2 February 2018

MOSFET characteristics

In general, any MOSFET shows three operating regions following below.
  1. Cut-off Region 
  2. Ohmic or Linear Region 
  3. Saturation Region 

Cut-off Region :

It is a region where MOSFET will be OFF because there will be no current flow through it. In this region, MOSFET acts as an open switch and thus when they are required to function as electronic switches. 

Ohmic Region : 

Ohmic or Linear region is a region where the current increases as the value of the voltage increases. If MOSFET is used in this region, it can be used as amplifiers. 

Saturation Region : 

In this region, despite the increase in voltage, the MOSFET has it's current constant and occurs when the voltage exceeds the pinch-off value. Under this condition, the device acts as a closed switch that saturates the value of current flows. This operating region is therefore selected whenever MOSFET is required to perform a switching operation. 

Now that we know this, let us analyze the prejudice in which these regions are experienced for each type of MOSFET. 

The transfer and output characteristics of the power MOSFET following below :

Transfer characteristics :

  • This feature shows the variation of the current  ID of the drain as a function of the VGS gate-source voltage. 
  • VGS is the minimum positive voltage to induce n-channel between gate and source. Therefore, for threshold voltage below VGS, the device is in the off state, a magnitude of is of VGST the order of 2  to 3 V. This is typical characteristics for n-type MOSFET. 
MOSFET characteristics

MOSFET output characteristics :

  • The output characteristics of MOSFET shown in the figure indicate the variation of the drain current ID as a function of the VDS drain-source voltage with the VGS gate-source voltage as a parameter.
  • For a low VDS value, the graph between Iand VDS is almost linear, indicating a constant value of on-resistance RDS = VDS/ID.
  • If VDS is increased for a given VGS, the output characteristics are relatively flat, indicating drain current is nearly constant.
  • The output characteristics of the load line A and B intersect. A indicates fully on condition and B fully off condition. MOSFET works as a switch either at A or B.
  • When power MOSFET is driven with large gate-source voltage, MOSFET is turned on, VDS is small. Here, MOSFET acts as a closed switch is said to be ohmic region.
  • When the device turns on, MOSFET traverses characteristics from cut off to the active region and then to the ohmic region.
  • When MOSFET turn off, it takes a backward journey from ohmic region to cut off state.

MOSFET characteristics