| dc.description.abstract | This thesis deals with the fabrication and characterization of millimeter-wave transistors based on AlInGaN/GaN heterostructure grown on a high-resistivity silicon substrate. The research includes the improvement of ohmic contact pre-processing, the design of Γ-gate structure, and the design of gate width. After evaluating the stability, yield, and trade-offs between RF characteristics, the gate length is finally defined as 150 nm. The Γ-gate structure is compared with the conventional T-gate structure to evaluate the influence of different gate structures on DC/RF characteristics by the small-signal and large-signal measurement. Additionally, transient characteristics are also measured to analyze the source of characteristic degradation and provide a reference for device reliability.
To achieve high stability and high yield for the Γ-gate structure, a double-layer photoresist structure of TDUR-P015/Dilute ZEP-A7 is used to expose the gate foot and head, and the Γ-gate head offset is controlled by the exposure mark. A Γ-gate structure with a gate length of 150 nm is used to improve the output power characteristics and linearity.
The DC characteristics of the high electron mobility transistors (HEMTs) fabricated in this paper showed that the current density of the Γ-gate and T-gate devices is 797.2 mA/mm and 755 mA/mm, respectively. The Γ-gate structure helped to smooth the peak electric field on the 2DEG channel, which more effectively dispersed the electric field at the edge of the gate to reduce leakage current. Specifically, the threshold voltage of Γ-gate structure shifts from -3.5 V to -3 V and increases the breakdown voltage from 80.4 V to 85.8 V compared to the T-gate structure. The small-signal characteristics with a gate length of 150 nm shows the fT/fmax of the Γ-gate and T-gate devices are 90/142 GHz and 83/130 GHz, respectively. This is attributed to the reduced gate capacitance in the Γ-gate structure. The large-signal characteristics of the devices were measured at 28 GHz with the devices biased in Class-AB condition. The saturated output power of Γ-gate and T-gate devices was 21.02/20.34 dBm, power gain was 10.22/9.73 dB, PAE was 11.87/11.78 %, and power density was 1.26/1.08 W/mm, respectively. Under Class-B operation, the saturated output power of Γ-gate and T-gate devices was 18.8/17.6 dBm, power gain was 10.01/10 dB, PAE was 13.5/12.6 %, and power density was 0.75/0.58 W/mm, respectively. These results show that the Γ-gate structure improves the power characteristics due to their higher transconductance. As to the linearity, the output third-order intercept point (OIP3) of Γ-gate and T-gate devices was 27.5/26.1 dBm at 28 GHz with Class-AB operation. Under Class-B operation, the OIP3 of Γ-gate and T-gate devices was 19.6/18.02 dBm, respectively. These results show that the Γ-gate device has better linearity than the T-gate device, because the Γ-gate device has a wider gate operation range and a flatter transconductance over a wider bias range. In the memory effect measurement results, the variation on IM3 of the Γ-gate and T-gate devices was 12.35 dBc and 11.8 dBc, respectively, under Class-B bias condition. This is because the Γ-gate device is more effective in suppressing the third-order signals, which causes the upper and lower signals to be asymmetrical. This indicates that the T-gate device exhibits more pronounced memory effect under Class-B operation. In the future, the large-signal characteristics could be further improved by increasing the thickness of the passivation layer to reduce the surface current and off-state leakage current, and increase the breakdown voltage of the devices. | en_US |