Threshold voltages of AlGaN/GaN metal-insulator-semiconductor devices with AlN or Al2O3 gate insulators

We investigated the threshold voltages of AlGaN/GaN metal-insulator-semiconductor (MIS) devices with AlN gate insulators deposited by electron cyclotron resonance (ECR) sputtering (AlN/AlGaN/GaN MIS devices), in comparison with those of AlGaN/GaN MIS devices with atomic-layer-deposited (ALD) A1
2
O
3
gate insulators (A1
2
O
3
/AlGaN/GaN MIS devices). The MIS devices with several gate insulator thicknesses were systematically fabricated and characterized, where the threshold voltages exhibit linear dependences on the insulator thickness for both AlN/AlGaN/GaN and A1
2
O
3
/AlGaN/GaN MIS devices. However, we found that the slope of the dependence, dominated by the insulator-semiconductor fixed charges, is positive for the AlN/AlGaN/GaN MIS devices, whereas being negative for the A1
2
O
3
/AlGaN/GaN. Moreover, we found that the sheet concentration of the two-dimensional electron gas at zero gate voltage is a decreasing function of the insulator thickness for the AlN/AlGaN/GaN MIS devices, while being an increasing function for the A1
2
O
3
/AlGaN/GaN MIS devices. This indicates that positive fixed charges at the AlN/AlGaN interface are suppressed in comparison with A1
2
O
3
/AlGaN interface. According to the suppression of the positive fixed charges, the AlN/AlGaN interface is negatively charged, leading to the behavior of the threshold voltage and the sheet concentration of the two-dimensional electron gas.