Explanation:

Junction Field Effect Transistor (JFET) is a three-terminal semiconductor device that is widely used for amplification and switching purposes. The operation of a JFET is based on the control of the channel resistance by the gate-source voltage. Here is the explanation of why the drain current is primarily controlled by the gate reverse bias in a JFET.

Overview of JFET:

A JFET is a voltage-controlled device that has a p-type or n-type channel between two p/n-type junctions. The voltage applied to the gate terminal controls the depletion width of the p-n junction, which, in turn, controls the resistance of the channel. When the gate-source voltage is zero, the depletion width is minimum, and the channel resistance is maximum.

When the gate-source voltage is negative, the depletion width increases, and the channel resistance decreases. As a result, the drain current increases. Conversely, when the gate-source voltage is positive, the depletion width decreases, and the channel resistance increases. Therefore, the drain current decreases.

Primary control of drain current:

The drain current in a JFET is primarily controlled by the gate reverse bias voltage. The following points explain why:

1. Channel width modulation:

When the gate-source voltage is negative, the depletion width increases, and the channel width decreases. As a result, the channel resistance decreases, and the drain current increases. This phenomenon is called channel width modulation.

2. Pinch-off voltage:

The pinch-off voltage is the voltage at which the depletion region of the p-n junction reaches the channel's center. Beyond this voltage, the channel is pinched-off, and the drain current becomes independent of the drain-source voltage. The pinch-off voltage is primarily determined by the gate-source voltage.

3. Gate-source voltage controls the depletion region:

The gate-source voltage controls the depletion width of the p-n junction, which, in turn, controls the channel's resistance. Therefore, the gate-source voltage is the primary control of the drain current.

Conclusion:

In conclusion, the drain current in a JFET is primarily controlled by the gate reverse bias voltage. When the gate-source voltage is negative, the depletion width increases, and the channel resistance decreases, leading to an increase in the drain current. Conversely, when the gate-source voltage is positive, the depletion width decreases, and the channel resistance increases, leading to a decrease in the drain current.