The expressions in the last slide derived for I_DS are valid for both the enhancement and the depletion mode devices. However, the threshold voltage for the nMOS depletion mode devices (generally denoted as V_td ) is negative .

Figure 2.9 depicts the typical current-voltage characteristics for nMOS enhancement as well as depletion mode transistors. The corresponding curves for a pMOS device may be obtained with appropriate reversal of polarity. For an n -channel device with = 600 cm²/ V.s, C₀ = 7 X 10^-8 F/cm² , W = 20 m, L = 2 m and V_th = V_T0 = 1.0 V, let us examine the relationship between the drain current and the terminal voltages.

Now, the current-voltage equation (2.2) can be written as follows.

If one plots I_DS as a function of V_DS , for different (constant) values of V_GS , one would obtain a characteristic similar to the one shown in Figure 2.9. It may be observed that the second-order current-voltage equation given above gives rise to a set of inverted parabolas for each constant V_GS value.