The above models are valid for the on-state regime, in which the gate bias, VG, is greater than the threshold voltage, VT. A further operating regime is described in which VGVT, and this is known as the sub-threshold regime. This is a low current regime, but one in which the channel current increases exponentially with gate bias (in contrast to the linear increase in the high-current, linear regime). Device operation in this regime is characterised by the sub-threshold slope, S, and this is commonly used as a figure of merit for TFT operation.