Threshold gate voltage and subthreshold swing of the ultrathin silicon-on-insulator field effect transistor: Analytical model

Threshold gate voltage vth and subthreshold swing s are both the key characteristics of a field effect transistor (FET) operating in the ON/OFF mode. In this mode, the electron concentration in the transistor channel is low. Therefore, in an ultrathin silicon-on-insulator FET, where the electron energy is quantized, electrons occupy only the lowest quantum subband and become two-dimensional (2D). This electron property allowed to derive analytical expressions for such single/double gate FET characteristics. The developed model relates the vth rise at the Si-film thinning with the increasing of electron energy in the channel by quantization. The limit of swing s is identical to that in bulk FETs, but its origin is different: the gate voltage drop entirely across the source-channel barrier with the electron Boltzmann distribution is the cause of the s limit. By contrast, in the bulk FET, the gate voltage drop across the surface barrier is a reason for this limit. The obtained expressions are also valid above the threshold gate voltage, if electrons remain 2D.