Modes of MOS TRANSISTOR

Explain the accumulation (Enhancement) mode, depletion layer and inversion layer of MOS transistor with diagram

1.  The MOS transistor is a majority-carrier device, in which the current in a conducting channel is controlled by gate voltage.
2.  In an nMOS transistor, the majority carriers are electrons.
3.  In a pMOS transistor, the majority carriers are holes.
4.  Figure 2 shows a simple MOS structure. The top layer of the structure is a good conductor called the gate.
5.  Transistor gate is polysilicon, i.e., silicon formed from many small crystals. The middle layer is a very thin insulating film of SiO2, called the gate oxide. The bottom layer is the doped silicon body.
6.  The figure 2 shows a p-type body, in which the carriers are holes. The body is grounded and voltage is applied to the gate.
7.  The gate oxide is a good insulator, so almost zero current flows from the gate to the body.

Accumulation (Enhancement) mode:

•  In Figure 2(a), when a negative voltage is applied to the gate, negative charges are formed on the gate. 
•  The positively charged holes are attracted to the region under the gate. This is called the accumulation mode. 

Depletion mode:

•  In Figure 2(b), when a small positive voltage is applied to the gate, positive charges are formed on the gate.
•  The holes in the body are repelled from the region directly under the gate, resulting in a depletion region forming below the gate.

Inversion layer:

•  In Figure 2(c), when a higher positive potential greater than threshold voltage (Vt) is applied, more positive charges are attracted to the gate.
•  The holes are repelled and some free electrons in the body are attracted to the region under the gate. This conductive layer of electrons in the p-type body is called the inversion layer.
•  The threshold voltage depends on the number of dopants in the body and the thickness tox of the oxide.

MOS structure demonstrating (a) accumulation, (b) depletion, and (c) inversion layer