First Demonstration of 4H-SiC RF Bipolar Junction Transistors on a Semi-insulating Substrate with fT/fMAX of 7/5.2 GHz Semi-insulating Substrate with fT/fMAX of 7/5.2 GHz (273.06 kB)

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First Demonstration of 4H-SiC RF Bipolar Junction Transistors on a
Semi-insulating Substrate with fT/fMAX of 7/5.2 GHz
Feng Zhao1, 2, Ivan Perez-Wurfl1, Chih-Fang Huang1, John Torvik1, and Bart Van Zeghbroeck1, 2
1
Boulder Advanced Technology Center*, Advanced Power Technology, Boulder, CO, 80301, USA
Department of Electrical and Computer Engineering, University of Colorado, Boulder, CO 80309, USA
2
Abstract — 4H-SiC RF BJTs on a semi-insulating (>105 Ω-cm)
substrate were designed and fabricated for the first time using an
n-p-n triple mesa-etch and interdigitated emitter-base finger
design. On-wafer small signal s-parameter measurements were
performed on a 4-finger device with 3 µm emitter stripe width
and 150 µm finger length. Both, the current gain and unilateral
power gain, were calculated from the measured s-parameters,
yielding an fT of 7 GHz and an fMAX of 5.2 GHz biased in
common-emitter configuration at JE = 10.6 kA/cm2 and VCE = 20
V. These are the highest RF figures of merit reported to date for
any SiC bipolar transistor. The calculated maximum available
power gain (GMAX) is 18.6-dB at 500 MHz and 12.4-dB at 1 GHz,
demonstrating the potential of 4H-SiC BJTs for both UHF and Lband applications.
Index Terms — 4H-SiC, RF BJTs, semi-insulating substrate,
fT, fMAX, GMAX, UHF, L-band.
II. DEVICE DESIGN AND FABRICATION
Emitter
Base
Collector
SiO2 passivation
n-emitter
p-base
Isolation mesa
n-collector
n-buffer layer
Semi-insulating 4H-SiC substrate
Fig. 1. The schematic cross-sectional structure of a 4H-SiC BJT
on a semi-insulating substrate.
I. INTRODUCTION
* Previously PowerSicel, Inc.
0-7803-8846-1/05/$20.00 (C) 2005 IEEE
Emitter pad
Collector
fingers
Collector
pad
Base
pad
Emitter pad
150 µm
Collector
“Bridge”
(Ti/Au)
Collector pad
4H-SiC bipolar junction transistors (BJTs) are promising RF
power devices for operation up to 1 GHz with the ability to
handle large power [1, 2] and to operate at a large collector
voltage [3]. More specifically, compared to its silicon
counterparts, SiC devices can be operated at 10 times the
voltage, for a given drift region thickness, due to the 10 times
larger breakdown field of SiC [4]. The attainable power
density is also higher due to the excellent thermal conductivity
of SiC and its wide energy band-gap. Previously, a 4H-SiC
BJTs was reported with up to 4 GHz fT and up to 1.8 GHz fMAX
[5, 6, 7]. In this work we have improved fMAX almost threefold.
Recently, high purity semi-insulating 4H-SiC wafers were
developed [8, 9] and are now commercially available. Devices
on semi-insulating substrates have been demonstrated [10, 11]
with improved RF performance due to the reduction of
parasitic components. In this paper, we report the first 4H-SiC
RF BJTs fabricated on a semi-insulating substrate with an
fT/fMAX of 7/5.2 GHz, and GMAX of 12.4-dB at 1 GHz and 18.6dB at 500 MHz. These are, to the best of our knowledge, the
highest values published to date for any SiC bipolar transistor.
50 µm
Fig. 2. Micrographs of a 4-finger RF BJT with front-collector“bridge” contacts and on-wafer RF pad layout.
The epitaxial structures (n-p-n) were grown by Acreo AB
5
(Sweden) on a 2-inch 4H-SiC semi-insulating substrate (>10
Ω-cm). The nominal thickness and doping density of each epilayer are listed in Table I. Bipolar transistors were fabricated
using a triple mesa-etch and an interdigitated emitter-base
finger structure. Each device was completely isolated by
etching the isolation mesa into the semi-insulating substrate. A
cross-sectional drawing of a single finger structure is shown in
2
Fig. 1. The emitter mesa area is 3×150 µm and the base mesa
2
area is 11×150 µm . The surface was passivated with a thin
layer of thermal oxide followed by a layer of deposited oxide.
Ni/Cr was used for the emitter and collector contacts, Ti/Al
for the base contacts, and Ti/Au for the wiring and pads as
well as the front-collector-“bridge” contacts as shown in Fig.
2. The fabrication process has been discussed in detail
elsewhere in the literature [12].
TABLE I
NOMINAL DOPING DENSITY AND THICKNESS OF EPI-LAYERS
Wafer (S.I.)
n-contact
n-emitter
p-base
n-collector
n-buffer layer
Substrate
Thickness
40 nm
100 nm
140 nm
1000 nm
700 nm
300 µm
Doping
19
-3
9×10 cm
19
-3
3×10 cm
18
-3
8×10 cm
15
-3
8×10 cm
19
-3
1×10 cm
18
-3
~10 cm
Dopant
Nitrogen
Nitrogen
Aluminum
Nitrogen
Nitrogen
Vanadium
h21 =
U=
III. RESULTS AND DISCUSSION
A DC characterization for the common-emitter
configuration was performed to qualify the RF transistors as
well as to identify the proper DC bias points for the small
signal measurements. A typical I-V is illustrated in Fig. 3. The
maximum DC current gain βmax is 11 and decreases at higher
bias due to the self-heating. The maximum emitter current
2
density JE is 10.1 kA/cm at VCE = 20 V with a corresponding
2
DC power dissipation of 200 kW/cm normalized to the
emitter mesa area. The breakdown voltage is greater than 100
V in spite of the 1 µm collector drift region.
250
IC (mA)
IB =20 mA
150
100
2 mA/step
50
IB =2 mA
0
0
5
10
15
VCE (V)
20
2 ⋅ s 21
s12 ⋅ s 21 + (1 − s11 ) ⋅ (1 + s 22 )
s 21 / s12 − 1
(1)
2
(2)
2 ⋅ k s 21 / s12 − 2 ⋅ Re( s 21 / s12 )
(3)
GMAX = s 21 / s12 ⋅ (k − k 2 − 1)
The DC current-voltage (I-V) properties of the transistors
shown in Fig. 2 were measured with an HP 4155C. The onwafer small signal RF measurements were performed using an
Agilent E5071B network analyzer with GSG probes. All
measurements were done at room temperature. The network
analyzer was calibrated using Short-Open-Load-Thru (SOLT)
standards. The s-parameters were measured from 8 MHz to 8
GHz at a collector-emitter voltage (VCE) of 20 V and at
multiple emitter bias current densities (JE).
200
The high frequency performance is characterized by onwafer small signal s-parameter measurements with the
remaining parasitic components de-embedded based on a
widely used correction procedure [13]. The AC common
emitter current gain |h21|, the unilateral power gain U, and the
maximum available power gain GMAX were calculated from
measured s-parameters using the following formulae [14, 15]:
25
Fig. 3. I-V characteristics of a 4-finger RF transistor. IB = 2, 4, …,
18 and 20 mA.
2
k=
2
1 − s11 − s 22 + s11 ⋅ s 22 − s12 ⋅ s 21
2 s12 ⋅ s 21
2
(4)
Where k is the Rollett stability factor. The de-embedded
frequency dependence of |h21|, U and GMAX from a 4-finger 4H2
SiC transistor biased at VCE = 20 V and JE = 10.6 kA/cm are
presented in Fig. 4. fT was extrapolated from the fitted line of
|h21|. fMAX was obtained from U and GMAX at the frequency
where the gain has decreased to 0-dB. The values of fT and fMAX
are 7 GHz and 5.2 GHz respectively, the highest numbers
reported to date for any SiC bipolar transistor. The
improvement of fT and fMAX is due to the reduction of parasitic
components achieved by the use of the semi-insulating
substrate. A more detailed discussion of this can be found
elsewhere [12]. The maximum available power gain GMAX was
calculated to be 12.4-dB at 1 GHz and 18.6-dB at 500 MHz,
showing the potential of SiC RF bipolar devices for UHF and
L-band applications such as radar, broadcast and wireless
communication.
It is worth noticing that the calculated U and GMAX follow the
expected 20-dB/decade slope, however, the slope of the fitline to |h21| is not 20-dB/decade, but 14-dB/decade. The latter
is explained by the back-injection current flowing from base
to emitter in homojunction bipolar transistors, resulting in the
small signal emitter injection efficiency significantly lower
than one [14]. With the increase of back-injection effects, the
slope of |h21| versus frequency decreases from 20-dB/decade to
10-dB/decade.
Current Gain |h 21| (dB)
30
25
Fit-line
The extracted fT and fMAX from the measured s-parameters of
two 4-finger RF transistors biased at various emitter current
densities are summarized in Fig. 5, with the calculated results
based on an ideal small signal transit-time model [16]:
20-dB/dec
20
1
= τ EC = τ E + τ B + τ SC + τ C
2πf T
15
10
VCE = 20 V
JE = 10.6 kA/cm2
5
=
fT =7 GHz
VT (C j , BE + C j , BC )
JE
0
0.01
0.1
1
Frequency (GHz)
Fit-line (20-dB/dec)
20
fMAX =5.2 GHz
VCE = 20 V
JE = 10.6 kA/cm2
0
0.1
1
Frequency (GHz)
10
2 µ n , BVT
+
x dep , BC
2v sat
+ RC C j , BC
fT
8πR B C j , BC
(5)
(6)
8
fT
6
4
fMAX
2
0
100
(b)
Maximum Available Gain
G M AX (dB)
2
10
Frequency (GHz)
Unilateral Power Gain
U (dB)
40
10
wB'
Where Cj,BE and Cj,BC are the base-emitter and base-collector
junction capacitance. The electron mobility in the base region,
µn,B, is 185 cm2/V-s [12] and the saturation velocity vsat is 2×107
cm/s [17].
(a)
30
f MAX =
10
+
1,000
10,000
100,000
2
Emitter Current Density (A/cm )
20
16
18.6-dB@500 MHz
Fig. 5. fT and fMAX extrapolated from s-parameter measurements at
different JE for two 4-finger RF transistors (open and filled symbols)
at two different locations on the wafer. Solid lines represent the
calculated values of fT and fMAX using the transit-time model.
Fit-line (20-dB/dec)
12
12.4-dB@1 GHz
8
IV. CONCLUSION
VCE = 20 V
JE = 10.6 kA/cm2
4
fMAX =5.2 GHz
0
0.1
1
Frequency (GHz)
10
(c)
Fig. 4. (a) Current gain |h21|; (b) unilateral power gain U; and (c)
maximum available power gain GMAX calculated from the measured sparameters versus frequency of a 4-finger 4H-SiC BJT.
4H-SiC RF BJTs on a semi-insulating substrate were
designed, fabricated and tested for the first time. On-wafer
small signal s-parameter measurements show a 7 GHz fT and a
5.2 GHz fMAX. With the improvement of fT and fMAX, the
maximum available power gain GMAX is 12.4-dB at 1 GHz and
18.6-dB at 500 MHz, showing the potential of these devices
for UHF and L-band applications.
ACKNOWLEDGEMENT
This work was funded in part by a NIST/ATP grant.
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