Semiconductor Technology
Improved InGaP/GaAs HBT technology
facilitates high linearity PAs
While innovative circuit techniques aid in improving the performance of highpower amplifiers, underlying RF power transistors play an equally important role
in achieving PA performance goals. This article reports on the improvement in a
recently developed InGaP/GaAs HBT for 24 V to 28 V linear PA operation. Key
improvements include adjacent-channel leakage ratio under WCDMA modulation,
ruggedness to sustain high VSWR, and reliability. Plus, it reports on lifetime tests
conducted to guarantee the performance of the improved HBT technology.
By Nan-Lei Larry Wang
G
allium arsenide (GaAs) heterojunction
bipolar transistor (HBT) has become
the dominant technology for handset power
amplifier (PA) applications due to its linearity and efficiency. Consequently, GaAs
HBT with the emitter made of indium gallium phosphide (InGaP) is widely used in
handset PAs, and is gaining acceptance in
infra-structure amplifier designs. To address
base station applications with power levels
above multi-Watt, operation voltage higher
than the common 3 V to 7 V now used must
be adopted. The base station PA application
commonly requires 24 V to 28 V operation, and occasionally even above 30 V[1,2].
An InGaP/GaAs HBT technology was developed in our labs for 28 V power amplification[3-5], which is compatible with standard MMIC technology.
To achieve high breakdown voltage in
InGaP/GaAs HBT, a thick collector is required: 3 m thickness is commonly used.
The thick collector presents challenges to
semiconductor processing in lithography and
step coverage of metal interconnection. Power
transistors demand large total device size.
Multiple fingers are arranged into a single
building block. Such building blocks can
deliver 2 W RF power in the 1 GHz to 3 GHz
frequency band. Multiple building blocks can
be arrayed into a single-power HBT for higher
power levels. The fT and fmax of the basic HBT
finger are 6.4 GHz and 25 GHz respectively.
In a typical lineup of power amplifiers, the
power stage is often a class B circuit for the
best efficiency, and the driver stage a class AB
for a trade-off of linearity and efficiency, and
the pre-driver stage may be a class A amplifier.
Working with InGaP/GaAs HBT, the goal is
to operate the driver and pre-driver stages in
near-class B operation for better efficiency,
while achieving a superior linearity at the
back-off power level.
Class B operation with high linearity can
18
Figure 1. Single-building block layout.
be achieved by a low-frequency low-source
impedance, or a dynamic bias circuit[7]. The
ruggedness is improved by increasing the
ballasting to withstand high output mismatch
of 10:1 VSWR with typical input power at
1 dB gain compression and overdrive condition of typical 8 dB gain compression into
50  load at 30 V.
Accelerated lifetime test at 315 oC junction temperature and 28 V bias was repeated.
More than 3000 hours lifetime test on HBTs
was achieved; the Gummel plot before and
after the 3000 hours lifetime test shows no
increase of the leakage current. The high
linearity power performance in class B
condition in the back-off power level, the
ruggedness under mismatch and overdrive
condition, and the long lifetime of the InGaP/
GaAs HBT technology makes it a contender
for the 28 V power amplifier application.
High-voltage HBT fabrication
InGaP/GaAs HBT for 28 V operation is
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identical to common 5 V counterpart in its
epitaxial layers except the collector thickness. Increasing collector thickness to 3 m
enables the base-collector breakdown voltage
to go more than 60 V. Likewise, monolithic
microwave integrated circuit (MMIC) for
28 V HBT is achieved with two interconnection metal layers, MIM capacitor, thin-film
resistor, spiral inductor, and through substrate
via holes. The substrate is thinned to 4 mils.
As a result, BVcbo of 70 V is achieved with
BVceo over 30 V. With 28 V as the bias voltage, the collector voltage in RF operation will
swing much above 28 V and exceed the BVceo.
Such condition does not present any concern
to the device operation or its reliability[8].
Power HBT design
Power HBT, like other semiconductor technology, is made of multiple small devices
strung in parallel. Thermal resistance design is
the first task for any power device. Sufficient
spreading of the active HBT fingers across
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Figure 2. The analysis of the third-order intermodulation distortion (IMD3) contributed by the
transconductance non-linearity in a bipolar transistor.
the IC die is done in the conventional MMIC
approach. Bipolar transistor is known to
require ballasting since Vbe has a negative
temperature coefficient.
The individual HBT finger size is balanced between the RF performance and
thermal resistance. Multiple HBT fingers
are linked into the basic building block.
The building block in the present design
delivers around 2 W RF power at 2 GHz.
Each building block has a MMIC prematch
circuit.
The bias circuit for the 28 V power HBT
is implemented on the same chip through the
current mirror approach. Excellent thermal
stability is achieved: less than 9% change
in the quiescent current is achieved over
–40 oC to +85 oC.
RF performance
Linearity improvement: The driver
stage for the power amplifier chain is often
biased toward class A in order to provide the
needed linearity. This approach sacrifices
the operation efficiency. It was found that a
low-frequency low-source impedance matching improves the linearity in near-class B
operation[4,6]. At the dc side of the choke, a
6 µF shunt capacitor is used to provide a low
impedance at the modulation frequency and
5 dB improvement in the IM3 is observed. The
improvement comes from the elimination of
the low-frequency component (ω1-ω2) at the
input, which if existing will mix with ω1 and
ω2 to generate the third-order distortion[6].
Further improvement on the linearity in
class AB or class B operations were achieved
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via a dynamic bias circuit. The major nonlinearity in the bipolar transistor is found to
come from the exponential I-V relationship.
Following the previously reported analysis[11],
HBTs are found to have similar behavior as
LDMOS in the third-order derivative of the
function Ic(Vin) as shown in Figure 2.
The curve of Ic-Vin along the load line follows the exponential relationship with the
ballasting resistor effect. The quiescent bias
point is at point I. For conventional class B
operation, the dc average voltage will remain
at point I regardless of the RF swing. With
the dynamic bias circuit, the time average
bias point is lifted up to conditions II then
III as the input power level is increased. The
RF voltage avoids swinging into the peaking
portion of the gm (Vin) curve as the bias point
is lifted by the dynamic bias circuit. Thus,
improving the linearity in near-class B
operation. Figure 2 shows the analysis of the
third-order intermodulation distortion (IMD3)
contributed by the transconductance nonlinearity in the bipolar transistor.
Memory effect was checked on the singlebuilding block HBT by varying the two-tone
frequency spacing from 1 MHz to 120 MHz.
Figure 3 demonstrates the test results for average power levels from 10 dBm to 30 dBm.
The test was done from 1 MHz to 120 MHz.
The curves are from 10 dBm to 30 dBm of the
average power (PEP is 3 dB higher). The IMD3
is fairly flat across this frequency band.
Figure 4 compares the 1BB power HBT
(with Psat ~ 33 dBm) tested under two-tone
and WCDMA signals at 2.14 GHz with and
without the dynamic bias circuit in the nearclass B bias condition. The improvement in
linearity by the dynamic bias circuit is most
noticeable over the output power range within
15 dB of the P1dB. At 20 dBm average Pout,
the ACLR of WCDMA signal improves over
10 dB. The IM3 improves by 15 dB at 23 dBm
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Figure 3. Two-tone test result of the building block HBT with dynamic bias circuit vs. the frequency separation.
20
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Figure 4. Comparison of linearity with and without the dynamic bias circuit.
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Figure 5. ACLR and efficiency vs. temperature for 2BB size HBT.
output power. At high power levels where
the peak power exceeds Psat, the clipping
of current or voltage waveforms becomes
the dominant non-linear effect and cannot
be improved by the dynamic bias circuit.
Temperature response of the dynamic
bias circuitry: The RF performance of
the 28 V HBT with dynamic bias circuit
scales well with the sizes up to four building blocks (or ~38 dBm saturated power).
The dynamic bias circuit also demonstrates
excellent temperature stability in RF operation
as evaluated over the –40 oC to +85 oC range.
A 2BB size HBT with the dynamic bias circuit
was evaluated over temperature with its
result shown in Figure 5. At ACLR = –50
dBc under the WCDMA signal, the output
power is maintained within 1 dB; the efficiency
at ACLR = –50 dBc is 16% over the same
temperature range.
22
The power HBT with dynamic bias circuit also performs well on other modulation
schemes.
Lifetime, reliability and ruggedness
Several rounds of accelerated lifetime
tests were conducted. At 28 V bias and 0.05
mA/µm2 quiescent bias current, 3000 hour
tests were repeated at a junction temperature
of 310 oC. This level of lifetime is expected
as the 28 V HBT operates at lower current
density than its 5 V counterpart since the current density is one key factor to the lifetime.
The collector sidewall in the 28 V HBT has not
shown any leakage in the accelerated lifetime
test. These tests demonstrate that 28 V HBT
not only maintains the same level of lifetime
as the 5 V InGaP/GaAs HBT[12], but also
is rugged against mismatch, RF power overdrive, and VCC fluctuations.
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The result is shown in Figure 6.
A Gummel plot was taken before and after
the 3000 hours accelerated life test. Figure 6
presents the outcome. Little change can be
noticed down to the nA range.
The power device, under normal usage,
may be subjected to mismatch or overdrive
conditions and it must be rugged enough
to survive such conditions. The ruggedness
is related to the level of ballasting. Under
the presently chosen ballasting level, the
three HBTs of 1BB, 2BB and 4BB were
individually tested for output mismatch. The
input power is held constant at the level
providing output power of P1dB under normal
operation; then the output load is changed
from 50  to 10:1 under all phases. The
test started at VCC = 24 V and went all
the way up to 30 V without any observed
device degradation.
With the peak-to-average ratio (PAR) of
the modulation signal in the 6 dB to 10 dB
range, it is not uncommon to have the peak
power level overdrive the amplifier way
beyond the 1 dB gain compression point.
Therefore, another test was conducted with
the RF overdrive under the normal matching
condition with the collector biased at 28 V.
For all three sizes, 8 dB gain compression
at 2.14 GHz was achieved without damage
or degradation with the sine wave CW
signal.
A small number of samples also underwent
an RF burn-in. The HBT hybrid amplifier
was driven to P1dB level at room temperature for more than 500 hours, and no
change can be noticed. All these experiments
demonstrate that a InGaP/GaAs HBT can
be a rugged, reliable, and linear device for
industrial applications such as a base station
amplifiers.
A 28 V solution
InGaP/GaAs HBTs were designed for the
28 V power application. The semiconductor
device structure and microfabrication procedure were introduced. The 28 V process
borrows many steps from its low-voltage
counterpart. Power transistor design to the
10 W level was discussed. Special attention
to the balance of the phase and magnitude
of the RF signal across all the HBT fingers
is essential to maintain the performance.
High linearity in class AB and near-class B
operation was achieved with the dynamic bias
circuit approach.
The combination of the high linearity RF
performance in the back-off power level, the
ruggedness in RF power overdrive and the
output mismatch condition, and the long
lifetime demonstrated that InGaP/GaAs
HBT technology is mature to serve the
28 V linear power operation in infrastructure
applications.
October 2006
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23
Buckle up for speed
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Shift gears to what’s possible. The
new Agilent MXA has broken all
speed limits with measurement
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The MXA signal analyzer helps
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you keep the lead with exceptional speed and performance.
Figure 6. Gummel plot at room temperature of the HBT before and after more than 3000 hours
accelerated lifetime test.
Stay ahead, find the edge, move
Acknowledgment
it forward.
To see how the new MXA lets you
perform at speeds never before
possible, go to www.agilent.com/
find/possible1. It’s signal and
spectrum analysis at the edge
of possibility.
Agilent MXA Midrange
Signal Analyzer
Marker Peak Search
< 5 ms
W-CDMA ACLR
(0.2 dB, standard
deviation)
� 14 ms
Measurement Mode
Switching
< 75 ms
Analysis Bandwidth
25 MHz
Absolute Amplitude
Accuracy
0.3 dB
W-CDMA ACLR
Dynamic Range
73 dB
u.s. 1-800-829-4444, ext. 5461
canada 1-877-894-4414, ext. 5461
www.agilent.com/find/possible1
The authors acknowledge the fruitful discussions with Walter Strifler and the technical
assistance of Huy Pham and Sindolfo Gacayan. The effort by C. Dunnrowicz in the early
phase of the project is appreciated. RFD
References
1. D. Hill, T.S. Kim, “28 V Low Thermal-Impedance HBT with 20 W CW Output
Power,” IEEE Trans. Microwave Theory and
Tech., vol. MTT-45, No. 12, pp. 2224-2228,
December 1997.
2. P. Kurpas et al, “High-voltage GaAs
Power-HBTs for Base-station Amplifiers,”
2001 IEEE MTT-S Int. Microwave Symposium Digest, pp. 633-636, June 2001.
3. N. L. Wang, C. Dunnrowicz, X. Chen, H.
F. Chau, X. Sun, Y. Chen, B. Lin, I. L. Lo, C.
H. Huang and M.H. T. Yang, “High-efficiency
28 V class AB InGaP/GaAs HBT MMIC Amplifier with Integrated Bias Circuit,” 2003 IEEE
MTT-S International Microwave Symposium
Digest, vol 3, pp, June 2003, pp. 2397-2402.
4. N. L. Wang, C. Dunnrowicz, W. Ma,
X. Chen, H. F. Chau, X. Sun, Y. Chen, B.
Lin, I. L. Lo, C. H. Huang and M.H. T. Yang,
“Linearity Improvement of Multi-Watts 24 V28 V InGaP/GaAs HBT by Low-frequency
Low-source Impedance Matching,” 2004 IEEE
MTT-S International Microwave Symposium
Digest, vol. 3, pp. 1721-1724, June 2004.
5. N.L.Wang, W. Ma, C. Dunnrowicz, X.
Chen, H.F. Chau, X. Sun, Y. Chen, B. Liin, I.L.
Lo, C.H. Huang, M.H.T. Yang, C.P. Lee, “28
V High-efficiency High-linearity InGaP/GaAs
Power HBT,” 2003 PA Workshop 6-2, UCSD.
6. J.H.K. Vuolevi, T. Rahkonen, J.P.A.
Manninen, “Measurement Technique for
Characterizing Memory Effects in RF Power
Amplifier,” IEEE Trans. MTT, vol. 49, No. 8,
pp. 1383-1389, August 2001.
7. N.L.Wang, W. Ma, S Xu, e. Camargo,
XP Sun, P. Hu, Z. Tang, H.F. Chau, A. Chen,
CP Lee, “28 V High-linearity and Rugged InGaP/GaAs Power HBT,” 2006 IEEE
MTT-S International Microwave Symposium
Digest, WE4-B, June 2006.
8. M. Rickelt, H-M Rein, E. Rose, “Influence of Impact-ionization-induced Instabilities on the Maximum Usable Output Voltage
of Si-Bipolar Transistors,” IEEE Trans. ED,
vol. 48, No. 4, pp. 774-779, April 2001.
9. D.E. Dalson et al, “CW measurement of
HBT Thermal Resistance,” IEEE Trans. Elec.
Dev. 39-10, pp. 2235-2239, 1992.
10. K. Goverdhanam, W. Dai, M. Frei, D.
Farrell, J. Bude, H. Safar, M. Mastrapasqua,
T. Bambridge, “Distributed Effects in Highpower RF LDMOS Transistors,” 2005 IEEE
MTT-S International Microwave Symposum
Digest, pp. 455-458, June 2005.
11. J. C. Pedro and N. B. Carvalho, “Intermodulation Distortion in Microwave and
Wireless Circuits,” Artech House 2003.
12. H.F. Chau et al, “Wafer Level Reliability Tests of InGaP HBTs Using High Current
Stress,” 2002 IEEE GaAs Mantech Digest.
ABOUT THE AUTHOR
Nan-Lei Larry Wang received his BSEE
from National Taiwan University, and
MSEE and PhD from UC Berkeley. He has
more than 20 years industry experience on
RF, microwave, millimeter-wave IC and
cellular phone RF transceiver design, which
includes work at Raytheon Research Division, Rockwell International Science Center
and Denso’s cellular phone design center.
He co-founded EiC Corp., which pioneered
the development of high-reliability InGaP/GaAs HBT for wireless infrastructure
base station and handset power amplifiers.
In 2004, the business was merged into
WJ Communications, where he is the vice
president of Advanced Power Devices.
© Agilent Technologies, Inc. 2006
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