Investigation of Non-ideal Characteristics in AlGaN∕ GaN High Electron Mobility Transistors

• Main Authors: Sheng Yu Liao, 廖笙佑
• Other Authors: L. B. Chang
• Format: Others
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/69352420627550685947

博士 === 長庚大學 === 電子工程學系 === 101 === In recent years, AlGaN/GaN high electron mobility transistor (HEMT) become an important technology because of their high power density, power handling capabilities, high temperature applications and high electronic velocities. Although the RF technology is rapid progress, the reliability issue uncertainly and the limiting operation performance should be discussed. In this dissertation, we presented the non-ideal effect on AlGaN/GaN HEMTs and used different measurement to suppress this phenomenon. First, the DC and RF power characteristics of GaN HEMTs under continuous wave (CW) and pulsed load-pull measurement are examined. The results give a comprehensive understanding of self-heating effects and allow improved heat dissipation by pulsed measurement. The RF power performance for pulsed mode was measured at 3.5 GHz, with 18.4 dB power gain and a large 3.5W/mm power output, under pulse load-pull, which is a 3.25 dB improvement, compared to CW operation. Then, we investigate the grain size effect of HEMT devices with ohmic contact metals of stacked Ti/Al/Ni/Au and Ti/Al/Mo/Au. In addition to a comparison of electrical characteristics, the ohmic contacts were also examined by a scratch test for the observation of adhesion behavior. The experimental results demonstrate that the metal grain size is strongly dependent to metal adhesion that may lead to bonding issue. Besides, the grain-induced lateral stress lowers the drive current and increase off-state current due to the degraded gate swing and transconductance of transistor switching characteristics. Subsequently, we investigate a gate leakage effect on AlGaN/GaN high mobility transistor using negative gate bias stress. After constant voltage stress, the increased off current is unavoidable due to Schottky barrier lowering and new defect center creation on the surface of AlGaN barrier layer. However, the gate-stressed HEMT device has a good immunity to sub-thresold swing and transconductance degradation, which can be attributed to a diluted vertical electric field by high-work-function Ni tuning to reduce large vertical electron tunneling current, which is much different to reported results using positive gate bias stress. Finally, we report AlGaN/GaN and lattice-matched AlInN/AlN/GaN HEMTs with 0.15μm T-gate length. Compared to conventional AlGaN/GaN HEMT, the AlInN/AlN/GaN one exhibits much higher current density and transconductance of 1558 mA/mm at Vd=2V and 330mS/mm, respectively. Besides, we find that the transconductance roll-off is significant in AlGaN/GaN, but largely improved in AlInN/AlN/GaN HEMT, suggesting that the high carrier density and lattice-matched epitaxial heterostructure is important to reach both large RF output power and high operation frequency, especially for an aggressively gate length scaling.