ELECTRONICS AND COMMUNICATION BITS EDU CAMPUS ELECTRONICS AND COMMUNICATION ELECTRONICs DEVICE & CIRCUITS Guided By: Mr. Ankit Dhimmar Prepared By: Rushina Shukla Yashasvi Savani Enrollment No.: 140050111066 140050111068

JFET JUNCTION FIELD EFFECT TRANSISTOR

Introduction Field Effect Transistor (FETs) are important devices such as BJTs. A field-effect transistor (FET) is a three terminal (namely drain, source and gate) semiconductor device in which current conduction is by only one type of majority carriers (electrons in case of an N-channel FET or holes in a P-channel FET). It is also sometimes called the uni-polar transistor. FET devices can be used to operate in amplifier circuits or other similar electronic circuits, with different bias considerations.

Systematic Symbol

Types of JFETs There are two types of JFET - n channel JFET - p channel JFET Three Terminals -Drain-D -Gate-G -Source-S

Systematic Symbol

N Channel JFET Major structure is n-type material (channel) between embedded p-type material to form 2 p-n junction. In the normal operation of an n-channel device, the Drain (D) is positive with respect to the Source(S). The current flows into the Drain(D), through the channel, and out of the Source(S) The resistance of the channel depends on the gate- to-source voltage (VGS), the Drain current (I0) is controlled by that voltage.

Symbol of N Channel

P Channel JFET Major structure is p-type material (channel) between embedded n-type material to form 2 p-n junction. Current flow: From Source (S) to Drain (D) Holes injected to Source (S) through p-type channel and flowed to Drain (D).

Symbol of P Channel

Working of JFET In a junction field-effect transistor, the controlled current passes from source to drain or from drain to source. In normal usage, a voltage is applied across the channel, with the drain being made positive with respect to the source. Thus, electrons will passes through the n-type channel from source to drain. Using the voltage applied and the resistance of the channel the magnitude of current flowing through the channel can be determined. Therefore, the effective width of the channel is restricted by the depletion region, and its effective resistance is higher in this part of the channel, than in any other part of the channel. If now the gate is forward biased with respect to the channel, the depletion region starts reducing thereby decreasing the flow of current through the channel. However, if the gate becomes reverse biased with respect to the channel, the applied electric field will increase the depletion region resulting in the narrowing of channel width, and the channel resistance is increased accordingly. Hence, it is seen that a small voltage applied to the gate can have a profound effect on the current flowing through the channel. If the applied reverse bias becomes very high, the channel width will become very narrow stopping the current flow completely

Characteristics of JFET The application of a voltage Vs. from drain to source will cause electrons to flow through the channel. It can be seen that for a given value of Gate voltage, the current is nearly constant over a wide range of Source-to-Drain voltages. But, when the Gate is made more negative, it depletes the majority carriers from a larger depletion zone around the gate reducing the current flow for a given value of Source-to-Drain voltage. Let us assume that Vgs= 0V (at this point of time gate is shorted to the source). Changing the Gate voltage modulates the current flow through the device. Now, if Vds is increased, drain current will also increase linearly and at some voltage between 2V to 6V, a saturation effect occurs and a "knee" develops in the characteristic curves. As drain current is increased, drain-to-source voltage also increases until avalanche effect due to diode breakdown takes place. Now, if gate voltage is introduced by making gate negative with respect to the source and the drain-to-source voltage is varied from zero up to breakdown, the curve is of similar shape but shifted down. This happens because negative voltage on the gate has reduced drain current. However, in case of P-channel type, in which holes (positive) are major current carriers application of positive gate could do the job of repelling holes. The transfer characteristic for the JFET could be used for visualising the gain from the device and identifying the region of linearity. Hence, the gain is proportional to the slope of the transfer curve. The Gate voltage at which the current becomes zero is called the "pinch voltage", VP.

CURVE

Applications of JFET JFETs can be used for several applications including: • High Input Impedance Amplifier • Low-Noise Amplifier • Differential Amplifier • Constant Current Source • Analog Switch or Gate • Voltage Controlled Resistor

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