|United States Patent
July 12, 1994
Heterojunction bipolar transistor with a specific graded base structure
The disclosed novel heterojunction bipolar transistor, to be referred to as
the enhanced diffusion transistor (EDT), comprises a base of composition
selected such that the base bandgap narrows from emitter towards collector
in substantially step-wise fashion, resulting in N (N.gtoreq.2)
substantially flat levels in the base bandgap. The height .DELTA..sub.j of
the steps in the bandgap is greater than kT (typically at least about 30
meV), and also greater than the threshold energy of an appropriate rapid
inelastic minority carrier scattering mechanism (e.g., optical phonon
scattering, plasmon scattering) in the base material. The presence of the
"steps" in the base bandgap of the EDT can, in consequence of the
resulting strongly accelerated diffusive minority carrier transport in the
base, lead to, e.g., improved high frequency characteristics, as compared
to otherwise identical prior art (N=1) transistors.
Luryi; Serge (Bridgewater, NJ)
AT&T Bell Laboratories (Murray Hill, NJ)
April 23, 1993|
|Current U.S. Class:
||257/197; 257/198; 257/592 |
||H01L 031/072; H01L 031/109; H01L 027/082|
|Field of Search:
References Cited [Referenced By]
|Foreign Patent Documents|
"Bipolar Transistor with Graded Band-gap Base"; Hayes et al; Electronics
letters, vol. 19, p. 410, 1983.
"Physics of Semiconductor Devices", by S. M. Sze, 2nd Edition, John Wiley &
Sons, 1981, Chapter 3.
"Subpicosecond InP/InGaAs Heterostructure Bipolar Transistors", by Y. Chen
et al., IEEE Electron Device Letters, vol. 10, No. 6, Jun. 1989, pp.
"Small-Signal Theory of the Transistor Transit-Time Oscillator
(Translator)", by G. T. Wright, Solid State Electronics, 1979, vol. 22,
"Frequency Dependence of the Unilateral Gain in Bipolar Transistors", by S.
Tiwari, IEEE Electron Device Letters, vol. 10, No. 12, Dec. 1989, pp.
"Negative Resistance Arising from Transit Time in Semiconductor Diodes", by
W. Shockley, The Bell System Technical Journal, vol. 33, No. 4, p. 799,
"A Unipolar Transistor With Negative Output Resistance", by N. Dagli,
Solid-State Electronics, vol. 33, No. 7, pp. 831-836, 1990.
"(GaAl)As/GaAs Heterojunction Bipolar Transistors With Graded Composition
in the Base", by D. L. Miller, Electronics Letters, vol. 19, p. 367, 1983.
"Bipolar Transistor With Graded Band-gap Base", by J. R. Hayes, Electronics
Letters, vol. 19, p. 410 (1983).
Primary Examiner: Hille; Rolf
Assistant Examiner: Fahmy; Wael
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, the base comprising
material of a second conductivity type that differs from the first
conductivity type and having a base bandgap; CHARACTERIZED IN THAT
the base is selected such that the base bandgap comprises N(N.gtoreq.2)
regions (designated 1st, 2nd, . . . , j-th, . . . , Nth) of substantially
constant bandgap width, with a substantially step wise change
(.DELTA..sub.j) in bandgap width between the j-th region and the (j+1)-th
region, the bandgap width decreasing monotonically, without increasing
anywhere, in the direction from emitter towards collector, with
.DELTA..sub.j >kT, where k is Boltzmann's constant and T is an absolute
transistor operating temperature, and furthermore, .DELTA..sub.j
>.DELTA..sub.inelastic, where .DELTA..sub.inelastic is an inelastic
minority carrier scattering threshold energy associated with the material
of the j'th region.
2. Article according to claim 1, wherein associated with the material of
the jth region is an optical phonon frequency .nu..sub.opt, and wherein
.DELTA..sub.inelastic =h.nu..sub.opt, where h is Planck's constant.
3. Article according to claim 1, wherein .DELTA..sub.inelastic is
associated with scattering of minority carriers by majority carrier
plasmons in the material of the j'th region.
4. Article according to claim 1, wherein T is about 300K.
5. Article according to claim 1, wherein .DELTA..sub.j .gtoreq.30 meV.
6. Article according to claim 5, wherein N=2.
7. Article according to claim 5, wherein N.gtoreq.5, said article
comprising means for providing to said base an electrical signal of
frequency approximately equal to .pi.f.sub.max, where f.sub.max is the
frequency at which the unilateral gain U of an otherwise identical
comparison transistor with N=1 is unity.
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 .ident..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 rate of 10 dB/decade, i.e. proportionally to
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. It is well known
that U at high frequencies decreases at the rate of 20 dB/decade, i.e.,
proportionally to inverse square of the frequency. 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. 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, inter alia, operate at even higher frequencies.
G. T. Wright, (see, for instance, Solid State Electronics, Vol. 22, p. 399,
1979) proposed extension of active transistor operation to frequencies
beyond the conventional cutoff frequencies. The proposal involved the
utilization of transit time resonances that arise from carrier drift in
the collector space charge region, resulting in a negative output
resistance of the transistor. The proposed model suggested for an ideal
transistor (i.e., a transistor without any parasitic extrinsic 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 the collector 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. Recent analysis shows that the indicated
difficulty in utilization of the collector transit-time effect arises
because of a relatively large decrease (by at least a factor of three) in
the magnitude of the common-base current gain, which is in principle
unavoidable if a necessary transit angle of order 180 degrees is acquired
in carrier transit across the collector space-charge region. The resultant
gain is so weak that it is practically damped by parasitic extrinsic
Almost 40 years ago it was suggested (W. Shockley, Bell System Technical
Journal, Vol. 33, p. 799) that active transistor behavior above the
conventional transit time cutoff could be obtained from the base transport
of minority carriers. A necessary condition for this is that the directed
minority carrier transport across the base is much faster than the
diffusive transport. In principle, this condition could be met in a
transistor with exponentially graded base doping profile. To the best of
my knowledge, no such device has ever been realized.
U.S. patent application Ser. No. 07/981,588, filed Nov. 25, 1992 and
incorporated herein by reference, discloses a "coherent" ballistic
transistor capable of providing gain at frequencies above the conventional
cut-off frequency. The coherent transistor employs the base transit angle
and therefore is much less susceptible to the parasitic damping than
previous proposals utilizing the collector transit angle. However, because
of the requirement that the minority carrier transport across the base be
ballistic, practical realizations of the disclosed are likely to be
restricted to low-temperatures and ultra-high frequencies (exemplarily
well above 100 GHz).
It would clearly be desirable to have available a transistor that can
operate at room temperature, exhibiting transit time resonances at
frequencies above the conventional cutoff frequencies. Moreover, it would
be desirable to be able to choose the resonant frequency in a wide range,
not necessarily above 100 GHz.
This application discloses such a transistor. The novel device, to be
referred to as the "enhanced diffusion" transistor (EDT), 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 of an appropriate prior art comparison
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 (typically n) conductivity type, and the
base comprises material of a second (typically p) conductivity type.
Associated with the transistor is a current gain .beta. and unilateral
power gain U. Significantly, the base region is selected such that the
base bandgap narrows from emitter towards collector in substantially
step-wise fashion, resulting in N(N.gtoreq.2) substantially flat levels in
the base bandgap. The steps are of height .DELTA..sub.j .gtoreq.kT (k is
Boltzmann's constant, T is the transistor operating temperature),
typically at least 30 meV, and greater than the threshold energy
(.DELTA..sub.inelastic) of an appropriate rapid inelastic carrier
scattering mechanism e.g., optical phonon scattering, plasmon scattering).
As will be described below, the presence of "steps" in the base bandgap
makes it possible to obtain active behavior of the transistor at higher
frequencies than in an otherwise identical conventional (N=1) transistor.
The improved high frequency behavior of the EDT is a consequence of
strongly accelerated diffusive minority carrier motion in the base, due to
the presence of the bandgap steps, and does not require "ballistic"
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically depicts the band diagram of an exemplary EDT; and
FIG. 2 illustrates current gain as a function of frequency for some
FIG. 1 schematically shows the band diagram of an exemplary EDT. Band
diagrams are well known to those skilled in the art. Numeral 10 designates
the emitter conduction band edge, 12 the collector conduction band edge,
13 the space charge region conduction band edge, and 111-11N the N
discrete conduction band levels in the base. The base contains N-1
substantially step-wise changes in composition, and consequently has N
distinct base band edge levels. Although FIG. 1 shows steps of essentially
equal height (i.e., .DELTA..sub.j) and levels of essentially equal width
(i.e., W.sub.j), this is not a requirement. Exemplarily, the emitter is of
composition Al.sub.0.4 Ga.sub.0.6 As and comprises a n.sup.++ (e.g.,
10.sup.19 Si/cm.sup.3) contact layer, a n.sup.+ (e.g., 10.sup.18
Si/cm.sup.3) intermediate layer, and a n (e.g., 10.sup.17 Si/cm.sup.3 ;
20-30 nm thickness) layer in contact with the base. Such emitters are
known. Furthermore, the collector exemplarily is GaAs and comprises a
substantially undoped space charge region (e.g., about 100 nm thickness)
and n.sup.+ (e.g., 5.times.10.sup.18 Si/cm.sup.3) collector region. The
(n.sup.++, e.g., 10.sup.19 Be/cm.sup.3) base exemplarily has 10 layers of
nominal compositions Al.sub.0.36 Ga.sub.0.54 As, Al.sub.0.32 Ga.sub.0.58
As, . . . , Al.sub.0.04 Ga.sub.0.96 As, respectively, resulting in
.DELTA..sub.j of about 60 meV. Numerals 14-16 refer to the emitter, base
and collector Fermi levels, respectively. It is known that in heavily
p-type base material essentially the entire bandgap discontinuity (e.g.,
.DELTA..sub.j) appears as a step in the conduction band edge.
The following discussion pertains to a transistor of the type exemplified
by FIG. 1, and is provided for pedagogical reasons. For simplicity's sake
it is assumed that all .DELTA..sub.j as well as all W.sub.j are equal. It
is assumed that .DELTA..sub.j is much greater than kT, preferably at least
2 or 3 kT. This condition is easily met, even for room temperature
operation, in, e.g., the GaAs/AlGaAs system, and assures that carriers are
effectively inhibited to return once they have "fallen off" a particular
If, furthermore, .DELTA..sub.j is greater than the threshold energy for a
rapid inelastic carrier scattering process (e.g., .DELTA..sub.j
.gtorsim.h.nu..sub.opt, where h is Planck's constant and .nu..sub.opt is
the optical phonon frequency in the relevant base material) then one can
consider the carrier transport on each level individually, and
characterize it by a transport factor .alpha..sub.j, where
.alpha..sub.j (.omega.)=[cosh(2i.phi..sub.j).sup.1/2 ].sup.-1,(1)
with .omega. being angular frequency, .phi..sub.j =.omega..tau..sub.j is
the phase acquired in transit across step j, and .tau..sub.j
=W.sub.j.sup.2 /2D is the step propagation time by diffusion, with D being
the minority carrier diffusivity in the base material. Equation (1)
represents a well-known expression for the transport factor in the absence
of recombination. For the acquired phase (.phi..sub.j) much less than
unity, it reduces to the following approximate expression:
which is valid neglecting cubic and higher order terms in .phi..sub.j.
Equation (2) shows that .vertline..alpha..sub.j .vertline. deviates from
unity quadratically in .phi..sub.j.
The base transport factor .alpha..sub.B is the product of the .alpha..sub.j
Under the above stated conditions, any given .alpha..sub.j is independent
of the others, and for sufficiently short W.sub.j the step transport
factors are given by Eq. (2). For N equal steps, the overall base
transport factor, (Eq. 3) reduces to
.alpha..sub.B =exp(-.phi..sup.2 /3N).multidot.exp(-i.phi.),(3a)
where .phi.=.SIGMA..phi..sub.j is the overall phase acquired in the base
transport. The key effect of the staircase base is the fact that for N<1
the exponential decrease in the magnitude of .alpha..sub.B is
substantially slower than it would be in a base of same width without
The intrinsic current gain .beta..sub.B is equal to .alpha..sub.B
/(1-.alpha..sub.B), and the absolute value of .beta..sub.B is greater than
unity at .phi.=2 .pi. (corresponding to a frequency of 2.pi.f.sub.T),
provided the absolute value of .alpha..sub.B at that frequency is greater
than about 0.5. This translates into the condition that N should be
greater than about 19. However, it will be shown below that advantageous
results can be attained for N<19, even as small as 2.
The above described effect results from an enhancement of the forward
diffusive transport of minority carriers, due to carrier thermalization at
every step. This thermalization typically provides the independence of
.alpha..sub.j 's and substantially restricts particles from returning to a
preceding level. It can be shown that, in the absence of recombination,
where J is the steady state current, e is the electron charge, D is the
minority carrier diffusivity, W=NW.sub.j is the total base thickness, and
n(0) is the minority carrier concentration of the beginning of the first
step (or any other step). Equation 4 shows that the presence of the steps
results in enhancement by a factor N of the diffusive minority carrier
flux, and an equal enhancement of the effective diffusion velocity, which
is now 2 D/W.sub.j.
FIG. 2 shows current gain .beta..sup.2 vs. dimensionless frequency
(.omega.W.sup.2 /2D), for a prior art transistor (N=1), and two analogous
EDTs (N=3 and 30). As can be seen, even the N=3 EDT has substantially
higher f.sub.T than the, otherwise identical, prior art transistor.
It can also be shown that an extended frequency peak in the unilateral
power gain U appears when .phi. is about equal to .pi.. Consideration of
the effect of (inevitably present) parasitic extrinsic impedances shows
that a peak appears in U at .phi..about..pi. if, for that value of .phi.,
.vertline..alpha..sub.B .vertline. is greater than .omega..tau..sub.x,
where .tau..sub.x is a, readily calculable, parasitics-limited transistor
time constant. For instance, for a particular known model of an
abrupt-junction EDT, .tau..sub.x =C.sub.C R.sub.x, where C.sub.C is the
capacitance of the intrinsic base-collector junction, and R.sub.x =R.sub.E
+R.sub.Ex +R.sub.Cx +R.sub.Cx (R.sub.E +R.sub.Ex)/(R.sub.B +R.sub.Bx),
with R.sub.E, R.sub.C and R.sub.B referring to the intrinsic value of
emitter, collector and base resistance, respectively, and subscript x
designating the corresponding parasitic extrinsic resistance.
If the condition .vertline..alpha..sub.B .vertline.>.omega..tau..sub.x is
met then, in addition to the above discussed "low" frequency regime in
which U>1, an EDT can be made active in the frequency range in which
.vertline..alpha..sub.B .vertline. sin (.phi.+.theta.')+.omega..tau..sub.x
.ltoreq.0, where .theta.'=.omega..tau..sub.c /2 is half of the collector
transit angle. Of course, this condition can only be met if, at the
frequency .omega., the transistor is not overdamped by the parasitics
(i.e., .omega..tau..sub.x <1). For example, if .omega..tau..sub.x
.about.0.5, then this condition will typically be met for N.gtorsim.5.
Although under these circumstances U<0, it is possible to obtain U>>1 by
adding a series resistance. See, for instance, N. Dagli, Solid State
Electronics, Vol. 33, p. 831 (1990).
Those skilled in the art will appreciate that in the disclosed novel
transistor minority carrier transport occurs (at room temperature, but not
excluding low temperature operation if desired) by a strongly accelerated
forward diffusion process, without requirement of ballistic transport
(however, a ballistic transport component may optionally be present). The
diffusive process can be adequate for achieving transit time resonance at
ultra-high (e.g.,>100 GHz) or conventional frequencies, depending on
design choices. Moreover, even without the resonance, use of the disclosed
accelerated diffusion process can enhance design flexibility for
high-performance HBTs, e.g., give greater flexibility in the choice of
base thickness and/or doping level.
In a particular exemplary embodiment an article according to the invention
comprises an EDT with N=2 or 3, the article comprising means for providing
to said EDT electrical signals or frequency below about 100 GHz. In
another exemplary embodiment the EDT has N.gtoreq.5, and the article
comprises means for providing to the transistor signals of approximate
frequency .pi.f.sub.max, where f.sub.max is the frequency at which the
unilateral gain (U) of an otherwise identical comparison transistor with
N=1 is unity.
Those skilled in the art will also recognize that, in the limit as N
becomes very large, a "staircase" transistor becomes a transistor with
continuously graded base (CGB). Such transistors are known. See, for
instance, D. L. Miller et al., Electronics Letters, Vol. 19, p. 367
(1983), and J. R. Hayes et al., Electronics Letters, Vol. 19, p. 410
(1983). Although a transistor with CGB is not a EDT according to the
instant invention, I have discovered that a transistor of the former type
offers the possibility of enhanced high frequency performance in a region
of the parameter space that is typically not of interest for prior art
It can be shown that in the CGB transistor,
.alpha..sub.B (.omega.)=exp(r)[ cos h(.lambda.)+(1+2i.omega..tau..sub.B
/r).sup.-1/2 sin h(.lambda.)].sup.-1 (5)
where .lambda.=(r.sup.2 +2i.omega..tau..sub.D).sup.1/2, and r=.tau..sub.D
/.tau..sub.B =Wv/2D. The characteristic diffusion time .tau..sub.B
=.omega..sup.2 /2D, and the drift time .tau..sub.B =W/v, where W is the
base width, D is the minority carrier diffusivity, and v is the minority
carrier drift velocity.
In the absence of grading, Eq. (5) reduces to .alpha..sub.B =cos h.sup.-1
[(2i.omega..tau..sub.B).sup.1/2 ], which corresponds to Eq. (1) extended
to the whole base (i.e., N=1). For r much greater than 1,
.lambda..about.r+i.omega..tau..sub.B +(.omega..tau..sub.B).sup.2 /2r, and
Eq. (5) reduces to
.alpha..sub.B (.phi.)=[exp(-.phi..sup.2 /2r)].multidot.exp-i.phi.,(6)
where .phi.=.omega..tau..sub.B (1-1/2r).
Those skilled in the art will recognize, based on the above analysis, that
the parameter 2 r in Eq. (6) plays the same role as 3N in Eq. (3a). Thus,
CGB transistors with larger r (typically r.gtorsim.8) can also exhibit
extended frequency operation at .phi.5/8.pi., functionally similar to the
situation in EDTs, but with differing underlying physics. Thus, it will be
desirable to design and manufacture novel CGB transistors having larger
than previously attained values of r, since such transistors can have
improved high frequency characteristics, as compared to, otherwise
identical, CGB transistors with prior art r value.
* * * * *