Understanding the Capacitor Model

The SPICE depletion capacitor model (CAPOP=0) uses a diode-like capacitance between source and gate, where the depletion region thickness (and therefore the capacitance) is determined by the gate-to-source voltage. A similar diode model is often used to describe the normally much smaller gate-to-drain capacitance.

These approximations have serious shortcomings such as:

1. Zero source-to-drain voltage: The symmetry of the FET physics gives the
conclusion that the gate-to-source and gate-to-drain capacitances should be
equal, but in fact they can be very different.

2. Inverse-biased transistor: Where the drain acts like the source and the
source acts like the drain. According to the model, the large capacitance
should be between the original source and gate; but in this circumstance,
the large capacitance is between the original drain and gate.

When low source-to-drain voltages inverse biased transistors are involved, large errors can be introduced into simulations. To overcome these limitations, use the Statz charge-conserving model by selecting model parameter CAPOP=1. The model selected by CAPOP=2 contains further improvements.

Model Applications

MESFETs are used to model GaAs transistors for high speed applications. Using MESFET models, transimpedance amplifiers for fiber optic transmitters up to 50 GHz can be designed and simulated.

Control Options

Control options that affect the simulation and design of both JFETs and MESFETs include:

Override a global depletion capacitance equation selection that uses the .OPTION DCAP=<val> statement in a JFET or MESFET model by including DCAP=<val> in the device's .MODEL statement.

Convergence

Enhance convergence for JFET and MESFET by using the GEAR method of computation (.OPTIONS METHOD=GEAR), when you include the transit time model parameter. Use the options GMIN, GMINDC, and GRAMP to increase the parasitic conductance value in parallel with pn junctions of the device.

Capacitor Equations

The option DCAP selects the equation used to calculate the gate-to-source and gate-to-drain capacitance for CAPOP=0. DCAP can be set to 1, 2 or 3. The default is 2.