A.1.2.1 Strong Inversion

When $\ensuremath{V_{\mathit{G}}}\xspace > \ensuremath{V_{\mathit{T}}}\xspace$ the electron density in the channel in thermal equilibrium is larger than the hole density in the bulk, $n>\ensuremath{N_{\mathit{A}}}\xspace$, which is called the strong-inversion condition. In this case the drain current in the linear region and saturation region can be approximately described using the same equations as for the structure in Fig. A.1:

$$\ensuremath{I_{\mathit{D}}}\xspace = \ensuremath{\beta }\xsp... ...th{V_{\mathit{GS}}}\xspace -\ensuremath{V_{\mathit{T}}}\xspace$$ (A.11)

$$\ensuremath{I_{\mathit{D}}}\xspace = \frac{1}{2}\ensuremath{... ...th{V_{\mathit{GS}}}\xspace -\ensuremath{V_{\mathit{T}}}\xspace$$ (A.12)

where the conductance parameter is defined as

$$\ensuremath{\beta }\xspace = \frac{W}{L} {\mu \ensuremath{C_{\mathit{ox}}}\xspace } .$$ (A.13)

Note, that these equations are valid only for devices with electrically long channels, i.e., $\ensuremath{L}\xspace \gg 2\ensuremath{X_{\mathit{d}}}\xspace$. Furthermore, as the depletion zone, which determines , actually depends on the electrostatic potential in the channel and on the source and drain voltages, these equations can be quite inaccurate for realistic devices with smaller dimensions and more aggressive operating conditions.