Steady state output v-i characteristics of a MOSFET

The MOSFET, like the BJT is a three terminal device where the voltage on the gate terminal controls the flow of current between the output terminals, Source and Drain.

The source terminal is common between the input and the output of a MOSFET.

The output characteristics of a MOSFET is then a plot of drain current (i D) as a function of the Drain –Source voltage (vDS) with gate source voltage (vGS).

With Gate-source voltage (VGS) below the threshold voltage (v GS (th)) the MOSFET operates in the cut-off mode.

No drain current flows in this mode and the applied drain–source voltage (v DS) is supported by the body-collector p-n junction. Therefore, the maximum applied voltage should be below the avalanche break down voltage of this junction (VDSS) to avoid destruction of the device.

Consequently this mode of operation is called “ohmic mode” of operation.

In power electronic applications a MOSFET is operated either in the cut off or in the

ohmic mode.

The slope of the vDS – iD characteristics in this mode is called the ON state

Resistance of the MOSFET (rDS (ON)).

In the active region the drain current is given approximately by

iD=K (VGS-VGS (TH) 2)

VDS=VGS-VGS (TH)

Therefore, iD=KVDS2

At this point the similarity of the output characteristics of a MOSFET with that of a BJT should be apparent.

Both of them have three distinct modes of operation, namely, (i)cut off, (ii) active and (iii) ohmic (saturation for BJT) modes. However, there are some important differences as well.