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Impact of Compositionally Graded Base Regions on the DC and RF Properties of Reduced Turn-On
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 10, OCTOBER 2004
1545
Impact of Compositionally Graded Base Regions
on the DC and RF Properties of Reduced Turn-On
Voltage InGaPGaInAsN DHBTs
Kevin S. Stevens, Member, IEEE, Rebecca J. Welty, Member, IEEE, Roger E. Welser, Barbara E. Landini,
Peter M. Asbeck, Fellow, IEEE, Shih-Chieh Hung, Wen-Pin Lu, and Shun-Ching Feng
AbstractBuilt-in drift elds are employed to enhance the
performance of GaAs-based heterojunction bipolar transistors
(HBTs) with reduced turn-on voltage. Specically, we explore in
detail the dc and RF device property improvements enabled by
using compositionally graded GaInAsN base layers. Experimental
results are compared to predictions of the standard drift-diffusion
base transport model employing a nite exit velocity. In large area
devices, graded base samples with built-in elds of
7 kV/cm (i.e.
40 meV over 500 
A) typically have a dc current gain 1.8 larger
than constant base composition samples. In small area devices,
the peak cut-off frequency is typically 10%15% higher than
constant composition samples. These results are shown to agree
reasonably well with predictions, thereby demonstrating that
analytical drift-diffusion based models can be extended to HBTs
with GaInAsN base layers.
Index TermsBipolar transistor, GaInAsN, graded base,
InGaPGaAs heterojunction bipolar transistors (HBTs), turn-on
voltage.
I. I
NTRODUCTION
G
aAs-BASED heterojunction bipolar transistors (HBTs)
have emerged over the past decade as a key technology
for both power ampliers (PAs) in wireless applications and
high-speed digital circuits in telecommunication applications.
However, there is an ever-increasing demand for performance
enhancements to the basic GaAs-based HBT. For wireless PAs,
lower turn-on, offset, and knee voltages are needed to improve
the minimum voltage of operation and power-added efciency
(PAE). For digital applications, increasingly higher frequency
of operation is desirable. Additionally, for both PA and digital
designs, improved temperature stability can minimize tempera-
ture compensation issues.
We are developing a graded GaInAsN base technology which
adds a new level of design freedom to GaAs-based HBTs [1],
[2]. The energy-gap of the base layer
is a key param-
eter controlling the turn-on voltage and beta temperature sta-
bility for an HBT [3]. The addition of both indium and nitrogen
Manuscript received April 13, 2004. This work was supported by the
AFRL/SND at WPAFB under STTR Contract F33615-99-C-1510. The review
of this paper was arranged by Editor M. Anwar.
K. S. Stevens, R. E. Welser, and B. E. Landini are with the Kopin Corporation,
Taunton, MA 02780 USA.
R. J. Welty and P. M. Asbeck are with the University of California at San
Diego, La Jolla, CA 92093 USA.
S.-C. Hung, W.-P. Lu, and S.-C. Feng are with the Advanced Wireless Semi-
conductor Company Tainan, Taiwan, R.O.C.
Digital Object Identier 10.1109/TED.2004.834905
to GaAs allows
to be reduced while minimizing strain. In
addition to
reduction, the double heterojunction nature of
the InGaPGaInAsNGaAs bipolar transistor leads to improve-
ments in offset and knee voltages. Compositionally grading the
GaInAsN base provides a means to engineer and improve both
the dc current gain (
or
) and the unity gain cut-off fre-
quency
.
In this paper, we explore in detail the impact of composi-
tional grading on the RF and dc properties of InGaPGaInAsN
double HBTs (DHBTs). We begin by reviewing the advantages
and shortcomings of constant composition GaInAsN DHBTs.
We then summarize predictions of existing drift-diffusion-based
analytical transport models in regard to the expected impact of
a linear compositional grade on collector current density, base
transit time, dc current gain, and RF properties. We present ex-
perimental data which shows agreement with the predictions,
thereby demonstrating that existing analytical models can be
reasonably applied to GaInAsN HBT structures.
II. R
EDUCED
T
URN
-O
N
V
OLTAGE
InGaPGaInAsN DHBTs
Due to the relatively large energy-gap of the GaAs base layer,
the turn-on voltage of a GaAs-based HBT is high compared
to other bipolar and eld-effect transistor (FET) technologies.
InGaAs ternary alloys have a lower energy-gap, but introduce
compressive strain due to a lattice mismatch with GaAs [4], [5].
The addition of N to InGaAs lowers the energy gap further (due
to a large bowing parameter) and minimizes strain by reducing
the lattice constant back toward GaAs. The GaInAsN quater-
nary material system is being pursued by a number of groups
for a variety of applications, including long wavelength lasers
[6], solar cells [7], and HBTs [8][10]. Very encouraging re-
sults have been reported in terms of turn-on voltage reduction
up to 400 mV using GaInAsN in the base layer of GaAs-based
HBTs. However, constant composition GaInAsN DHBTs have
suffered from degraded minority carrier properties compared to
conventional GaAs HBTs. This degradation can be quite severe
in low
samples, leading to low beta, high base sheet resis-
tance
, and reduced
[8], [9].
We have been growing InGaPGaInAsN DHBT struc-
tures on production metalorganic chemical vapor deposition
(MOCVD) systems [3]. The basic InGaPGaInAsN DHBT
structure is identical to a conventional InGaPGaAs HBT ex-
cept for the use of GaInAsN and associated interfacial layers in
the base layer. The Gummel plot comparison in Fig. 1 illustrates
0018-9383/04$20.00 © 2004 IEEE 1546
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 10, OCTOBER 2004
Fig. 1.
Gummel plots from large area devices
(75 2 75 m ) on three different transistor structures: a conventional InGaPGaAs HBT (diamonds) and two
constant InGaPGaInAsN DHBTs (circles and triangles) with reduced
E . Both the collector (solid shapes with line) and base (open shapes) currents are shown.
To maintain a dc current gain
35, the base sheet resistance increases from 125 to 275 to 700
= in the lower V samples. The collector current ideality factor
(n ) is 1.0 to 1.02 on all three devices.
the resulting pathway for
reduction using the GaInAsN ma-
terial system. Specically, Fig. 1 compares the Gummel plots
from a standard InGaPGaAs HBT and two InGaPGaInAsN
DHBTs with increasing In and N content. The conventional
GaAs-based HBT has a turn-on
at
A/cm ,
which has been reduced by 100 and 200 mV to
1.0 and
0.9 V in the two GaInAsN DHBTs shown here. This turn-on
reduction is governed by the base sheet resistance and the
base energy-gap, with
and 1.26 eV for the
two GaInAsN samples shown in Fig. 1 [3]. Greater reductions
in
are possible as
is further lowered by increasing In
and N content. Constant composition GaInAsN DHBTs also
exhibit reduced offset and knee voltages, more stable
versus
temperature characteristics, and improved linearity [10], [11].
While GaInAsN DHBTs provide many advantages com-
pared to conventional GaAs HBTs, a degradation in minority
carrier lifetime is typically observed. Fig. 1 illustrates this via
decreasing
ratios as turn-on
is reduced. We note
that a signicant reduction in this ratio is observed even though
the maximum N content is under 0.4% in these samples. While
work continues to improve the minority carrier lifetime in the
GaInAsN material system, a graded base structure provides a
means to reduce base transit time and thereby increase both the
dc and RF gain.
In this paper, we compare constant and graded composition
GaInAsN DHBTs with standard InGaPGaAs HBTs. Large
area device
m
results are presented for a number
of different sets of structures. Each of these sets consists of
comparisons between graded base structures and constant
composition structures grown with conditions that match the
base-emitter endpoint, the base-collector endpoint, or interme-
diate points along the grade. Most of the graded base structures
have an energy-gap grade
on the order of 40 meV, as
will be discussed in the upcoming sections. Indium compostion
in all samples falls roughly between 3% and 8%.
Two sets of structures were processed into small area devices
(
m ,
m and
m ) at UCSD and AWSC
(Table I). The graded base structures for these small area devices
have
meV. All the GaInAsN base structures in
Table I exhibit
V at
A/cm .
III. G
RADED
B
ASE
C
ONCEPT AND
B
ACKGROUND
Although the graded base concept was introduced by
Kroemer in 1954 [12][14], [39] and has been successfully
exploited in Si Ge
HBT technologies [15], its application
to GaAs-based HBTs has been limited [4], [5], [16], in part
due to the difculty of C-doping In-containing compounds. In
an n-p-n bipolar transistor, grading the base layer energy-gap
from a larger value at the baseemitter junction to a smaller
value at the basecollector interface produces a quasi-electric
eld which accelerates electrons across the base layer, thereby
increasing the average electron velocity. This increase in ve-
locity relative to a conventional GaAs base can be quantied
by a velocity factor VF which is a function of
and base
thickness
, dened as
VF
(1)
in which
VF
(2)
VF
(3)
where
is the average electron velocity across the base,
is
the base transit time,
is the diffusion constant for electrons in
the base, and
is the exit veloci