V-I Characteristics of JFET

The voltage V_GS applied to the Gate controls the current flowing between the Drain and the Source terminals. V_GS term as the voltage applied between the Gate and the Source whereas V_DS refers to the voltage applied between the Drain and the Source.

Junction Field Effect Transistor is a voltage controlled device, “NO current flows into the gate!” then the Source current ( I_S ) flowing out of the device equals the Drain current flowing into it and therefore ( I_D = I_S ).

The characteristics curves, as shown above, have four different regions of operation for a JFET and these regions are given as:

1. Ohmic Region– When V_GS = 0 the depletion layer of the channel is very small and in this region, JFET acts like a voltage controlled resistor.
2. Cut-off Region– This is also known as the pinch-off region in this region Gate voltage, V_GS is sufficient to cause the JFET to act as an open circuit as the channel resistance is at maximum.
3. Saturation or Active Region– In this region, JFET becomes a good conductor and is controlled by the Gate-Source voltage, (V_GS) while the Drain-Source voltage, (VDS) has very small or no effect.
4. Breakdown Region– in this region the voltage between the Drain and the Source, (V_DS) is high enough to cause the JFET’s resistive channel to break down and pass uncontrolled maximum current.

The characteristics curves for a P-channel junction field effect transistor are similar to n channel characteristics shown above, except that the Drain current I_D decreases with an increasing positive Gate-Source voltage, V_GS.

For normal operation, the Drain current is zero when V_GS = V_P., V_GS is biased to be somewhere between V_P and 0.so that we can calculate the Drain current, I_D for any given bias point in the saturation or active region as follows:

Drain current in the active region.

Drain-Source Channel Resistance.