Surface trapping effects observed in AlGaN/GaN HFETs and heterostructures

We have observed very slow surface potential transients in AlGaN/GaN HFETs induced by high drain and gate bias stresses. The surface potential has been observed to change as much as 2.5 V near the gate. It is proposed that the change in surface potential is caused by electrons that tunnel from the gate and get trapped at the surface states. The increase in the net negative charge at the surface raises the surface barrier, which in turn reduces the 2DEG concentration. Simultaneous measurements of surface electrostatic potential and drain current indicate that the transients have similar transient responses and are therefore related. A spatial map of the surface potential change after stress with respect to time shows that maximum changes occur close to the gate. Exposure to UV laser light completely eliminates the transients, while large reduction in transient magnitude has been observed for devices passivated with SiN_x. Contrary to the gate and drain stress, UV laser exposure of AlGaN/GaN heterostructure samples has been observed to reduce the surface barrier, which slowly increases following a stretched exponential type transient after the laser is switched off. Such an observation is explained by the creation of electron-hole pairs, which decrease the net charge dipole across the AlGaN barrier and lower the barrier. The transient response is modeled by thermionic emission of electrons from the interface, which recombine with the holes trapped at the surface.