High Frequency Power Metamorphic HEMT
C. S. Whelan, K. Herrick, J. Laroche, K. W. Brown, F. Rose, Y. Zhang, P. Balas,
R. E. Leoni III, W. E. Hoke, S. Lichwala, J. Kotce, T. E. Kazior
Raytheon RF Components
362 Lowell St. Andover, MA 01810, cwhelan@raytheon.com
Abstract - By tailoring the device’s material, geometry
and processing, our device designers have fabricated a
state-of-the-art high frequency Metamorphic HEMT
device with a Gmax of 12 dB, a power density of 360
mW/mm, and PAEs exceeding 30% at 95 GHz. This
device had been utilized to create a range of W-band
amplifiers with excellent performance, including a 266
mW PA operating at 90 GHz.
I. INTRODUCTION
12000
1.2
We have successfully exploited the freedom of
metamorphic device tailoring with our 4”, 60% indium,
low noise device technology that is currently in
production. This paper describes a new pursuit and the
results obtained by optimizing the material and
processing to address the need for low cost, high gain,
power amplifiers at W-band.
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1.0
II. DEVICE AND PROCESSING
8000
0.8
6000
0.6
4000
InP
Devices
GaAs
Devices
0.4
Strained Delta Ec (eV)
2/V-s)
300K Mob. of Electrons (cm
(cm2/V.s)
GaAs based metamorphic HEMT (MHEMT)
technology has emerged as an attractive, low cost
alternative to InP HEMTs. The strain-induced
imperfections caused by high indium content layers on
GaAs are eliminated in metamorphic devices by
providing a properly grown lattice-grading buffer
between the substrate and active device layers. With
this limitation overcome, it is now possible to provide
the superior performance of InP-based devices with the
cost advantages of highly manufacturable 4-inch GaAs
wafers that can easily be integrated on existing GaAs
fabrication lines [1]-[20].
20% to 85% and measured such properties as band gap,
mobilities, Ft, NF, gain, PAE, power densities and onand off-state breakdown voltages. Figure 1 shows that
the measured mobility of channel electrons increases
with increasing channel In content, due mainly to the
reduction in electron effective mass and reduced
intervalley scattering (increasing EL-EGamma). These
improvements give rise to higher channel velocities,
improving Ft. By straining the Schottky layer (with
higher Al contents than lattice matched to InxGa1-xAs),
one can achieve improve confinement further, reducing
NF and improving maximum current density (Figure 1).
This collection of data has formed the basis to optimize
a metamorphic GaAs HEMT for nearly any frequency
and application.
0.2
2000
MHEMTs
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Indium Content Channel [In(x)Ga(1-x)As]
Figure 1. As indium is added to the channel, both the mobility
and well depth increase (when a strained InAlAs layer is
used).
We have fabricated metamorphic HEMT devices with
channel indium contents (InxGa1-xAs) ranging from
The MHEMT devices are mesa etched for isolation
using a sulfuric based etchant. A series of metals
containing Au/Ge are evaporated and annealed to form
an ohmic contact, with contact resistance of
approximately 0.17 Ohm-mm.
Following ohmic
formation, first recess and gate etching are performed
selectively using a self-aligned gate method. 0.15
micron Ti/Pt/Au gates with wide T-tops (to reduce gate
resistance) are then evaporated. Finally, silicon nitride
is used to passivate the devices. The devices are
thinned to 50 microns before being plated with gold.
The devices employ individual source vias to reduce
source inductance and improve thermal dissipation
from the channel of the device.
The DC performance data for our new power MHEMT
2 fingers
4 fingers
6 fingers
8 fingers
2 fingers
4 fingers
6 fingers
8 fingers
10 fingers
13.0
2 fingers
4 fingers
6 fingers
8 fingers
10 fingers
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
2 fingers
4 fingers
6 fingers
8 fingers
10 fingers
bigger peripheries with increasing gate fingers, the
devices’ Gmax only degrades by 1.5 dB. Due to
optimized channel geometries, the devices lose virtually
no gain as the unit gate width is increased from 37.5
microns to 60 microns. Even very large 8 finger x 60
micron unit gate width devices (480 micron total
periphery) show 11 dB of maximum available gain at
94 GHz. This ability to scale is very promising for
building large W-band power amplifiers with high gain
per stage and efficiencies.
12.0
Gmax (dB)
31
01
31 4 2
01
31 4 4
07
31 0 2
07
0
31 4
09
31 7 2
09
31 7 4
18
0
31 2
18
40 0 4
05
40 6 2
05
40 6 4
10
6
40 4
15
40 5 2
15
40 5 4
34
72
Standard Deviation (V)
device shows a Gm of 850 mS/mm, an Imax of 700
mA/mm, a Vpo of –0.8V and a Vdg BRK = 8V. The
excellent uniformity of approximately 50 mV standard
deviation in pinch off voltage across nearly (30) 4”
wafers is due to both the high selectivity of the gate
etch and the precision of the MBE growth process
(Figure 2).
Lot and Wafer #
Figure 2. An average standard deviation of 50 mV is
achieved across nearly (30) 4” wafers.
11.0
10.0
9.0
37..5
37.5 um
um
40 um
50 um
60 um
8.0
7.0
The materials layers of the Schottky and
channel are carefully selected, with strain
considerations, to optimized the high frequency
performance of the devices, while maximizing on-state
breakdown voltage.
By exploiting metamorphic
growth, the lattice constant of the device needs not be
that of InP, thus giving the device designer a large
degree of freedom in optimizing each material layer
individually.
6.0
5.0
Figure 4. Gmax of power MHEMT devices biased at 2.0V at
94 GHz.
III. W-BAND AMPLIFIERS
Using our new power MHEMT material and
device processing, we achieved state-of-the-art W-band
power results. Single stage W-band pre-matched FETs
with 150 microns of periphery were used to
characterize the process and as building blocks for
amplifier design.
Pulsed IV data is overlaid upon static IV
curves in Figure 3, and shows no current collapse, a
necessity for achieving good power and efficiency. The
device also demonstrates an Imax of 700 mA/mm and
pulsed operation of up to 6V at 200 mA/mm. The high
on-state breakdown (for a high indium content device)
is achieved through channel recess optimization and
material management to reduce impact ionization.
S-parameters of these single stage pre-matched
devices were taken on our 110 GHz ANA at 2.5V and
200 mA/mm and are shown in Figure 5. This 0.15
micron gate length power device shows 12 dB of
measured gain at 104 GHz from a single stage.
Static Vs Pulsed IV curves
TS236-19 C031014-02 4x37.5um Test FET, Bias: Vd = 2.5V, Vg=-2V
Pulsed is Yellow
M124-1A CKT 3 (2.5V 30 mA)
120
15
100
10
60
40
20
0
0
1
2
3
4
5
6
7
S-parameters (dB)
Ids (mA)
80
5
0
-5
Vds (Volts)
Figure 3. Pulsed IV overlaid upon static IV curves.
-10
Figure 4 shows a Gmax at 94 GHz of various
geometry power MHEMT devices operating at 2V and
approximately 300 mA/mm. As the devices scale to
-15
90
95
100
105
110
Frequency (GHz)
Figure 5. S-parameters of a single stage pre-matched device.
300
25
250
20
200
15
150
10
100
5
50
0
Pout (mW)
Pout(dBm), Gain(dB), Compression(dB), PAE(%)
These same power matched devices were then
measured for power at 95 GHz, as shown in Figure 6a.
The 150 micron device shows a Psat of 17.5 dBm with
27% power added efficiency and 5 dB of compressed
gain (2.5 dB of compression). This equates to 360
mW/mm. This same circuit was biased for maximum
PAE and produced 16 dBm of power and 32% PAE
(Figure 6b). Load pull results of the same 150 micron
device itself have yielded in excess of 35% PAE.
30
0
0
2
4
6
8
10
12
14
16
18
20
Pin (dBm)
M67-4 Power MHEMT 150um Pre-matched Single Stage
Figure 7. Power, gain and efficiency of a single stage, 90 GHz
power amplifier.
30
Pout-c
Gain
Eff
25
V. CONCLUSION
20
15
A new power MHEMT process developed
specifically for high frequency performance shows
more than 12 dB of gain per stage at 100 GHz, 360
mW/mm power density and 30% power added
efficiency at 95 GHz.
10
5
0
-5
-10
-15
-10
-5
0
5
10
15
Pin (dBm)
6a.
M67-4 Power MHEMT 150um Pre-matched Single Stage
35
30
25
20
15
10
5
0
-5
-15
-10
-5
0
Pin (dBm)
5
10
15
6b.
Figure 6a & b. Power, gain and efficiency of a 150 micron
device measured at 95 GHz tuned for maximum power (a)
and maximum PAE (b).
These single stage building blocks were used
to create a large, single stage W-band power amplifier.
Operating at a Vds of 3.3V, the single stage PA
demonstrated 6.5 dB of small signal gain, 17% PAE
and 266 mW at 1 dB compression, all at 90 GHz
(Figure 7).
VI. REFERENCES
[1] “Raytheon, OMMIC launch new MHEMT processes,”
Compound Semiconductor, page 11, Oct. 2001.
[2] W.E. Hoke, et al., JVST B 17, p. 1131 (1999).
[3] D. Lubyshev, et al., JVST B 19, p. 1510, 2001.
[4] M. Chertouk et al, IEEE EDL, vol. 17, no. 6, p. 273-275,
1996.
[5] M. Kawano et al, IEEE Microwave and Guided Wave
Letters, Vol. 7, No. 1, pp. 6-8, 1997.
[6] S. Halder, et al., Proc. of 2001 Intern. Micro. Symp., p.
1885.
[7] W. K. Liu, 2001 InP and Related Materials Conf. Proc.,
p. 284.
[8] W.E. Hoke, et al., JVST B 19, p. 1505, 2001.
[9] P.M. Smith, et al., Proc. of 2001 GaAs IC Symp., p. 7.
[10] A. Cappy, et al., InP and Related Materials Conf. Proc.,
p. 192.
[11] P.F. Marsh et al., Proc. of 2001 GaAs REL Workshop, p.
119.
[12] M. Chertouk, et al., 2000 GaAs MANTECH Conf. Proc.,
p. 233.
[13] C.S. Whelan, et al, 2000 InP and Related Materials
Conf. Proc.
[14] C.S. Whelan, et al., 2001 CS-MAX Conf. Proc.
[15] P. Balas, et al, 2002 CS-MAX Conf. Proc.
[16] R. Leoni, et al, 2002 GaAs MANTECH Proc.
[17] C. S. Whelan, et al, 2002 LEOS Conf. Proc.
[18] P. Marsh, et al, 2003 TWHM Conf. Proc.
[19] R. E. Leoni III, et al, 2003 GOMAC Conf. Proc.
[20] K. Herrick, et al, 2003 MTT Conf Proc.
Pout-c
Gain
Comp
Eff
mW