Metal Semiconductor Field-Effect Transistor (MESFET) Model
Model Kind
Transistor
Model Sub-Kind
MESFET
SPICE Prefix
Z
SPICE Netlist Template Format
@DESIGNATOR %1 %2 %3 @MODEL &"AREA FACTOR" &"STARTING CONDITION" ?"INITIAL D-S VOLTAGE"|IC=@"INITIAL D-S VOLTAGE", @"INITIAL G-S VOLTAGE"|
Parameters (definable at component level)
The following component-level parameters are definable for this model type and are listed on the Parameters tab of the Sim Model dialog. To access this dialog, simply double-click on the entry for the simulation model link in the Models region of the Component Properties dialog.
| Area Factor | specifies the number of equivalent parallel devices of the specified model. This setting affects a number of parameters in the model. |
| Starting Condition | set to OFF to set terminal voltages to zero during operating point analysis. Can be useful as an aid in convergence. |
| Initial D-S Voltage | time-zero voltage across Drain-Source terminals (in Volts). |
| Initial G-S Voltage | time-zero voltage across Gate-Source terminals (in Volts). |
Parameters (definable within model file)
The following is a list of parameters that can be stored in the associated model file:
| VTO | pinch-off voltage (in Volts). (Default = -2.0). |
| BETA | transconductance parameter β (in A/V 2 ). (Default = 1.0e-4) |
| B | doping tail extending parameter (in 1/V). (Default = 0.3). |
| ALPHA | saturation voltage parameter (in 1/V). (Default = 2). |
| LAMBDA | channel-length modulation parameter λ (in 1/V). (Default = 0). |
| RD | drain ohmic resistance (in Ohms). (Default = 0). |
| RS | source ohmic resistance (in Ohms). (Default = 0). |
| CGS | zero-bias G-S junction capacitance (in Farads). (Default = 0). |
| CGD | zero-bias G-D junction capacitance (in Farads). (Default = 0). |
| PB | gate junction potential (in Volts). (Default = 1). |
| KF | flicker noise coefficient (Default = 0). |
| AF | flicker noise exponent (Default = 1). |
| FC | coefficient for forward-bias depletion capacitance formula (Default = 0.5). |
Notes
- The model for the MESFET is based on the GaAs FET model of Statz et-al.
- The values for the Initial D-S Voltage and Initial G-S Voltage only apply if the Use Initial Conditions option is enabled on the Transient/Fourier Analysis Setup page of the Analyses Setup dialog.
- The Area Factor affects the following model parameters:
- transconductance parameter (
BETA) - doping tail extending parameter (
B) - saturation voltage parameter (
ALPHA) - drain ohmic resistance (
RD) - source ohmic resistance (
RS) - zero-bias G-S junction capacitance (
CGS) - zero-bias G-D junction capacitance (
CGD)
- If the Area Factor is omitted, a value of 1.0 is assumed.
- The link to the required model file (
*.mdl) is specified on the Model Kind tab of the Sim Model dialog. The Model Name is used in the netlist to reference this file. - Where a parameter has an indicated default (as part of the SPICE model definition), that default will be used if no value is specifically entered. The default should be applicable to most simulations. Generally you do not need to change this value.
Examples
Consider the MESFET in the above image, with the following characteristics:
- Pin1 (Drain) is connected to net D
- Pin2 (Gate) is connected to net G
- Pin3 (Source) is connected to net S
- Designator is Q1
- The linked simulation model file is NMESFET.mdl.
If no values are entered for the parameters in the Sim Model dialog, the entries in the SPICE netlist would be:
*Schematic Netlist:
ZQ1 D G S NMESFET
.
.
*Models and Subcircuit:
.MODEL NMESFET NMF(LEVEL=6, )
In this case, there are no parameter values specified in both the Sim Model dialog and the model file. The SPICE engine would therefore use the parameter default values inherent to the model.