Effects of Al thickness in Ti/Al/Ti/Au ohmic contact on AlGaN/GaN heterostructures are studied. Samples having Al thickness of 30, 90 and 120 nm in Ti/Al/Ti/Au have been investigated by electrical and X-ray photoelectron spectroscopy (XPS) depth profile analysis. It is found that thick Al samples show lower resistance and formation of Al-based alloy under the oxidized Al layer.

Low-power dry-etching process has been adopted to study the influence of dry-etching on Ohmic contact to p-GaN. When the surface layer of as-grown p-GaN was removed by low-power SiCl4/Cl2-etching, no Ohmic contact can be formed on the low-power dry-etched p-GaN. The same dry-etching process was also applied on n-GaN to understand the influence of the low-power dry-etching process. By capacitance-voltage (C-V) measurement, the Schottky barrier heights (SBHs) of p-GaN and n-GaN were measured. By comparing the change of measured SBHs on p-GaN and n-GaN, it was suggested that etching damage is not the only reason responsible for the degraded Ohmic contacts to dry-etched p-GaN and for Ohmic contact formatin, the original surface layer of as-grown p-GaN have some special properties, which were removed by dry-etching process. To partially recover the original surface of as-grown p-GaN, high temperature annealing (1000C 30 s) was tried on the SiCl4/Cl2-etched p-GaN and Ohmic contact was obtained.

Single-crystal LiNbO3 and LiTaO3 piezo-electric resonators were developed for surface-mount technology (SMT) used in electronic equipment manufacturing. Using an energy-trapping design, a shear-mode piezoelectric resonator chip is bonded directly to the board with conductive resin and covered with a ceramic cap. The process occasionally produces nonlinear resonators, however, which led us to study the frequency characteristics of impedances for the abnormal samples. Their input impedances at the resonant frequency depended on the driving voltage. The insulator between the thin film metal electrode on the crystal strip or the thick film electrode on the ceramic base, in conjunction with silver balls in the adhesive resin, apparently caused the problem. Assuming that the insulator makes diode contacts, which show stable nonohmic phenonena or cause a discharge in a conductor causing a drastically changing in the impedance, we proposed the following corrective action：subject the nonohmic contacts to a high-voltage frequency-swept signal near the resonant frequency. The samples subjected to the high voltages recovered metalic contact and maintained even after severe thermal cycle testing.

We fabricated and investigated HEMTs with nonalloyed ohmic contacts using highly conductive n+-ln0.5Ga0.5 As contact layers. We optimized the growth condition of n+-In0.5 Ga0.5As contact layers by MOVPE. Using WSi/W nonalloyed ohmic electrodes, we fabricated InGaP/InGaAs/GaAs pseudomorphic HEMTs with thermal stability of 500 for 30 minutes. In order to examine the scalability of HEMT devices, we tried to reduce the total size of HEMT devices to 3.2 µm using nonalloyed ohmic electrodes, which is the smallest value as far as we know. We could reduce the nonalloyed ohmic contact length Loh to 0.4 µm without degrading the device characteristics. Reducing the n+-In0.5Ga0.5As contact length LIGA to l µm however, decreased the transconductance gm by about 20%. We found that the scaling of the conventional nonalloyed HEMT structure is limited by LIGA.