Accumulation hole layer in p-GaN/AlGaN heterostructures

We present the results on piezoelectric and pyroelectric doping in AlGaN-on-GaN and GaN-on-AlGaN heterostructures and demonstrate $\mathrm{p-}\mathrm{GaN/AlGaN}$ structures with accumulation hole layer. Our results indicate that polarization charge can induce up to ${\text{5×10}}^{13}\hspace{0.17em}{\mathrm{cm}}^{\text{−2}}$ holes at the AlGaN/GaN heterointerfaces. We show that the transition from three-dimensional (3D) to two-dimensional (2D) hole gas can be only achieved for hole sheet densities on the order of ${10}^{13}\hspace{0.17em}{\mathrm{cm}}^{\text{−2}}$ or higher. At lower densities, only 3D-hole accumulation layer may exist. These results suggest that a piezoelectrically induced 2D-hole gas can be used for the reduction of the base spreading resistance in AlGaN/GaN-based heterostructure bipolar transistors.