Important performance parameters of a FET device are static and dynamic drain resistance, transconductance (gm) and amplification factor (μ).
Static drain resistance (RD) is defined as the ratio of the drain-source voltage (VDS ) to the drain current (ID).
It is defined as the ratio of change in the drain-source voltage to the change in the drain current at a constant gate-source voltage.
The typical values of rd lie in the range of 0.1 to 1 M Ω for JFET and in the range of 1 to 50 k Ω for MOSFETs.
. Transconductance (gm) is defined as the ratio of the change in the drain current to the change in the gate-source voltage for a constant drain-source voltage
Transconductance varies with the applied-gate source voltage (VGS) as given below
Where, gm0 is the transconductance at zero gate-source voltage. The value of gm is in the range of 0.1 to 10 mA/V for JFETs and between 0.1 and 20mA/V or more for MOSFETs.
Amplification factor (μ) is defined as the ratio of the change in the drain-source voltage to the change in the gate-source voltage for a constant value of drain current.
Amplification factor is given by the product of the dynamic drain resistance and the transconductance of the JFET
