, respectively. The base has width
W.sub.B, voltage V.sub.BE is applied between base and emitter, and a
voltage-V.sub.BC is applied between base and collector. Minority carriers
(i.e., electrons in the instant case) are injected into the base over a
(desirably abrupt) energy barrier of height .DELTA.. An analogous band
diagram can readily be drawn for a p-n-p HBT according to the invention.
FIG. 2 shows the square of the intrinsic common emitter current gain as a
function of frequency (in units of .omega..tau..sub.B), for .DELTA.=5 kT,
and .DELTA. and W.sub.B selected such that .tau..sub.B =1 ps. As can be
seen, the gain peaks occur approximately at f.sub.n, their magnitude
decreasing with frequency as 1/f.sub.n.sup.2. It can be shown that, under
the stated conditions, the maximum current gain of the nth peak
(.beta..sub.n) is approximately equal to
See also A. A. Grinberg, et al., Applied Physics Letters, Vol. 60, p. 2770,
1992 (incorporated herein by reference), for a discussion of high
frequency current roll-off in a HBT with collisionless propagation of
minority carriers across the base.
FIG. 3 shows intrinsic current gain vs. frequency, all curves including the
effect of collector delay .tau..sub.c =1 ps. Curve 30 corresponds to the
(unphysical) case of a transistor with zero base delay, 31 to a transistor
with diffusive base delay .tau..sub.D =2 ps, and 32 to an analogous CT
with .DELTA.=10 kT and .tau..sub.B =2 ps. The figure clearly demonstrates
the existence in the CT of large gain in the frequency range in which not
only the diffusive transistor but even the transistor with no base delay
at all, are completely damped.
We shall next include in our discussion the effects of (unavoidable)
extrinsic impedances. FIG. 4 represents an appropriate equivalent circuit
of an abrupt junction HBT, wherein dashed line 40 encloses the intrinsic
portion of the transistor, and E, B and C refer to emitter, base and
collector, respectively. The intrinsic parameters R.sub.E and C.sub.E are
the differential resistance and capacitance of the emitter/base junction,
respectively, C.sub.C and g.sub.A are the collector junction capacitance
and the Early conductance, respectively, .alpha..sub.B and .xi..sub.c are
the base and collector transport factors, respectively, and R.sub.B is the
intrinsic base resistance. C.sub.CX is the extrinsic collector
capacitance, and R.sub.BX, R.sub.CX and R.sub.EX are the parasitic base,
collector and emitter resistances.
Analysis of the equivalent circuit for the case of a CT reveals an
unexpected result, namely, the desirability of a relatively large W.sub.B.
Frequently, the coherency condition can still be met at temperatures below
77K for W.sub.B =100 nm or even larger, and it will frequently be
desirable to design a CT such that W.sub.B is relatively large,
possibly.gtoreq.100 nm. Large W.sub.B is typically desirable because it
allows the minority carriers to acquire an optimum phase delay
(typically>.tau.) at frequencies within the contemplated frequency range
(e.g., 100 GHz-1 THz). Furthermore, relatively large W.sub.B allows one to
attain relatively low base resistances R.sub.B and R.sub.BX. This is a
significant advantage, as those skilled in the art will appreciate. The
above expressed preference for relatively large W.sub.B is to be compared
to the general prior art teaching to minimize W.sub.B in "ballistic"
The analysis of the equivalent circuit also indicates that 2.pi.fC.sub.c
(R.sub.E +R.sub.EX +R.sub.CX +R.sub.x.sup.eff) desirably is less than 1,
where R.sup.eff =R.sub.CX (R.sub.E +R.sub.EX)/(R.sub.B +R.sub.BX). This
result indicates that even for .LAMBDA.>>W.sub.B, the upper limit of the
frequency range in which the transistor can exhibit gain is limited by the
As those skilled in the art will know, a phase delay is associated with the
current transport through any bipolar transistor. The phase delay can be
expressed as the sum of the injection phase delay .phi. and the drift
delay .theta. in the base/collector junction, with .phi.=.phi..sub.E
+.phi..sub.B, where .phi..sub.E and .phi..sub.B are the total transit
angles of emitter and base, respectively. It is a significant aspect of
the invention that a CT will typically be designed such that
.phi..gtorsim..theta., with .phi..sub.B .congruent.2.pi.f.tau..sub.B
.gtorsim..pi. for frequencies above f.sub.T and f.sub.max. This implies
design choices that are contrary to the prior art teachings. For instance,
these conditions suggest rather large values of W.sub.B (frequently
.gtorsim.100 nm), relatively small values of .DELTA., and use of
relatively large effective mass minority carriers. All of the above
referred to phase angles can be determined for a given design. For
instance, .theta.=W.sub.c /v.sub.s, where W.sub.c is the width of the
collector depletion region, and v.sub.s is the saturated velocity in that
depletion region. The emitter phase angle .phi..sub.E is defined only in
the limit .phi..sub.E <<.pi. (typically .phi..sub.E .ltorsim..pi./4), and
in that limit is approximately equal to 2.pi.fR.sub.e C.sub.e.
FIG. 5 schematically shows relevant aspects of an exemplary CT, wherein
numerals 50-56 refer, respectively, to the collector contact, collector,
collector depletion region, base, emitter, emitter contact and base
contact. Numeral 540 refers to the emitter/base space charge layer. The
emitter stripe width L.sub.E, base width W.sub.B and collector depletion
layer width W.sub.C are also indicated. The relevant characteristics of a
CT as shown in FIG. 5 were determined from the equivalent circuit of FIG.
4, using the following parameter values: R.sub.E =5.OMEGA..multidot..mu.m,
R.sub.B =25.OMEGA..multidot..mu.m, R.sub.BX =25.OMEGA..multidot..mu.m,
R.sub.EX =20.OMEGA..multidot..mu.m, R.sub.CX =20.OMEGA..multidot..mu.m,
C.sub.C =0.5 fF/.mu.m, C.sub.E =10 fF/.mu.m, and C.sub.CX =1 fF/.mu.m;
these parameters are given per 1 .mu.m of emitter stripe width Z and are
based on the assumed dimensions W.sub.B =0.1 .mu.m, W.sub.C =0.1 .mu.m,
L.sub.E =0.5 .mu.m, the injection energy .DELTA.= 14.4 meV, and the base
layer resistivity .rho..sub.B =0.001.OMEGA..multidot.cm. Furthermore, it
was assumed that the device temperature is 4.2K. These parameters can
substantially be obtained in, for instance, a p-n-p Si-Ge heterostructure
with a wide gap p-type Si emitter, abruptly adjoining a narrow gap n-type
Ge.sub.x Si.sub.1-x (x.about.0.1) base. For a heavy hole mass m=0.5
m.sub.o, one gets V.sub.B .about.10.sup.7 cm/s, where m.sub.o is the free
electron mass and v.sub.B is hole velocity in the base.
FIG. 6 shows results of the numerical analysis. In particular, it shows the
absolute values of current gain and unilateral power gain, both as a
function of frequency. As can readily be seen, the conventional f.sub.T of
the exemplary transistor is about 100 GHz. The figure shows, however, that
the transistor is active up to frequencies of about 2.pi.f.sub.T. The
analysis revealed that the current gain is largely damped away by the
parasites (although a trace of the peak is clearly seen near f.tau..sub.B
.about.1), and that the unilateral power gain U in the region between the
two peaks in .vertline.U.vertline. is actually negative, indicating that
the transistor is active and the real part of the output impedance
z.sub.22.sup.e is less than zero in that frequency region.
An abrupt junction CT of design substantially as shown in FIG. 5 is made as
follows: on a conventional single crystal Si substrate is grown by
conventional MBE an epitaxial layer sequence that comprises a 200 nm thick
n-type (10.sup.19 cm.sup.-3 B) collector layer, a 100 nm thick
substantially undoped (.gtorsim.10.sup.16 cm.sup.-3) collector depletion
layer, a 100 nm thick p-type (10.sup.19 cm.sup.-3 As) Si.sub.1-x Ge.sub.x
(x.apprxeq.0.1) base layer, a 5 nm thick light p-type (.ltorsim.10.sup.17
cm.sup.-3 As) Si emitter/base space charge layer, and a 200 nm thick
n-type (10.sup.19 cm.sup.-3 B) emitter layer. The wafer is patterned by
conventional lithography and etching to define a HBT, and emitter, base
and collector contacts are provided, all as known in the art. The HBT is
cooled to 4.2K and conventional measurements are carried out. The
measurements show that the device is a CT, with .beta. and U substantially
as shown in FIG. 6. Measurements at 15K show little change in behavior.
This temperature can readily be reached by means of a commercially
available re-circulating He-refrigerator.
* * * * *