IV Characteristics of MOSFET

I-V Characteristics of MOSFET (Output Characteristics)

1. The I-V characteristics (output characteristics) of power MOSFET is shown in Figure.

• The source terminal is common between the input and output of the MOSFET. The output characteristics is a graph of drain current i_D versus drain to source voltage V_DS for different values of gate to source voltage V_GS as shown in Figure.
• The saturation, cut-off and ohmic regions of the characteristics are also shown in Figure In the power electronic applications where the MOSFET is used as switch, the device must be operated in the cut-off and ohmic region when turned off and on respectively.
• The operation in the saturation region should be avoided to reduce the power dissipation in the on state. (The on state voltage across the MOSFET is high in the saturation region).

2. The MOSFET is in the cut-off state when the gate source voltage V_GS is less than the threshold voltage V_GS(th).

• The device must withstand to the applied voltage and to avoid the breakdown the drain to source breakdown voltage should be greater than the applied voltage. The breakdown takes place due to the avalanche breakdown of the drain body junction.

3. When a larger positive gate to source voltage is applied the power MOSFET goes into the ohmic region where the drain to source voltage V_DS(on) (on) is small.

• In this region of operation the power dissipation can be kept reasonably low, by minimising the on state voltage.
• In the saturation region the drain current i_D is almost independent of the drain to source voltage V_DS. It is only dependent on the gate to source voltage V_GS.
• In the saturation region the gate voltage V_GS is greater than the threshold voltage V_GS(th) and the drain current increases with increase in VGS.
  

Conclusions from the static characteristics

   The important conclusions from the I-V characteristics are as follows

1. The MOSFETs are voltage controlled devices i.e. the output current can be controlled by varying the gate to source voltage (V_GS).
2. With increase in V_GS the drain current will increase.
3. The gate to source voltage ( V_GS ) should be large enough to drive the MOSFET into ohmic region. Practically the minimum V_GS required is about 12 V. If V_GS is less than 12 V, the MOSFET will operate in the active region which is not desired.
4. When the forward voltage V_DS applied to the MOSFET exceeds the breakdown voltage B V_DSS, the avalanche breakdown takes place. Operation above the breakdown voltage must be avoided, to protect the MOSFET.
5. The second breakdown does not exist in MOSFETS.