Performance and physical mechanisms in SIMOX MOS transistors operated at very low temperature

The performance and the physical properties of SIMOX (separation by implantation of oxygen) MOS transistors are studied from room to liquid helium temperatures with particular emphasis on the behavior of carrier mobility, threshold voltage, subthreshold swing, leakage current, and kink effect. Various SIMOX substrates, such as partially depleted films annealed at low or high temperature and ultrathin films (100 nm), are analyzed and compared. Enhancement- and depletion-mode devices with different doping levels, channel lengths, and geometries are considered. The front and back channels are activated independently in order to assess the electrical quality of both interfaces. Comparison with bulk Si transistors reveals a number of interesting features of SIMOX devices, which are explained using comprehensive models. The advantages of low-temperature operation of SIMOX transistors are related to the decrease in subthreshold swing and leakage, increase in mobility, and reasonable shift of the threshold voltage. The performance of ultra-thin-film devices is excellent over the whole range or temperatures, whereas partially depleted transistors exhibit optimum performance at 77 K