FET
FET has square law characteristics. It has negative temperature coefficient. High immunity against radiation. FET has least noise. Pinch off voltage of FET is 5 volt. FET is classified in to JFET ( junction field effect transistor) and MOSFET (metal oxide semiconductor FET). MOSFET has highest input impedance. Quiescent current of FET is ID. Both are available in N channel and P channel form
JFET (junction FET)
JFET is voltage controlled device. Zero drift current in JFET is 0.623V. In JFET gate is heavily doped region, gate source voltage is always reverse biased, so gate current is zero. Equivalent to transistor at saturation is JFET is ohmic. Input impedance of JFET is approximately unity. In N channel JFET source is connected to negative terminal and positive terminal connected to drain so that drain current will flow from drain to source. Input noise is less in n channel JFET than p channel. Larger transconductance in n channel. When Vgs = 0 and Vds is increased from zero drain current increases lineary with the voltage (obeys ohmic law), when Vds reaches a critical voltage called pinch off voltage drain current became constant, this current is called zero gate voltage drain current represented by Idss, where ss stands for shorted to source
When Vds is zero and value of Vgs increased negatively as a result depletion region increases and reduces the channel width at a certain point channel vanishes and that point is called cut off point. Drain characteristic is the relation between drain current and drain voltage with different Vgs. Transfer characteristic is the relation between drain current and Vgs with different Vds (drain voltage). Transfer characteristic is similar to transconductance characteristic of a BJT. AC drain resistance is the ratio between change in drain voltage and change in drain current at constant Vgs. AC drain resistance is also called dynamic drain resistance
Transconductance is ratio between change in drain current and change in Vgs at constant drain voltage. Transconductance is the slope of transfer characteristics. Transconductance also called forward transconductance or forward transadmittance. Unit of Transconductance is siemens or mho. Amplification factor is the ratio between change in Vds and change in Vgs at constant drain current. Amplification factor is also defined as the product of transconductance and drain resistance. Voltage gain of JFET depends on amplification factor. Pinch off voltage is equal to cut off voltage. DC drain resistance is the ratio between drain voltage and drain current. Voltage divider bias is preferably used in JFET