3.2 Super-Linear Power MOSFET Physics

RF power transistors must be operated at relatively large drain voltages with drain current saturation (i.e. with high output resistance in the drain I-V characteristics). As discussed above, the well accepted theory for current saturation in MOSFETs, based up on channel pinch-off, leads to a square-law relationship between the drain current and gate voltage. This is detrimental for RF amplification because the non-linearity introduces undesired harmonics at the output of power amplifiers. This section describes a new mode of operation that has been discovered ^[6] for MOSFETs that enables obtaining linear transfer characteristics. This new mode of operation is based up on maintaining the MOS channel of the MOSFET in the linear regime of operation while achieving current saturation by utilizing the saturated velocity-field curve for semiconductors. It is worth pointing out that this new mode of operation is distinct from the linear transfer characteristics reported for sub-micron transistors when the electrons in the inversion layer undergo velocity saturation ^[2]. In the case of these submicron transistors, the velocity saturation for electrons in the channel occurs only if the drain voltage is sufficiently high to push the longitudinal electric field above 5 10 ⁴ V/cm. At these high electric fields, the on-set of impact ionization creates hot electrons at the vicinity of the gate oxide. These hot electrons can add charge in the gate oxide resulting in undesirable shifts in the threshold voltage. Further, a sub-micron channel length cannot support the...