This paper examines in detail the phenomenon of Substrate Enhanced Electron Injection (SEEI). By using floating gate devices less aggressively scaled than the MOSFETs of Bude (1995) we are able to: (1) develop criteria to separate SEEI from the coexisting channel hot electron (CHE) injection; (2) point out a direct proportionality between the gate (I/sub G/) and the substrate (I/sub B/) currents that provides a signature of SEEI; (3) reconcile SEEI with reported mechanisms of optical minority carrier generation in the substrate.