During purchasing a JFET for a particular application we need to check the specifications of the device. These specifications are provided by manufacturers. The followings are the parameters used to specify a JFET and these are

- Gate Cut Off Voltage (V
_{GS(off)}) - Shorted Gate Drain Current (I
_{DSS}) - Transconductance (g
_{mo}) - Dynamic Output Resistance (r
_{d}) - Amplification Factor (μ)

## Gate Cut Off Voltage

At a fixed drain voltage, the drain current (I_{D}) of a JFET depends on the gate to source voltage (V_{GS}). If the gate to source voltage decreases from zero in n channel JFET, the drain current also gets decreased accordingly. The relation between gate to source voltage and drain current is given below. After a certain gate to source voltage (V_{GS}), the drain current I_{D} becomes zero. This voltage is known as Cut Off Gate Voltage (V_{GS(off)}). This voltage numerically equals the pinch-off drain to source voltage (V_{p}). In the case of p channel JFET if we increase the gate terminal voltage from zero the drain current gets decreased and after a certain gate to source voltage, the drain current becomes zero. This voltage is the cut off gate voltage for p channel JFET. It is the gate cut off voltage for p channel JFET.

## Shorted Gate Drain Current

When the gate terminal is grounded (V_{GS} = 0) and positive drain to source voltage (V_{DS}) is being increased slowly in case of n channel JFET, the drain current gets increased linearly. But after pinch-off voltage (V_{p}), the drain current would not be increased further and gets a constant value. This is the maximum drain current that can flow through the channel when the gate terminal is in ground potential. This current is fixed for a JFET and this is called shorted gated drain current and generally denoted by I_{DSS}.

## Transconductance

Transconductance is the ratio of change in drain current (δI_{D}) to change in the gate to source voltage (δV_{GS}) at a constant drain to source voltage (V_{DS} = Constant).

This value is maximum at V_{GS} = 0. This is denoted by g_{mo}. This maximum value (g_{mo}) is specified in a JFET data sheet. The transconductance at any other value of gate to source voltage (g_{m}) can be determined as follows. The expression of drain current (I_{D}) is

By partial differentiating the expression of drain current (I_{D}) in respect of gate to source voltage (V_{GS})

At V_{GS} = 0, the transconductance gets its maximum value and that is

Therefore, we can write,

## Dynamic Output Resistance

This is the ratio of change of drain to source voltage (δV_{DS}) to the change of drain current (δI_{D}) at a constant gate to source voltage (V_{GS} = Constant). The ratio is denoted as r_{d}.

## Amplification Factor

The amplification factor is defined as the ratio of change of drain voltage (δV_{DS}) to change of gate voltage (δV_{GS}) at a constant drain current (I_{D} = Constant).

There is a relation between transconductance (g_{m}) and dynamic output resistance (r_{d}) and that can be established in the following way.