Junction Field Effect Transistor (JFET)

Definition: The Unipolar transistor is a type of transistor in which electric current flows due to only one type of charges i.e. either holes or electrons.

The Unipolar transistor is categorized into two main types:

• Junction Field Effect Transistor (JFET)
  • N-channel JFET
  • P-channel JFET
• Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
  • N-channel MOSFET
  • P-channel MOSFET

Construction of JFET

It consists of one long n-channel with two p-layers inserted at the sides. The n-channel forms the conducting path for the charges.

Note the direction of the arrow in the symbol, the inward direction of arrow indicates N-channel JFET and the outward direction of arrow indicates P-channel JFET.

We can also consider the working of JFET in terms of bipolar transistor. For this we can consider that –

1. Drain (d) of JFET is similar to collector (c) in bipolar transistor.
2. Source (s) of JFET is similar to emitter (e) in bipolar transistor.
3. And finally, Gate (g) of JFET is similar to base (b) for bipolar transistor.

Parameters of JFET

There are three important parameters of JFET, they are as follows –

Dynamic drain resistance: It is defined as the ratio of change in the drain voltage V_ds to the corresponding change in the drain current I_d, at constant V_gs.

r_d = ΔV_ds / ΔI_d …… at constant ΔV_gs

Mutual Conductance or Transconductance: It is defined as the ratio of change in drain current I_d to the corresponding change in the gate-source voltage V_gs, at constant V_ds.

g_m = ΔI_d / ΔV_gs …… at constant ΔV_ds

Amplification factor: It is defined as the ratio of change in the drain-source voltage to the corresponding change in gate-source voltage, at constant drain current I_d.

μ = ΔV_ds / ΔV_gs …… at constant ΔI_d

Generally, the amplification factor is also given as –

μ = r_d × g_m