Until now, SmartSpice has supported the TriQuint model designated TOM using the Level=5 model parameter. SmartSpice now supports the TriQuint-2 MESFET model designated TOM-2. This model is accessible as a standard MESFET model, using the Level=7 model parameter. It is an incremental improvement based on TriQuint?s original MESFET model. All current analyses are currently supported (dc, ac, transient and noise).

Physical Effects

Refinements to improve accuracy in the knee and subthreshold region have been added. Particular attention has been given to temperature effects. TOM-2 retains the desirable features of TOM-1 while improving performance in the subthreshold characteristics near the cut-off and knee regions. Additional temperature coefficients are included relating to the drain current and major deficiencies in the behavior of the capacitance as a function of temperature have been corrected.

Simulation Model

The parameters of the TOM-2 MESFET model are represented in Table 1.

Parameter	Description	Units	Default	Area
BETA	Transconductance parameter	A/V2	0.1	( W e f f / Leff). M
VTO	Quadratic equation gate threshold voltage	V	-2.5	-
VTO	Quadratic equation gate threshold voltage	V	-2.5	-
ALPHA	Coefficient of vds	1/V	2.0	-
GAMMA (GAMDS)	Static feedback parameter	-	0	-
DELTA	Output feedback parameter	1/(A.V)	0.2	-
Q (VGEXP)	Power law parameter	-	2.0	-
VBI	Gate diode built-in potential	V	1.0	-
VMAX	Gate diode capacitance limiting voltage	V	0.95	-
LAMBDA	Channel length modulation parameter	1/V	0	-
K1	Bulk bias sensitivity for thresh-old voltage	-	0	-
VBITC	Temperature coefficient for VBI	1/K	0	-
ALPHATCE	Temperature coefficient for ALPHA	1/K	0	-
GAMMATC	Temperature coefficient for GAMMA	1/K	0	-

Table 1. Parameters of the TOM-2 TriQuint MESFET model.

The model parameters BETA, VTO, ALPHA, GAMMA, DELTA, Q are common for both the original and improved TriQuint models. An important aspect of modeling the MESFET is a correct description of the behavior as a function of temperature.

Figure 1. Ids versus Vds for different Vgs voltages.

The basic equations including the current source equations and the capacitance equations can be found in [1]. The essentials of the subthreshold model are described in [2] where the formulae are used to describe the CMOS subthreshold behavior.

Figure 2. Ids versus Vgs for Vds=1V for different temperatures.

DC Results

Tests have been performed for a variety of temperatures and device widths to check proper scaling and temperature dependencies. Drain-source current Ids, gate-drain and gate-source capacitances respectively Cgd and Cgs (CAPMOD and CAPDC=1) are represented versus bias for the depletion mode. A comparison between TOM-1 and TOM-2 dc characteristics (Ids-Vds) are plotted in Figure 1. The drain current is plotted versus Vgs for different temperatures in Figure 2.

Figure 3. Cgs versus Vds for different Vgs.

Improved accuracy in the knee region is observed in the TOM-2 model characteristic. Similar results can be found in reference [3].

Figure 4. Cgd versus Vds for different Vgs..

Reference

SmartSpice/UTMOST Modeling Manual Volume 2.
M. Godfrey "CMOS device modeling for subthreshold circuits" IEEE Transactions on Circuits and Systems II : Analog and Digital Signal Processing, 39(8), August 1992.
N. SCHEINBERG, R. BAYRUNS, P. WALLACE "An accurate MESFET Model for linear and microwave circuit design" IEEE Journal of Solid-State Circuits, vol. 24, n. 2, April 1989.