Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – With lattice constant mismatch
Reexamination Certificate
2001-02-27
2004-02-24
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
With lattice constant mismatch
Reexamination Certificate
active
06696710
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to transistors, and, more particularly, to a heterojunction bipolar transistor (HBT) having an improved emitter-base junction.
BACKGROUND OF THE INVENTION
Heterojunction bipolar transistors (HBTs) have become state of the art, particularly in npn form, for applications in which high switching speeds and high frequency operation are desired. The emitter in an HBT has a bandgap wider than the bandgap of the base, thus creating an energy barrier in the valence band at the emitter-base junction that inhibits the unwanted flow of holes from the base region to the emitter region. This arrangement increases the emitter injection efficiency, current gain and operating frequency of the HBT.
First generation commercial HBTs were based on a gallium-arsenide (GaAs) substrate and semiconductor materials lattice matched to GaAs. Next generation HBTs are likely to be based on an indium-phosphide (InP) substrate and semiconductor materials lattice matched to InP. Typically, the base of such an HBT is fabricated from either the indium-gallium-arsenide (InGaAs) material system or the gallium-arsenide-antimonide (GaAsSb) material system, with the collector and the emitter fabricated from, for example, InP, aluminum-indium-arsenide (AlInAs) or InGaAs. HBTs that are fabricated using GaAsSb as the base material and AlInAs as the emitter material offer certain advantages over HBTs in which the base material is GaAsSb and the emitter is material is InP. For example, the conduction band energy line-up between AlInAs and GaAsSb (emitter-base) provides certain advantages, such as higher current-density operation, and hence higher frequency operation. Unfortunately, it is difficult to grow a good AlInAs/GaAsSb interface.
FIG. 1
is a graphical illustration showing an energy band diagram
11
of a conventional InP emitter/GaAsSb base/InP collector HBT under modest forward electrical bias on the emitter-base junction. The vertical axis
12
represents the energy level and the horizontal axis
14
represents distance. That is, the thickness of the material that respectively comprises the emitter region
22
, the base region
24
and the collector region
26
. A heterojunction bipolar transistor (HBT) with a GaAsSb base and InP collector has a type-II band lineup at the collector-base junction
32
as shown. The energy discontinuity &Dgr;E
c
in the conduction band
16
is about 0.18 electron Volts (eV) and the energy discontinuity &Dgr;E
v
in the valence band
18
is about 0.76 eV. This is an essentially ideal band lineup for this junction for the following reasons. A small ballistic energy &Dgr;E
c
is imparted to collected electrons and there is a large valence-band discontinuity &Dgr;E
v
at the metallurgical base (base-collector junction
32
) that minimizes hole injection into the collector region
26
even at low or positive collector bias. Since the wide-bandgap InP extends throughout the collector region
26
, avalanche breakdown is minimized.
Other variations of HBTs lattice-matched to InP, but with a base layer different from GaAsSb fail to offer these advantages. For example, the use of the same structure but with an InGaAs base has the large valence-band discontinuity &Dgr;E
v
at the metallurgical base and the benefit of the wide-bandgap InP, but presents a barrier to electron collection, which could result in undesirable stored charge in the base. This compromises the frequency response and maximum current of the device. Any scheme to eliminate this barrier compromises the desired features of the large valence-band discontinuity &Dgr;E
v
at the metallurgical collector-base junction, and the benefit of the wide-bandgap InP.
Furthermore, in HBTs having a GaAsSb base and InP emitter (as shown in
FIG. 1
) the type II band lineup leads to two undesirable features. Both are related to the discontinuity in the electron concentration across the heterojunction of exp(−q&Dgr;E
c
/kT), where q is the electron charge, &Dgr;E
c
is the conduction band discontinuity, k is Boltzmann's constant, and T is the absolute junction temperature. Since &Dgr;E
C
is approximately 0.18±0.1 eV, the ratio of electron concentration across the discontinuity is in the range of 2×10
−5
to 5×10
−2
at room temperature.
The first undesirable feature is lowered current gain. Below some limiting injection level, it can be shown that interface recombination at the metallurgical junction (the emitter-base junction
28
) depends on the electron concentration on the emitter side of the junction and on the interface trap properties.
The interface current density j
interface
=qn
emitter
v
interface
, where n
emitter
is the electron density on the emitter side of the interface and where v
interface
is the interface recombination velocity. The interface recombination velocity v
interface
=&sgr;
n
v
thermal
N
traps
+K
s
—
i
—
rad
p
base
, where &sgr;
n
is the cross-section for capture of an electron by an interface trap, v
thermal
is the thermal velocity of electrons, N
traps
is the trap concentration as a density per unit area, K
s
—
i
—
rad
is a constant that describes the proportionality of spatially indirect radiative recombination at the interface, and p
base
is the hole concentration on the base side of the interface. The total interface recombination velocity is thus due to recombination through traps, and through spatially indirect radiative recombination. The material interface, as it can be practically grown, will not be electrically perfect. For example, there may be impurities or imperfections at the interface that lead to spatially localized states inside the energy gap. Electrons or holes that land in these spatially localized states cannot move around (unlike electrons or holes in the conduction or valence bands), and these spatially localized states have a potential energy between that of the valence and conduction bands. These spatially localized states can alternately trap electrons and holes, thereby providing a path for recombination. This is conceptually similar to Schockley-Read-Hall recombination. Spatially indirect recombination is band-to-band recombination between electrons that are localized on one side of a type-II heterojunction (in this example the InP side) and holes that are localized on the other side (in this example the GaAsSb side). The recombination is referred to as spatially indirect because the electrons and holes are separated according to classical physics. According to quantum physics the electrons and holes are not perfectly localized. They are represented by wave functions that slightly overlap. Therefore, some recombination occurs. Both of these effects are known to those having ordinary skill in the art.
The injection current density j
injection
=qn
base
v
base
, where n
base
is the injected electron concentration on the base side of the emitter-base junction and where v
base
is the electron velocity through the base. The ratio j
injection
/j
interface
=v
base
n
base
/v
interface
n
emitter
represents an upper limit to the current gain of the transistor. The ratio of electron density on either side of the metallurgical junction leads to an effective multiplication of the interface recombination velocity by exp (q&Dgr;E
c
/kT), directly affecting the current gain.
The second undesirable feature in an HBT having a GaAsSb base and InP emitter is a reduction of the current at which current gain compression occurs. In typical HBT's, a relatively low emitter doping N
e
is used to reduce the emitter-base capacitance. For example, the use of a 4-8×10
17
cm
−3
emitter doping places a hard upper limit, of N
e
exp (−q&Dgr;E
c
/kT), on the injected electron density in the base. This is illustrated by the energy band diagram
51
in
FIG. 2
, which represents the InP/GaAsSb/InP HBT of
FIG. 1
at a strong forward bias on the emitter-base junction
68
. As this bias is approached, the emitter capacitance becomes quite large and the freq
Houng Yu-Min
Moll Nicolas J.
Agilent Technologie,s Inc.
Fahmy Wael
Farahani Dana
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