The built-in bipolar junction transistor (BJT) of a BIMOS fabricated in 28 nm ultra-thin body and BOX (UTBB) fully-depleted silicon-on-insulator (FD-SOI) high-k metal gate technology is investigated in common-emitter mode and in built-in metal-oxide-semi-conductor field effect transistor (MOSFET) off-state. In the weak V-BE regime, field-effects dominate, generating a negative base current and making the current gain beta(0) meaningless. For V-BE high enough, the BJT works normally but with a very low gain below 1.