Illustration of prominent failure mechanisms in AlGaN/GaN HEMTs as a function of physical effects. This work limits its study to the electric field driven effects particularly in the “OFF” state.
The solid line indicates the step-voltage stress from −5 to −20 V applied to the gate terminal where source and drain are connected to ground (biasing shown in the inset). The measurements are performed at the end of the 10 min stress period (indicated by dashed line) by measuring , vs , drain current noise, and gate current noise.
Top figure (a) shows the increase in threshold voltage during stress. The inset in (a) depicts the prestress and poststress drain current transfer characteristics at . The slow threshold voltage recovery is shown in bottom figure (b).
The evolution of the gate leakage current measured at during stress is shown at each stress-step. An overall decrease is seen (indicated by the arrow) as stress is increased.
Normalized drain current spectra for prestress and poststress cases. An increase by a factor of 4 is seen indicated by the arrow. This increase was found to temporary and it fully recovered to its prestress levels a few weeks later as the dc threshold voltage recovered.
Normalized gate current noise for prestress and poststress cases. An increase by a factor of 10 can be seen (indicated by the arrow). This shift was found to be permanent and did not recover.
Gate current noise measured at 77 K for two dc biasing in triode region and saturation at a . A drastic increase in noise is seen in saturation with a distinct Lorentzian at a corner frequency of 30 Hz.
Conduction band diagram of the cross-section at the gate-drain edge (shown in the device structure on the right). Electrons tunneling in and out (shown in solid arrows) contribute to the increased dc gate leakage current and gate current noise. New trap states are created (indicated by dashed circle) in the AlGaN barrier which are above the metal Fermi level . P1 (dashed-dotted arrow) shows the detrapping of electrons from the existing traps which contribute to recovery of threshold voltage after stress. P2 (dotted arrow) shows the trapping of electrons which tunnel from the gate terminal during stress. They lead to an increase in the threshold voltage.
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