A.1.2.2 Subthreshold Current

When the gate voltage is below the threshold voltage, which is the so-called weak-inversion condition, the electron density at the interface decreases to a small but finite value, which depends exponentially on the channel potential. Thus, even when the device is actually ``turned off'' a small current which is essentially a diffusion current, flows from drain to source when $\ensuremath{V_{\mathit{DS}}}\xspace \ne 0$:

$$\ensuremath{I_{\mathit{D}}}\xspace = \ensuremath{\beta }\xsp... ...t{DS}}}\xspace }{\ensuremath{U_{\mathit{T}}}\xspace }}\right) ,$$ (A.14)

where $n = 1+(\ensuremath{X_{\mathit{d}}}\xspace /\ensuremath{t_{\mathit{ox}}}\xspace ... ...epsilon _{\mathit{s}}}\xspace /\ensuremath{\epsilon _{\mathit{i}}}\xspace ) > 1$ is a non-ideality factor. Thus, the drain current in subthreshold operation depends exponentially on . The required voltage to change by one decade is $\Delta V = \ensuremath{S}\xspace = n\ensuremath{U_{\mathit{T}}}\xspace \ln(10)$ which is the so-called gate swing.