Time-dependent degradation of hydrogen-terminated diamond MESFETs

In this paper, we discuss the changes in the electrical performance induced by operating time in hydrogen-terminated diamond MESFETs for high power and high frequency applications. During the single stress step an increase of the current flowing in the sample is visible, possibly caused by the self-heating of the sample, as supported by temperature-dependent measurements, and by charge detrapping processes. In the full experiment the drain current was found to decrease, whereas the gate current remains below the detection limit. In the characterization phase, we detected an increase in on-resistance, a decrease in the saturation current, a shift in the threshold voltage and a decrease in the transconductance peak. We found a time-dependent behavior for all these parameters, showing a further worsening up to 10 minutes after the end of the stress step. The time-dependent behavior is related to the creation of defects inside the structure and not to the self-heating, since the dynamic variation was found to increase as a consequence of stress, whereas the power dissipation decreases. The increase in the concentration of defects with activation energy of 0.30 eV was confirmed by ON-resistance and threshold voltage transient spectroscopy. The variations in on-resistance and threshold voltage are not correlated in the full duration of the stress, suggesting that the generation of defects has (i) a different impact or (ii) a different generation rate in different parts of the device, with (iii) a possible role of the worsening of the contacts. Furthermore, a decrease in electroluminescence with higher magnitude than the decrease in drain current was found, compatible with an increased carrier-defect scattering.