The effects of oxide stress waveform on MOSFET performance

Three different waveforms were used to study the behavior of n-MOSFETs after dynamic oxide stress. It is shown that the transport and detrapping properties of holes generated near the anode can account for the differences observed after unipolar and bipolar oxide stressing. Bipolar stress yields a lower rate of bulk charge trapping and longer time to breakdown than unipolar stress. The difference is dependent on both field and frequency. At the same time, bipolar stress leads to enhanced interface trap generation. Since the gate-drain overlap region of MOSFETs in circuits is subject to bipolar stress, DC transistor stress tests may underestimate the MOSFET degradation rate. The observation that interface trap density does not saturate during bipolar stress as readily as during unipolar stress is particularly alarming. Bidirectional tunneling current creates a larger number of interface traps than does unidirectional gate current. Even though unipolar stressed devices suffer more bulk trapping and TDDB (time-dependent dielectric breakdown) degradation than do bipolar stressed devices, the degradation in transconductance, mobility, and threshold voltage is worse in bipolar stressed devices.