In this note, Poly Notes Hub discusses the Junction Field Effect Transistor ( What is JFET? ), the Construction of JFET, the Working Principle of JFET, and the Uses of JFET.
Author Name: Arun Paul.
Introduction of FET
There are two types of FET ( Field Effect Transistor ).
- Junction Field Effect Transistor (JFET)
- Metal Oxide Field Effect Transistor (MOSFET)
In the field effect transistor, the output characteristics are controlled by the input voltage and not by the input current. For this reason, it is called Voltage Controlled Device. Below we discuss about JFET in details.
What is JFET ?
A junction field effect transistor is a three-terminal semiconductor device in which current conduction is by one type of carrier i.e., electrons or holes.
The JFET was developed at about the same time as the transistor but in came into general use only in the late 1960s. In a JFET, the current conduction is either by electrons or holes and controlled using an electric field between the gate electrode and the conducting channel of the device. The JFET has high input impedance and low noise levels.
Construction of JFET
A JFET consists of a p-type or n-type silicon bar containing two PN Junctions at the sides as shown in Fig. The bar forms the conducting channel for the charge carriers. If the bar is of n-type, it is called n-type JFET as shown in Fig. The two PN junction-forming diodes are connected internally and a common terminal called a gate is taken out. Other terminals are source and drain taken out from the bar as shown. Thus a JFET has essentially three terminals viz., gate (G), source (S), and drain(D).
Working of JFET
- When a voltage is applied between drain and source terminals and voltage on the gate is zero (see Fig i), the two pn Junctions at the sides of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the depletion layers. The size of these layers determines the width of the channel and hence the current condition through the bar.
- When a reverse voltage is applied between the gate and source (see Fig ii), the width of the depletion layers is increased. This reduces the width of the conducting channel, thereby increasing the resistance of the n-type bar. Consequently, the current from the source to the drain is decreased. On the other hand, if the reverse voltage on the gate is decreased, the width of the depletion layers also decreases. This increases the width of the conducting channel and hence the source to drain current.
Symbol of JFET
Below shows the schematic symbol of Junction Field Effect Transistor ( JFET )
Difference between BJT and JFET ( BJT Vs JFET )
| BJT | JFET |
|---|---|
| 1. It is a current controlled device. | 1. It is a voltage controlled device. |
| 2. It is a bipolar transistor. | 2. It is a unipolar transistor. |
| 3. The input circuit is always forward biased. | 3. The input circuit is always reverse biased. |
| 4. Noise level is LOW. | 4. Noise level is HIGH. |
Conclusion | Diploma Engineering Notes | Poly Notes Hub
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