The operation can be explained with two different operating conditions:
Operation with V_GS = 0 and Operation when V_GS is positive.
Operation with V_GS =0 volt:
If V_GS = 0 and a positive voltage is applied between its drain and source (positive $V_DS$), then due to the absence of the n-type channel, a zero drain current will result. This is exactly opposite to what happens in the depletion-type MOSFET, where $I_D$ = $I_DSS$ at $V_GS$ = 0.
Operation when $V_GS$ is positive:
Refer to fig. 2.2, where both $V_GS$ and $V_DS$ are positive. The positive potential at the gate terminal will repel the holes present in the p-type substrate as shown in Fig. 2.2.
This results in creation of a depletion region near the $SiO_2$ insulating layer. But the minority carriers i.e. the electrons in the p-type substrate will be attracted towards positive gate terminal and gather near the surface of $SiO_2$ as shown in Fig. 2.2.
As we increase the positive $V_GS$, the number of electrons gathering near the $SiO_2$ layer will increase.
The electron concentration near $SiO_2$ layer increases to such an extent that it creates an induced n-channel which connects the n-type doped regions.
The drain current then starts flowing through this induced channel. The value of $V_GS$ atwhich this conduction begins is called as the "threshold voltage" and is indicated by $V_T$ or $V_GS$ (TH).
For output characteristic Refer Chap2, Q10
The expression for I_D is a nonlinear relation and it is valid only for $V_GS\gtV_TN$.
The dotted curve in fig 2.9 represents the boundary between the saturation region and the non-saturation region.
In the ideal MOSFET, the drain current is constant for $V_GS\gtV_(DS(sat))$that means when the MOSFET is in the saturation region. The expression for$ V_(DS(sat))$ is given by, $V_(DS(sat))$= $V_GS- V_T$.
The region for which V_DS<v_(ds(sat))is known="" as="" the="" non-saturation="" region.="" in="" the="" saturation="" region,="" the="" drain="" current="" is="" independent="" of="" the="" drain="" to="" source="" voltage="" $V_DS$="" which="" is="" evident="" from="" the="" following="" expression.="" <="" p="">
Expression for $I_D$ different regions:
The expressions for $I_D$ of an E-MOSFET are different for different regions of operation.
We know that the expression for drain current in the saturation region is,
$I_D=k_n[V_GS-V_TN]^2$
The region for which $V_DS$<$V_(DS(sat))$ is called as the nonsaturation or triode region. The ideal current voltage relation in the non-saturation region is given by
$I_D=k_n[2(V_GS-V_TN)V_DS-D_DS^2]$
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