dc.description.abstract | This thesis deals with the fabrication and characterization of AlInGaN/GaN on-Si high electron mobility transistors (HEMTs) for millimeter-wave applications, and the development of a T-gate process by electron beam lithography. In this paper, the TDUR-P015/ Dilute ZEP-A7 double-layer photoresist structure is used to expose the gate foot and head to achieve a highstability and high-yield T-gate process. Moreover, the gate length is reduced to 90
nm through a thermal reflow process to enhance current gain and high-frequency characteristics. This thesis also discusses the influence of the GaN cap layer with different thicknesses on high-frequency devices. In addition to the transfer and output characteristic curves, the transient characteristics of the device are also measured, and the correlation with the dislocation density of epitaxy and device characteristics are analyzed by X-ray diffraction experiment.
The electrical characteristics of high electron mobility transistors with 2 and 5 nm GaN cap layer are investigated in this work. Devices with 140 nm gate length have a current density of 876.1 and 999.6 mA/mm, respectively. When the gate length was reduced to 90 nm by a thermal reflow process, the current density can be increased to 1035 and 1082 mA/mm, respectively. The device with an 5 nm GaN cap layer exhibits better DC characteristics than that with a 2 nm cap layer. This is attributed to the thicker GaN cap, which can more effectively disperse the electric field at the edge of gate and reduce the gate leakage current. The device on/off ratio was increased from 106 to 107. In addition, the de embedded small signal measurements show that for the 2-nm and 5-nm cap devices with a gate length of 90 nm, the fT/fmax are 100.4/110.9 GHz and 130.4/144.3 GHz, respectively. The higher cut-off frequency can be attributed to the reduction of gate capacitance of the 5-nm cap device. These results are among the best reported data for devices with the same gate length. The large signal performance of the devices was measured in the Class AB bias condition at 10 GHz and 28 GHz. the PAE for the 2/5-nm cap device is 19.29/21.77%, the power gain is 17.22/16.74 dB at 10 GHz. When operated at 28 GHz, the PAE is 17.15/12.38%, and the power gain is 11.31/11.43 dB, respectively. The large signal
performance can be further improved after reducing the off-state leakage current in the future. | en_US |