2.2 MOS Device Current -Voltage Equations

This section first derives the current-voltage relationships for various bias conditions in a MOS transistor. Although the subsequent discussion is centred on an nMOS transistor, the basic expressions can be derived for a pMOS transistor by simply replacing the electron mobility by the hole mobility and reversing the polarities of voltages and currents.

As mentioned in the earlier section, the fundamental operation of a MOS transistor arises out of the gate voltage V_GS (between the gate and the source) creating a channel between the source and the drain, attracting the majority carriers from the source and causing them to move towards the drain under the influence of an electric field due to the voltage V_DS (between the drain and the source). The corresponding current I_DS depends on both V_GS and V_DS .

2.2.1 Basic DC Equations

Let us consider the simplified structure of an nMOS transistor shown in Figure 2.8, in which the majority carriers electrons flow from the source to the drain.

Fig 2.8: Simplified geometrical structure of an nMOS transistor

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