EFFECT OF SILICON FILM THICKNESS ON THRESHOLD VOLTAGE OF SOS-MOSFETS

The variation in threshold voltage tVth of silicon-on-sapphire MOSFETs (SOS-MOSFETs) is experimentally and theoretically studied as a function of the epitaxial film thickness t in the thickness range of 0.18-0.80 μm. Threshold voltages of both n- and p-channel MOSFETs are found to shift drastically toward the more enhancement mode as t decreases. This phenomenon is not explained by a variation of the doping concentration in the film nor by a change in the SiSiO2 interface charge. A model of two deep levels (upper acceptorlike and lower donorlike) due to defects distributed exponentially in the film is presented, from whcih the Vth are numerically obtained as a function of t. This treatment explains well the experimental shifts of Vth. © 1979.