This paper, for the first time, shows that the work-function variation (WFV) in emerging metal-gate devices results in significant fluctuation in the gate-oxide electric field, and hence fluctuation in bias temperature instability (BTI) characteristics (both NBTI and PBTI). We modify the existing NBTI and PBTI models in order to accurately characterize the BTI characteristics of the metalgate devices. It is shown that the impact of the oxide electric field on threshold voltage degradation is substantially underestimated if WFV is neglected. Moreover, in FinFET devices, work-function variation Induced electric field (which is independent of the gate-source bias) not only results in fluctuation in the BTI characteristics, but also causes variation in the recovery process. It is highlighted for the first time that WFV induced BTI fluctuation can have significant impact on the performance and reliability characteristics of digital circuits such as SRAM cells and Domino logic gates.