Threshold voltage and sub-threshold slope variation with gate-length in Al2O3/InAlAs/InGaAs QW FETs

During the last 2-3 years there has been an intensive research on InGaAs quantum well channels [1-4] as a possible alternative to the traditional Si-SiO₂ FETs. Both Al₂O₃ [1-3] and HfO₂ [4] are used as gate oxides. The fabricated devices with available current-voltage characteristics in the literature are of long gates. Typically the gate lengths Lg vary from 200 nm to 10 μm. These devices however have to be scaled to the 20nm regime if they are to replace the Si-SiO₂ system. A theoretical calculation tool that would show the trend from the micrometer values of Lg to the tenths of nm values would be valuable. In this paper we examine theoretically the Al₂O₃/InGaAs system with a variable gate length Lg. The gate metal is tungsten (W) as employed in the fabricated devices. We will see that it is a very good choice for having the threshold value V_T near zero volts. Furthermore, in accordance with experiment, a cap layer of InAlAs-which also acts as a supply layer-is used so that finally our system consists of the layers: W/Al₂O₃/AlGaAs/InGaAs. This is shown in figure 1 together with the relevant lengths.