Reliability tests of N-channel metal-oxide-semiconductor field-effect transistors (NMOSFET's) with oxide thickness ranging from 3.3 nm to 1.7 nm are performed and analyzed in this work. New device failure mechanism due to gate-to-drain leakage path formation is observed, and it severely degrades the off-state performance of devices with sub 2 nm gate oxides. Among the device parameters monitored, on-state conduction current and off-state drain leakage are the two most decisive parameter, which dominate NMOSFET's functional reliability. A new unified functional reliability model is proposed and lifetime predictions due to respective device parameters can be achieved.