Electronics Engineering (ELEX) Board Practice Exam

In a JFET, what controls the drain current?

1. Drain-source voltage

2. Reverse-bias at the gate

3. Forward-bias at the gate

4. Gate-drain voltage

The correct answer is: Reverse-bias at the gate

In a Junction Field Effect Transistor (JFET), the drain current is primarily controlled by the reverse-bias voltage applied at the gate. The gate is typically reverse-biased to create an electric field that influences the width of the conductive channel formed between the source and drain. This electric field affects the number of charge carriers in the channel and, consequently, modulates the flow of current when a voltage is applied across the drain and source. When the reverse bias at the gate increases, the depletion region widens, thus narrowing the channel and reducing the drain current. Conversely, reducing the reverse bias allows the channel width to increase, allowing more current to flow from the drain to the source. This characteristic makes the JFET a voltage-controlled device, where the gate voltage directly regulates the current flowing through it. Other choices do not directly control the drain current in the same way. The drain-source voltage affects the overall current in a JFET but is not the primary controlling factor of the current itself like the gate voltage. Forward-bias at the gate is generally not applied in regular JFET operations, as it would lead to undesirable conducting states. Gate-drain voltage, while it may influence the device's operation in specific configurations, does