Field Effect Transistor

FET

Classification of FET

JFET Schematic Symbols

N-channel JFET

Water Analog for JFET

Operation of n-channel JFET

Cont.

VGS = 0 V; VDS = Vp

VGS = -V; No voltage applied B/w the Drain & Source

VGS = -1V, VDS > 0 V

JFET Working

Operation of n-channel JFET Summary

Operation of n-channel JFET Summary

Drain Characteristics

Drain Characteristics of n channel JFET

Cont.

Important terms

1. Pinch-off Voltage (Vp)

2. Shorted-Gate Drain Current (IDSS) or Drain to Source current with gate shorted

3. Gate Source Cut-off Voltage (Turn off voltage)(VGS (off))

Drain Characteristics

• There are 4 important region:1.Ohmic Region2.Saturation Region3.Breakdown Region4.Cut-off Region

Transfer Characteristics of n-channel JFET

Transfer Characteristics

Amplification Factor (µ)

• It is the ratio of change in drain-source voltage (ΔVDS) to the change in gate-source voltage (ΔVGS) at constant drain current (ID)

•

Other Formulas

P channel JFET Working

P channel JFETTransfer & Drain Characteristics

Important Relationships (JFET versus BJT)

JFET as Voltage Variable Resistance

JFET as Voltage Variable Resistance

• The resistance of the device b/w drain & source for VDS < VP is a function of the applied voltage VGS

Merits & Demerits of JFET

Merits• Very High Input

Resistance• Better Thermal Stability• Produces less noise• Long Life & Small Size• Higher power gain• Relatively immune to

radiation

Demerits• Low Voltage gain

compared to transistor Amplifier

• Expensive• Low gain bandwidth

product

JFET BJTUnipolar device Bipolar deviceVoltage Controlled Device Current Controlled DeviceHigh Input Resistance (order of MΩ) Input Resistance Low Compared to JFET

Doesn’t Suffer from minority carrier storage effects (higher switching speeds & cut off frequencies)

Suffers from minority carrier storage effects (lower switching speeds & cut off frequencies)

Negative Temperature Coefficient at high current levels ( prevents from thermal breakdown). Better thermal Stability

Positive Temperature coefficient at high current levels (leads to thermal breakdown)

Less noisy than BJT or Vacuum Tube More Noisy than a JFETSimpler to Fabricate as an IC & occupies less space on IC chip than BJT

Comparatively difficult to Fabricate as an IC and occupies more space on IC chip

FET are expensive than BJT Cheaper to produce than FETsFET can tolerate a much higher level of radiation

Performance is degraded by neutron radiation

Poor Fanout & Low Power consumption Good Fanout & High Power Consumption

Fan-out Meaning

Application of JFET

• Used as a Buffer Amplifier, low noise Amplifier• Used as a Voltage Variable Resistor• Used in digital Circuits, in Computers• Used in Oscillator circuits• Used in low frequency amplifiers in hearing

aids & inductive transducer

Small Signal Low Frequency JFET Model

Small Signal High Frequency JFET Model

Reference

• Electronic Devices & Circuits by S Salivahanan• Electronic Devices & Circuit Theory by Robert

Boylestad and Louis Nashelsky