MOSFET Basics

MOSFET regions of operation

For MOSFET basic we are directly goes to Long channel I-V characteristics. I recommended guys who felt difficulties in MOSFET basic refer Chapter 2 in CMOS VLSI design by Neil Weste and david money harris.  I also use same concept from that book.

(I) Cutoff Region

The Gate to source voltage Vgs is less than threshold voltage (It is the minimum voltage required to turn ON the transistor). The source and drain have free electrons, the body has free holes but no free electrons. 

Suppose the source is grounded. The junctions between the body and the source or drain are zero biased or reverse biased, so little or no current flows (Id=0).

(II) Linear Region

The gate to source voltage Vgs > Vth. Now an inversion region of electrons (majority carrier) called channel connects the source and drain, creating a conductive path and turning ON (Yesss we need this;)).

The potential difference between drain and source is Vds = Vgs - Vgd.

• If Vds = 0 (i.e. Vgs = Vgd), There is no electric field tending to push current from drain to source.
• When small positive potential Vds is applied to drain current Ids flows through the channel from drain to source. Current increase with both drain and gate voltage. This mode of operation known as linear, resistive, triode and non saturated region.

(III) Saturation Region

If Vds becomes sufficiently large that Vgd<Vt, the channel is no longer inverted near the drain and becomes pinched off. However, conduction is still brought about by drift of electrons under the influence of the positive drain voltage.

As electrons reach the end of the channel, they are injected into the depletion region near the drain and accelerated towards the drain. 

Above this drain voltage the current Ids is controlled only by the gate and ceases to be influenced by the drain this mode is called saturation.  

For the current equation refer same book i refer early of this part.

Summary of Current equation in all three regions,

where ,