Why the Universal Mobility Is Not

Examples taken from ultrathin silicon-on-insulator (SOI) transistors tend to contradict the universality of mobility-field dependence. We revisit the meaning of the effective field concept and its implications on the universal mobility curve (UMC). Poisson-Schroedinger simulations point out the inappropriateness of the standard definitions of effective field when dealing with SOI or double-gate devices. Different carrier distributions can lead to the same value of the effective field breaking the foundation of the universality. The presence of two different gate stacks, the coexistence and coupling of two channels, and the spreading of carriers in the body are interesting nonlocal effects that are not accounted for by the UMC. Selected practical results showing the UMC failure in SOI metal-oxide-semiconductor field-effect transistors are presented. The actual behavior of the effective mobility is illustrated, shedding light on the limitations of the universal mobility/effective field representation.