EDC | FET | JFET | 2 Marks

FET (Field Effect Transistor)

FET is a  voltage controlled, unipolar, three terminal device.

Why FET is called a unipolar device?
In UJT, either holes or electrons is responsible for conduction of current (ID) through the channel and hence called unipolar device.
In n-channel JFET, free electrons (charge carrier) in the channel alone is responsible for flow of current (drain current) whereas in p-channel JFET, holes (charge carriers) present in the channel alone is responsible for flow of current.

Why is the term "Field Effect"?
In JFET, the drain current ID is controlled by electric "field" applied across gate and source.

Types of FET

• JFET (Junction Field Effect Transistor)
• MESFET (MEtal Semiconductor FET)
• MOSFET (Metal Oxide Semiconductor FET)

JFET Biasing

• Self bias
• Voltage-divider bias

JFET Characteristics

• Transfer Characteristics (ID Vs VGS)
• Drain Characteristics (ID Vs VDS)

Pinch-off voltage
In drain characteristics,   at a constant VGS (VGS=0), the  value of VDS at which the drain current becomes constant for further increase in VDS is called pinch-off voltage.

Cut-off voltage
In drain characteristics, the value of VGS that keeps ID approximately zero is called cut-off voltage.

Characteristic parameters of JFET

1. Transconductance (gm)
2. Input resistance and capacitance
3. Drain and source resistance (rd)
4. Amplification factor (μ)
5. Power dissipation (PD)

Applications of JFET

• Can be used as switch or as amplifier as like BJTs.
• Since input impedance is high and output impedance is low, it can be used as buffer in measuring instruments.
• It has low noise capability ad hence can be used in RF amplifiers, FM tuners and other communication devices.
• Input capacitance of FET is very low and hence usedd in cascade amplifiers in measuring and test equipments.
• Used in oscillators, low frequency amplifiers and digital circuits.