SPICE-aided Modelling of SiC MESFETs
Janusz Zar^bski, Damian Bisewski Gdynia Maritime University, Department of Marine Electronics, Poland
Key words: SiC MESFETs, modelling, SPICE.
Abstract; In the paper the d.c. characteristics of the SIC MESFET operating in the wide temperature range are investigated. The transistor CRF24010 offered by Cree Inc. is considered. The characteristics obtained from measurements and SPICE simulations performed with the use of Raytheon-Statz model are compared.
Modeliranje SiC MESFET transistorjev s programom SPICE
Kjučne besede: SiC MESFET tranzistorji, modeliranje, SPICE
Izvleček: V prispevku raziščemo d.c. karakteristike SIC MESFET tranzistorja v širokem območju temperatur. Merili smo tranzistor CRF 24010 firme Cree Inc. in primerjali izmerjene vrednosti s tistimi, ki smo jih dobili s simulacijami s programom SPICE in Raytheon-Statz modelom.
1.	Introduction
MESFETs are very popular high frequency devices (RF transistors) which have found applications in radiocommuni-cation circuits, as: amplifiers, mixers, oscillators, etc. Commonly used MESFETs made of gallium arsenide (GaAs) are known since 1968. In 1998 the first MESFET made of silicon carbide (SIC MESFET) was worked out in Cree Labs., whereas since 2002 such devices have been commercially available /1 /.
Computer-aided design of the circuits mentioned above, requires the credible, experimentally verified models ofthe considered devices, acceptable by proper computertools, as e.g. SPICE /2/. The built-in SPICE models of a MESFET have been worked out for GaAs devices.
In the paper the usefulness of the built-in Raytheon-Statz model for describing SIC MESFETs is investigated. The estimation of accuracy of this model is performed by the comparison of the measured and simulated device characteristics. The transistor CRF24010 offered by Cree Inc. /3/ was chosen for investigations. In the Raytheon-Statz based simulations the values of the model parameters were obtained from measurements.
2.	The Raytheon-Statz model
The network form of the Raytheon-Statz model is presented in Fig. 1 /2/.
The main device current Id is of the form /2/: in the cut-off region (ugs-VTO<0):
ldrain = 0	(D
in the linear and the saturation regions {ugs-VTO>0): Idrain = BETA •(! +LAMBDA-Uds) A	(2)
where: ugs - the gate-source voltage, uds - the drain-source voltage, VTO - the pinchoff voltage, BETA - the transconductance coefficient, LAMBDA - the channel-length modulation coefficient, whereas the parameter A is given by /2/:
Kt
A = (uGs-VTOf
1 + B-(ugs-VT0)
(3)
where: B - the doping tail extending parameter. In turn, the parameter Kt is given by the formula /2/: in the linear region:
3
0<uds<
ALPHA
K, =1-
1-u
ALPHA
\3
DS
in the saturation region:
Uds
ALPHA
Kt=l
where: ALPHA - the saturation voltage parameter.
D'
RD
Uds
(4)
(5)
(6) (7)
Fig. 1. The network form of the Raytheon-Statz model
The currents of the Schottky diodes (D? and Ds in Fig. 1) are described by the ideal components only, expressed as follows /2/:
iGs=IS'
'GD
= IS-
exp
exp
u
GS
N-Vt
"GD
N-Vt
-1
-1
(8)
(9)
where: IS - the saturation current, N - the emission coefficient, Vt - the thermal potential.
In the model, the parameters: BETA, IS and VTO are dependent on the temperature T. The thermal dependence of BETA(T) and VTO(T) are given by /2/:
BETA (T) = BETA ■ i,o	.(x-xnom) (^ q)
VTO(T) = VTO + VTOTC-(T-Tnom) (11)
where: Tnom - the nominal temperature, VTOTC - the pin-choff voltage temperature coefficient, BETATCE - the transconductance exponential coefficient.
The thermal dependence of the saturation current IS(T) is given by/2/:
IS(T) = ISexp
T
TNOM

EG(T)
N-Vt
XTI
T
TNOM j
(12)
where: XTI - the temperature exponent. The temperature dependence of the band-gap voltage (barrier height) is written in the formula /2/:
EG(T) = 1,16-
0,000702-T^ T-M108
(13)
As it is seen, in the model under consideration, the temperature dependence of the band-gap energy Eq (13) is given just for the silicon (Si), therefore in the case of modelling transistors made of other semiconductors, e.g. gallium arsenide (GaAs), silicon carbide (SIC), Eq (13) is inaccurate.
3. The investigation results
To verify the Raytheon-Statz model, the current-voltage characteristics of CRF24010-101 SIC transistor in the wide temperature range were measured. The catalog admissible values of the parameters of the investigated device are given in Table 1 /3/.
TABLE 1. The catalog parameter values of CRF24010-101 transistor
Parameter	Catalog value
UdS max	120 VDC
Uqs max	-20, -F3 VDC
P ^ max	low
fmax	2,7 GHz
T' wax	250 °C
The measured and simulated characteristics are shown in Figs. 2-7, where the points joined by the broken lines and the solid lines denote the measured and calculated results, respectively. The values of the model parameters are given in Table 2.
TABLE 2. The Raytheon-Statz model parameter values of CRF24010-101 transistor
Parameter	Value
VTO	-9,42 V
BETA	0,0136 AA^^
LAMBDA	6,53 V'
ALPHA	0,136 V'
B	0,034 V"'
VTOTC	-l,16mV/°C
BETATCE	0,02 %/ °C
IS	7E-14A
N	1,25
In Fig. 2 and 3 the transfer characteristics at Uds=5V in the temperature range 295-433K (Fig, 2) as well as at T=295K for two values of the drain-source voltage (Fig. 3) are presented. As seen, the value of the threshold voltage decreases with the increase of the temperature and the drain-source voltage. For ugs<VTO the main current of the MESFET is of a relatively high value due to the current flowing through the Schottky diodes.
-9,7 -9,5 -9,3 -9,1 -8 9 "GS [V]
Fig. 2. The transfer characteristics at Uds=5V
-10,8 -10,4 -10 -9,6 -9,2 uos [V]
Fig. 3. The transfer characteristics at T=295K
The temperature dependence of the threshold voltage Vto(T) at two values of the drain-source voltage is presented in Fig. 4. As seen, these characteristics are quasi-line-ar and the temperature coefficients of changing Vto are: VTOTC=-1,16mV/K at Uds=5V and VTOTC=-0,68mV/K at Uds=20V.
-9,3
-9,4 -
>
b -9,5 H O
H >
-9,6 -
-9,7
1 CRF24010 1	
.VTOTC=-0,68mV/K	
■	tJ^^V
	■
■	
VT0TC=-l,16mV/K ^	Uds=5V
	
-10,3
-10,7
290
340
T[K]
390
440
Fig. 4. The temperature dependence of the threshold voltage VTO(T)
In Fig. 5 the output characteristics in the range of the drain-source voltage up to 45V at the ambient temperature T=295K for two gate-source voltage values Ugs: -9V and -11V are shown. It is visible, that the acceptable agreement between simulation and measured results is observed at Ugs=-9V only. The value of the drain current corresponding to the characteristic measured at Ugs=-1 1V increases strongly, what probably results from influence of the drain-source voltage on the transistor threshold voltage (see Fig. 3).
10'
10"-
C
10"
10-"
Ur,c,=-9V
CRF24010
\ Ugs,=-11V
T=295K
10
20	30	40
UDS [V]
Fig. 5. The output characteristics at Ugs: -9Vand -11V
In turn, in Fig. 6 the qualitative discrepancy between measurements and simulations in the avalanche range of the investigated device are observed. It should be noted, that at the point B (Uds=110V) the transistor was damaged, in spite of that its operation point was inside SOA.
20 16 ^
CRF24010
T-297K
=L
12 ^ 8 ^ 4 -0
Ur,..=-20V
■ Calculations
•-t—
30

60
Uds [V]
90
120
Fig. 6. The output characteristics at T=297K
In Fig. 7 the current-voltage characteristics of the Schottky diode (Di) operating at the forward bias (Fig. 7a) and at the reverse bias (Fig. 7b), corresponding to five temperature values are presented.
0,4
0,5
0,6	0,7
UGS [V]
0,8
0,9
10-
10"
10"^
<
^ 10-'
10"^
10"
T=295K
-j352K|-
^391K
HÄ}-=—--
--^
T=295K __________- ■ M'F
CRF24010
-25	-20	-15	-10	-5	0
UGS [V]
b)
Fig. 7. The characteristics of the Schottky diode Di
As seen, the model fits well to measurements at the forward bias of the diode, whereas unacceptable discrepancies between theoretical and experimental results (differences higher then even 12 orders) occur at the reverse bias of the diode.
4. Conclusion
In the paper the usefulness of the Raytheon-Statz model for describing the SIC MESFET was estimated for the first time. The estimation of accuracy of the Raytheon-Statz model of the SiC MESFET (Cree Inc.) was performed by comparison of the measured and simulated characteristics in the wide range of the temperature.
The considered model takes into account the thermal dependences of: the pinchoff voltage, the saturation current and the band-gap energy. On the other hand, as seen from the investigation results (Figs. 2-7) to get the better agreement between measured and calculated characteristics, the dependencies: Vto(Uds), Ubr(Ugs,T) as well as the Schottky barrier lowering effect existing in the reverse biased diodes should be included in the considered model, which afterward could be implemented to SPICE as a sub-circuit.
References
/1/ Östling M., "Silicon Carbide Devices for High Frequency and Higin Power - A State of tine Art View", RadioVetenskap och Kommu-nil<ation RVK, Linl^öping, 2005.
/2/ "PSPICE A/D Reference Guide Version 10.0", Cadence Design Systems Inc., June 2003.
/3/ http ://www. cree, com/products/pdf/crf24010. pdf.
Prof. Janusz ZarQbski Gdynia Maritime University Department of Marine Eiectronics Morsl<a 83, 81-225 Gdynia, POLAND, Tel. ++48 58 6901599, fax ++48 58 6217353 E-mail: zarebsl<l@am.gdynia.pl
Prispelo (Arrived): 11.12.2006 Sprejeto (Accepted): 15.06.2007