Double-charge-sheet model for thin silicon-on-insulator films

A simple algorithm is proposed that facilitates the calculation of surface potentials and charge densities at the front and back interfaces in thin silicon-on-insulator (SOI) layers by decoupling of the potentials and charges at the two interfaces. An expression relating the front surface potential and inversion charge to the front and back gate biases is derived and compared with a numerical solution of Poisson's equation. The charge-sheet model agrees well with the simulation results over the front-surface bias range from weak to heavy inversion and with the back silicon surface biased into accumulation, depletion, and inversion. The results are shown to be reasonably accurate for all doping densities of common interest and for SOI film thicknesses above approximately 20 nm. An extension of the model to a nonequilibrium system is used to derive an expression for the drain current in a fully-depleted SOI MOSFET. Other applications of the model include a closed-form analytical solution for the threshold voltage and a calculation of the interface-state trapped charge.