We analyze the effects of stress in off-state condition at increasing drain bias. The main variations are an increase in on resistance (Ron) and in threshold voltage (Vth) and a decrease in the peak transconductance value, whereas the gate diode remains stable during the test. The Ron and Vth variations are correlated, suggesting a common degradation mechanism for both effects.
By means of sampled filling and recovery measurements, it is possible to highlight a trapping process occurring in off-state condition and dependent on the drain filling bias. This process is related to deep levels located both in the access regions and in the region under the gate, since it results in both Ron and Vth shifts. By means of temperature-dependent recovery measurements, a dominant thermal activation energy of 0.30 eV was found. The good fit quality according to the “stretched exponential” model indicates that the deep levels are extended defects or energy mini-bands.
The amplitude of the recovery transient collected after the same filling condition increases after stress at higher drain voltage, confirming that the deep levels causing the detected dynamic variation are increasing in concentration as a consequence of the stress. The good correlation between the amplitude and the Ron variation suggests that the detected variation in concentration is the root cause for the degradation of the device.