Solid State Electronic Devices, 7th Edition

6.5 The Mos Field-Effect Transistor

The MOS transistor is also called a surface field-effect transistor, since it depends on control of current through a thin channel at the surface of the semiconductor (Fig. 6–10). When an inversion region is formed under the gate, current can flow from drain to source (for an n-channel device). In this section we analyze the conductance of this channel and find the I_D – V_D characteristics as a function of gate voltage V_G. As in the JFET case, we will find these characteristics below saturation and then assume I_D remains essentially constant above saturation.

6.5.1 Output Characteristics

The applied gate voltage V_G is accounted for by Eq. (6–​28) plus the voltage required to achieve flat band:

$$V_G=V_{FB}-\frac{{\text{Q}}_s}{{\text{C}}_i}$$