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.

We fabricated and evaluated a second-order ΣΔ ADC with a polarity alternating feedback (PAF) comparator based on 0.4 µm InGaP/InGaAs enhancement and depletion mode high electron mobility transistors (E/D HEMT) technology. We propose a PAF technique for enhancing the sampling frequency and have applied the technique in the design of ADC circuit. The ADC has a signal-to-noise ratio (SNR) of 43 dB when operating at a differential clock frequency of 4.9 GHz, and has a power dissipation of 400 mW.