JFETs are formed by diffusing a gate diode between the source and drain, while MESFETs are formed by applying a metal layer over the gate region, and creating a Schottky diode. Both technologies control the flow of carriers by modulating the gate diode depletion region. These field effect devices are called bulk semiconductor devices and are in the same category as bipolar transistors. Compared to surface effect devices such as MOSFETs, bulk semiconductor devices tend to have higher gain because bulk semiconductor mobility is always higher than surface mobility.
Enhanced characteristics of JFETs and MESFETs, relative to surface effect devices, include lower noise generation rates and higher immunity to radiation. These advantages have created the need for newer and more advanced models.
Features for JFET and MESFET modeling include:
The Star-Hspice GaAs model LEVEL=3 (See A MESFET Model for Use in the Design of GaAs Integrated Circuits, IEEE Transactions on Microwave Theory, Vol. MTT-28 No. 5) assumes that GaAs device velocity saturates at very low drain voltages. The Star-Hspice model has been further enhanced to include drain voltage induced threshold modulation and user-selectable materials constants. These features let you use the model for other materials such as silicon, indium phosphide, and gallium aluminum arsenide.
The Curtice model