The effects of fixed bulk charge on the characteristics of metal-oxide-semiconductor transistors

The theory of the characteristics of the MOS transistors is developed based on a model in which both the bulk charge due to the ionized impurity in the semiconductor substrate and the difference between the electrostatic potential and the voltage drop in the channel are included. A detailed comparison of the theory is made with experimental data of gate capacitance, drain current voltage characteristics, and transconductance characteristics on both N-channel and P-channel silicon devices with thin (2000 A) and thick (6200 and 8400 A) oxides under the gate electrode. The correlation is good using the surface mobility as the adjustable parameter. Mobility reduction in the saturation transconductance characteristics is predicted in the theory and demonstrated in the experimental data. It arises entirely from the bulk charge, which modifies the device characteristics, and is not associated with some basic surface scattering phenomena, which further reduce the mobility. It is also demonstrated experimentally that to evaluate a physically meaningful surface mobility from the conductance of the channel, the interface surface state charge Qsscannot be assumed constant in the devices used in this study.