Parameters of a JFET

There are several parameters describing a JFET but am going to present these:

1.   Shorted Gate Drain Current (I_DSS)
2.   Pinch Off Voltage (V_P)
3.   Gate Source Cut Off Voltage V_GS(off)
4.   a.c. Drain Resistance
5.   Transconductance
6.   Amplification Factor

Shorted Gate Drain Current (I_DSS)

This is the drain current with source short circuited to gate (ie V_GS = 0) and drain voltage (V_DS) equal to the pinch off voltage. This is sometimes called Zero Bias Current. When V_DS is increased beyond V_P, the depletion layers expand at the top of the channel. The channel therefore, now acts as a current limiter and holds drain current constant at I_DSS.



Note that since I_DSS is measured under shorted gate conditions. It is the maximum drain current that one can get with normal operation of JFET.
There is a maximum drain voltage V_DS(max) that can be applied to a JFET. If the drain voltage exceeds V_DS(max) the JFET would breakdown.
The region between V_P and V_DS(max) is called constant current region or active region. In active region, JFET behaves as a constant current device. For proper working of JFET it must be operated in the active region.

Pinch Off Voltage (V_P)

This is the maximum drain source voltage at which the drain current becomes essentially constant.

Gate Source Cut Off Voltage V_GS(off)

This is the gate source voltage where the channel is completely cut off and the drain current becomes zero.
V_GS(off) will always have the same magnitude value as V_P. For Example if V_P = 6V, V_GS(off) = -6V.

a.c. Drain Resistance

This is the ratio of change in drain source voltage (ΔV_DS) to the change in drain current (ΔI_D) at a constant gate source voltage

Equation ===> ΔV_GS/ΔI_D at constant V_GS

Transconductance

This is the ratio of the change in drain current (ΔI_D) to the change in gate source voltage (ΔV_GS) at constant drain source voltage.

Equation ===> ΔI_D/ΔV_GS at constant V_DS

Amplification Factor

This is the ratio of change in the drain source voltage (ΔV_DS) to the change in the gate source voltage (ΔV_GS) at constant drain current.
Amplification factor of a JFET indicates how much control the gate voltage has over drain current than the drain voltage.

Equation ===> ΔV_DS/ΔV_GS

It is worthy to note this equations

I_D = I_DSS(1 - (V_GS/V_GS(off)))²
g_m = g_mo[1 - (V_GS/V_GS(off))]²
g_mo = 2I_DSS/|V_GS(off)|