JFET (JUNCTION-GATE FIELD-EFFECT TRANSISTOR)

JFET (JUNCTION-GATE FIELD-EFFECT TRANSISTOR)

JFETs are transistors that are used to control or moderate current. They are used in low-noise, low-signal level analog applications, and sometimes used in switching applications.

How it works:

A JFET consists of an input contact called the source and an output called the drain. The whole assembly is placed in a doped semi-conducting material. The electric current flow from source to drain is restricted when a voltage is applied to the gate. JFETs are always in ‘ON’ state until a gate voltage is applied. The gate polarity, when compared to the p-n junction of the terminals, is always in reverse bias. When there is no voltage at the gate, current flow between the source and drain is free. When voltage at the gate is increased, the space available for current to flow is decreased.

Design considerations:

The JFET gates should always be reverse biased, as forward bias voltages greater than 50mA may burn the JFET.
Voltage between gate and source: This is maximum when gate voltage is zero.
Gate source pinch-off voltage: the voltage at which the gate source voltage is cut off.
Drain current: current flowing into the drain.
Source current: current flowing out of the source.
Capacitance: JFETs should have a low capacitance (less than 3pF).
Power dissipation: power consumption of the device.
Temperatures: Considerations have to be done for the lead temperature (around 300 deg C), storage and operating junction temperatures (between –65 to 200 deg C).
Turn off and turn on time: Measured in nanoseconds. Should be least (between 3 to 15 ns).

A JFET transfer tracer is used in labs to analyze its transfer characteristics.

Types:

JFETS can be symmetric, where connection to source and drain can be interchanged, or asymmetric, where the two terminals cannot be interchanged. They are available in hermetically sealed round or triangular cans with surface mounts.

Applications:

JFETs are used in analog switches, choppers, normally ‘on’ switches, commutators and current limiters. Other areas are: high-speed integrators, digital-to-analog converters, sample-and-hold circuits, JFET-input operational amplifiers, octal low-power JFET-input amplifiers, etc.