Effect of AlN Cap Layer on Polarization Coulomb Field Scattering in AlGaN/GaN Heterostructure Field Effect Transistor

In this study, AlGaN/GaN heterostructure field-effect transistors (HFETs) with an AlN cap layer and a GaN cap layer were fabricated. The devices were of different sizes. Capacitance-voltage (<i>C-V</i>) and current-voltage (<i>I-V</i>) curves were measured. Based on two-dimensional (2D) scattering theory, electron mobility corresponding to polarization Coulomb field (PCF) scattering and other primary scattering mechanisms was quantitatively determined. The influence of the AlN cap layer on PCF scattering in AlGaN/GaN HFETs was studied. It was found that the AlN cap layer suppresses the inverse piezoelectric effect (IPE) in the AlGaN barrier layer because of its greater polarization and larger Young's modulus, thereby reducing the generation of additional polarization charge (APC) under the gate. In addition, the 2D electron gas (2DEG) density (<i>n</i>_2DEG) under the gate of the samples with an AlN cap layer is higher. Both factors help reduce PCF scattering intensity. Moreover, mobility analysis of samples with different gate-drain spacings (<i>L</i>_GD) showed that PCF scattering is less affected by <i>L</i>_GD variations in devices with AlN cap layers. This study offers new insights into the structural optimization of AlGaN/GaN HFETs.