Operation of p-Channel and n-Channel JFET

Operation of p‒Channel and n‒Channel JFET

Operation of n‒Channel JFET

• In JFET, the p‒n junction between gate and source is always kept in reverse biased conditions. Since the current in a reverse biased p‒n junction is extremely small, practically zero; the gate current in JFET is often neglected and assumed to be zero.

• Let us consider the circuit shown in Fig. 5.4.1. As shown in Fig. 5.4.1, voltage V_DD is applied between drain and source. Gate terminal is kept open. The bar is of n‒type material.

Fig. 5.4.1 JFET with gate open and V_DD is applied between drain and source

• Due to the applied voltage, the majority carriers i.e. the electrons start flowing from the source to the drain. This flow of electrons makes the drain current, I_D.

• The majority carriers (electrons in n‒channel JFET and holes in p‒channel JFET) move from source to drain through the space between the gate regions. This space is commonly known as channel. The width of this channel can be controlled by varying the gate voltage.

• Fig. 5.4.2 (a) shows that an n‒channel JFET with the gate directly connected to the source terminal. When drain voltage V_DS is applied, a drain current I_D flows in the direction shown. Since the n‒material is resistive, the drain current causes a voltage drop along the channel. This voltage drop reverse biases the pn junctions, and causes the depletion regions to penetrate into the channel. Since gate is heavily doped and the channel is lightly doped, the width of the depletion region will mainly be spread in the channel shown in Fig. 5.4.2 (a). This penetration depends on the reverse bias voltage.

• The depletion region width is more at the drain side as compared to source side because near the junction, voltage at drain side is more than the voltage at the source side. This shows that reverse bias is not uniform near the junction; it gradually increases from source side to drain side.

• The depletion region does not contain charge carriers, the space between two depletion regions is available for the conducting portion of the channel. If we externally apply reverse bias voltage to the gate, the reverse bias will further increase and hence increase the penetration of the depletion region, which reduces the width of the conducting portion of the channel. As width of the conducting portion of the channel reduces, the number of electrons flowing from source to drain reduces and hence the current flowing from drain to source reduces.

• If we go on increasing the reverse bias voltage to the gate as shown in Fig. 5.4.2 (b) and 5.4.2 (c), depletion regions will increase more and more and stage will come when the width of the depletion regions will be equal to the original width of the channel, leaving zero width for conducting portion of the channel. This will prevent any current flow from drain to source and hence cut off the drain current. The gate to source voltage that produces cut‒off is known as cut‒off voltage and it is denoted by V_{GS (off).}

• When the gate is shorted to source, there is minimum reverse bias between gate and source p‒n junction, making depletion region width minimum and conducting channel width maximum. In this case maximum drain current flows which is designated by I_DSS.

• From above discussion it is cleared that the gate to source voltage controls the current flowing through channel and hence FET is also called voltage controlled current source.

Operation of p‒Channel JFET

• The p‒channel JFET is constructed in exactly the same manner as the n‒channel JFET but with reversal of the p‒and n‒type materials as shown in Fig. 5.4.3.

• All current directions and voltage polarities are reversed.

• For V_GS = 0, channel width is maximum. By increasing positive gate to source (V_GS) voltage, the channel width is reduced.

Review Questions

1. Explain the operation of n‒channel FET.

2. Sketch a block diagram to represent an n‒channel FET. Indicate voltage and current direction and the device operation.

3. Explain JFET as voltage controlled current source.

4. Explain the working of p‒channel FET.

5. With neat diagram explain the construction, working principle of p‒channel JFET.

6. Illustrate the working mechanism of JFET with necessary diagram.