IGFET circuit performance-n-channel versus p-channel

The n-channel insulated-gate field-effect transistor offers a factor of 2 to 3.4 mobility advantage (depending on crystal orientation and substrate doping level) over p-channel devices. In addition, several advantages result from the fact that the work function difference between an aluminum gate and the silicon substrate is about -0.8 volt for a p substrate compared with about zero for an n substrate. In particular, this results in a low threshold voltage that allows the use of a substrate bias to adjust the threshold voltage over a useful design range resulting in an added flexibility in choice of thresholds and substrate doping, a reduction in the effect of source-substrate bias on device threshold, decreased junction capacitance, and larger parasitic thick-oxide thresholds for a given insulator thickness. The speed, power, and density advantages of the n-channel device are illustrated for logic and memory circuits using representative n- and p-channel device designs.