GaN-based MESFETs and DC-MOSFETs
R. Gaska
Sensor Electronic Technology, Inc., 21 Cavalier Way, Latham, NY 121 10
M. Asif Khan, X. Hu, G. Simin, and J. Yang
Department of ECE, University of South Carolina, Columbia, SC 29208
J. Deng, S. Rumyantsev, and M. S. Shur
CIEEM and Department of ECSE, Rensselaer Polytechnic Institute, Troy, NY 12180
We present experimental results, which show that GaN MESFET and MOSFET
technology can demonstrate the performance comparable to that of GaN/AlGaN HFETs for
highly doped narrow channel devices.
The device structures were grown by low-pressure MOCVD over (0001) sapphire
substrates. Growth of approximately 1.5 pm thick semi-insulating GaN was followed by
deposition of 0.1-0.2 pn Si-doped n-GaN. The Hall mobility in the samples was 85-100 cm2Ns,
electron concentration varied from 2xlOI7 cm-3 to 6~10' ~ cm3. PECVD deposited 10-15 nm thick
Si02 was used as an insulating layer for doped channel GaN-based MOSFETs. The devices with
source-drain spacing of 5 pm and gate length of 2 pn were fabricated. The threshold voltage for
MESFETs and DC-MOSFETs ranged from 1.5 V to 10 V, and from 4 V to 20 V, respectively.
The maximum drain currents up to 300 d m m and transconductances up to 60 mS/mm were
measured for 100 pm wide devices. The Schottky gate turn-on voltage for MESFET devices was
close to 1 V, which is approximately two times lower than for AlGaN/GaN HEMTs. The gate
leakage current in DC-MOSFETs was more than three orders of magnitude lower than in
MESFETs.
The long-channel GaN MESFETs that we fabricated exhibited the cutoff frequency times
the gate length product of 1 1.6 GHz-micron. This number is comparable with the 16.4 GHz-
micron value demonstrated recently for AlGaN/GaN MOS-HFETs on Si c substrates and 18.2
GHz-micron demonstrated for AlGaN/GaN HFETs on sapphire substrates. The cutoff frequency
improves with increasing the channel doping. Our model based on the parameters extracted from
the fabricated devices predicts that shorter and thinner channel, higher doped devices should
yield the values of the transconductances and of the maximum current comparable to those
demonstrated for AlGaN/GaN HFETs and AlGaN/GaN MOS-HFETs.
Recent results for AlGaN/GaN MOS-HFETs demonstrated a good quality of SiOZ/AlGaN
interface. We now show that the quality of the GaN-Si02 interface is also sufficiently good for
demonstrating GaN DC-MOSFETs with a reasonable performance. For these devices, we
measured the cutoff frequency-gate length products up to 6.6 GHz-micron. However, these
devices exhibit more pronounced hysterisis and other trapping effects, especially at high applied
biases. Their advantage compared to GaN MESFETs is an extremely low gate leakage current,
which is of the same order as for AlGaN/GaN MOS-HFETs.
We will also present results on the comparison of the noise and switching performance of
GaN MESFETs
In conclusion, our experimental results and model predictions show that GaN MESFETs
and GaN DC-MOSFETs might find applications for power devices in X-band and above.
43
1000 7 4
800
600
400
200
0 -
-
-
-
-
0 2 4 6 8 1 0
Voltage (V)
Fig. 1 . C-V characteristics of three epilayers
( 1) Doped channel 0.1 pm, n=2x1 0i7cm-3
(2) Doped channel 0.2pm, n=S~l O ~cm-~
Doped channel 0.2pm, n=8x10i7cm
h 10
N
-10 -8 -6 -4 -2 0
Thr eshol d Vol t age ( V )
f,, and ft of GaN MESFETs versus threshold
voltage
A C
I E 20
-0 2 4 6 8 10
Vds ( V )
GaN MESFET I-V characteristics.
Top curve for gate bias 1V. Step - 1V.
12 r
L
11 -
10 -
- 9 -
N .
I 8 -
h e -
o . s 5 -
a ,
* 7 :
C I .
a 4 -
g 3 :
L 2 -
0 10 20 30 40 50
Current ( mA )
-4 of MESFET
fmax of MESFET
ft of MISFET
fmax of MISFET
f,, and ft versus drain current
44
