MOSFET operation

MOSFET operation:

  1. If the Gate voltage applied is less than the Threshold Voltage, little or NO current flows and the device is said to be OFF. This mode of operation is called cutoff.

  2. If the Gate voltage applied is more than the threshold voltage, an inversion region called channel connects Source and Drain. The transistor is now turned ON. 

  3. The number of carriers increase thus increasing the conductivity with increase in the Gate voltage

  4. The potential difference between Drain and Source is Vds = Vgs- Vgd. If there is no Drain voltage applied i.e. Vds=0, then Vgs=Vgd. Now there is no potential difference and thus there is no electric field to push current from Drain to Source.

  5. If a small potential Vds is applied, current Ids flows through the channel from Drain to Source - mode of operation called Linear or resistive or triode 

  6. The current increase with increase in both Drain voltage and Gate voltage

  7. If the Drain voltage increases beyond a certain value, then VGD<VT the channel ceases to exist near the Drain - pinched off. 

  8. Even under these conditions, current still flows. This current is due to the drift of electrons as the Drain voltage is positive.

  9. If the Drain voltage is further increased, the current Ids is controlled only by the Gate voltage and the Drain voltage has no influence. This mode of operation is called Saturation mode.

The table below summarizes the three modes of Operation



Mode Of Operation








Vds is small



Vgs>Vt and Vds is large

Current Source, Saturation

Now let us look at the P Channel MOSFET operation. As is known the operation is exactly opposite to that of the N Channel MOSFET.

Figure 16 shows the P Channel MOSFET.

Note: From now onwards, let us denote N Channel MOSFET as nMOS and P Channel MOSFET as pMOS.

The n+ type body is connected to a high voltage, this makes the p-type junctions of Source and Drain normally reverse biased. Now if we make the Gate voltage as high no current flows in the device between Drain and Source. 

The p-type Channel is formed between Source and Drain when the Gate voltage is made less than the Threshold Voltage Vt. 

It may be noted here that the threshold voltages of nMOS and pMOS need not be of same value both in magnitude and polarity. So to distinguish between these two subscripts n or p is added in the threshold voltage - Vtn and Vtp.

The MOSFET is a symmetrical device, but by convention it might be stated that the majority carriers flow from Source to Drain.

The charge of electrons is negative, therefore the Source of nMOS transistor is more negative of the two terminals (SOurce and Drain). 

Similarly the charge of holes is positive, therefore the Source of the pMOS transistor is more positive of the two terminals.