MOSFET

The characteristics of depletion mode D MOSFETs is the same as for JFETs, and so the square law (110) holds, and the curves of Figure 124 can be used. However, D MOSFETs can also be operated in enhancement mode with positive values of v_GS, see Figure 125. Formulas (111) and (112) also apply.

**Figure 125:** n-channel D MOSFET characteristics.
$\begin{figure} \begin{center} \epsfig{file=images/fetimg21.eps}\end{center}\end{figure}$

E MOSFETs can only operate in enhancement mode. When v_GS=0, there is no conducting channel and only an extremely small leakage current I_DSS can flow. In an n-type E MOSFET, positive values of v_GS larger than V_GS(th) form the conducting channel between source and drain. The characteristics are shown in Figure 126.

**Figure 126:** n-channel E MOSFET characteristics.
$\begin{figure} \begin{center} \epsfig{file=images/fetimg22.eps}\end{center}\end{figure}$

E MOSFETs are also square-law devices, and we write

i_DS = K( v_GS - V_GS(th))²

(113)

where the constant K can be determined from the data sheets for the device.

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