AlGaN barrier thickness dependent surface and interface trapping characteristics of AlGaN/GaN heterostructure

The aluminium gallium nitride (AlGaN) barrier thickness dependent trapping characteristic of AlGaN/GaN heterostructure is investigated in detail by frequency dependent conductance measurements. The conductance measurementsin the depletion region biases (?4.8 V to ?3.2 V) shows that the Al_0.3Ga_0.7N(18 nm)/GaN structure suffers from both the surface (the metal/AlGaN interface of the gate region) and interface (the AlGaN/GaN interface of the channel region) trapping states, whereas the AlGaN/GaN structure with a thicker AlGaN barrier (25 nm) layer suffers from only interface (the channel region of AlGaN/GaN) trap energy states in the bias region (?6 V to ?4.2). The two extracted time constants of the trap levels are (2.6–4.59) μs (surface) and (113.4–33.8) μs (interface) for the Al_0.3Ga_0.7N(18 nm)/GaN structure in the depletion region of biases (?4.8 V to ?3.2 V), whereas the Al_0.3Ga_0.7N (25 nm)/GaN structure yields only interface trap states with time constants of (86.8–33.3) μs in the voltage bias range of ?6.0 V to ?4.2 V. The extracted surface trapping time constants are found to be very muchless in the Al_0.3Ga_0.7N(18 nm)/GaN heterostructure compared to that of the interface trap states. The higher electric field formation across the AlGaN barrier causes de-trapping of the surface trapped electron through a tunnelling process for the Al_0.3Ga_0.7N(18 nm)/GaN structure, and hence the time constants of the surface trap are less.