Steady state output i-v characteristics of a MOSFET
Steady state output i-v 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 are then a plot of drain current (iD) as a function of the Drain –Source voltage (vDS) with gate source voltage (vGS) as a parameter.
- With gate-source voltage (VGS) below the threshold voltage (vGS (th)) the MOSFET operates in the cut-off mode. No drain current flows in this mode and the applied drain–source voltage (vDS) 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.
- When VGS is increased beyond vGS(th) drain current starts flowing. For small values of vDS (vDS < (vGS – vGS(th)) iD is almost proportional to vDS. 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)).
- Note that rDS (ON) reduces with increase in vGS. This is mainly due to reduction of the channel resistance at higher value of vGS. Hence, it is desirable in power electronic applications, to use as large a gate-source voltage as possible subject to the dielectric break down limit of the gate-oxide layer.
- At still higher value of vDS (vDS > (vGS – vGS (th)) the iD – vDS characteristics deviates from the linear relationship of the ohmic region and for a given vGS, iD tends to saturate with increase in vDS.
- The exact mechanism behind this is rather complex. It will suffice to state that, at higher drain current the voltage drop across the channel resistance tends to decrease the channel width at the drain drift layer end.
- In addition, at large value of the electric field, produced by the large Drain – Source voltage, the drift velocity of free electrons in the channel tends to saturate. As a result the drain current becomes independent of VDS and determined solely by the gate – source voltage vGS. This is the active mode of operation of a MOSFET.
- 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.