Output Characteristics

The output  or drain  characteristic of N - Channel  power MOSFET. When gate  is made positive with respect to the source an N type  channel  is formed  close to the gate. In  this case N ⁺type  substrate  and the N source terminal. When  V_gs  is zero  or negative no channel exists  and the drain current is  zero with the increase in gate  voltage the channel resistance is reduced and therefore the drain current ID  increases thus the  drain current ID remains fairly  constant  over a wide range of V_ps levels. Drain terminal  beings  at the bottom  of the power  MOSFET. It can have a considerable large area for any given  of the  device which  allows  more  power  dissipation.

                                                   Figure  output  characteristics of N channel power MOSFET

When Vas is zero  or negative and the  drain  is positive  with  respect to  source the  junctions  between the P layer  and the N layer is  reverse biased. The  depletion region  at eh junction penetrates deep  into the N layer  and thus  punch through  from  drain to source is avoided.

The  lower  most  curve is for  VGS ( Th.)  when VGS< VGS(Th.)  drain  current  is almost  zero when VGS> VGS(Th.)  the device is  ON. This  device has  three  regions ohmic region  active region and breakdown  region. The  rising part of  curve (from  BDS= 0 to VDS=  few volts)  is the ohmic region. The  device behaves as a resistor  when  operated in this region. The  drain  current is almost constant when the device operates in the active region.ehn BDS excess the rated value avalanche breakdown occur and the device is in the breakdown  region. V_gs  can be positive and negative. The  characteristics of P channel  power MOSFET  are similar to those of N channel  MOSFET except that the current  directions and voltage  polarities are reversed.