| Summary: | 博士 === 國立交通大學 === 材料科學與工程系所 === 95 === High performance InAlAs/InGaAs metamorphic high electron mobility transistors (MHEMTs) have been fabricated and characterized for high frequency applicatiions. The performance of the MHEMTs was improved by optimizing the device structure and reducing the gate length using several novel gate-shrinking techniques. The epi-structure, layout design and electrical measurements of the MHEMTs were also discussed.
In this dissertation, several novel gate shrinking processes for MHEMTs fabrication were developed. For cost-effective production of submicron MHEMTs, a 0.15-μm Γ-shaped gate MHEMT technology using Deep UV lithography and a tilt dry-etching technique was developed and demonstrated for the first time. The fabricated 0.15-μm MHEMT using this novel technique shows a drain-source current of 680 mA/mm and transconductance of 728 mS/mm. The cutoff frequency fT and maximum oscillation frequency fmax of the MHEMT are 130 GHz and 180 GHz, respectively. In addition, a 0.1 μm T-gate was achieved by thermally reflowing the bi-layer E-beam resist using hotplate and the 0.1-μm T-gate was applied to the MHEMT manufacture. Comparing with 2 step lithography of the conventional E-Beam T-gate process, the reflowed gate process is a much simpler, relatively inexpensive and flexible process.
Under 100-nm scale, a low-noise MHEMT using 90-nm sidewall T-gate process was also successfully fabricated. The noise figure of the 160μm-width MHEMT was 0.69dB and the associated gain was 9.76dB at 16GHz. Moreover, a 70-nm In0.52Al0.48As/In0.6Ga0.4As power MHEMT with double δ-doping for power application was also fabricated and evaluated. The device has a high transconductance of 827 mS/mm, high saturated drain-source current of 890 mA/mm, high fT of 200 GHz, and a high fmax of 300 GHz was achieved due to the nanometer gate length and the high Indium content in the channel. When measured at 32 GHz, the device demonstrates a maximum output power of 14.5 dBm with P1dB of 11.1 dBm and the power gain is 9.5 dB. The excellent DC and RF performance of the 70-nm MHEMT shows a great potential for the Ka band power applications.
In addition, electrical characteristics and thermal stability of the Ti/Pt/Cu contact on InAlAs Schottky layer of the MHEMT were investigated. The Ti/Pt/Cu Schottky contact had comparable electrical properties compared to the conventional Ti/Pt/Au contact after annealing. As judged from the material analysis, the Ti/Pt/Cu on InAlAs after 350°C annealing showed no diffusion sign into the InAlAs. After 400°C annealing, the interfacial mixing of Cu and the underlying layers occurred and resulted in the formation of Cu4Ti. The results show that the Ti/Pt/Cu Schottky contact using platinum as the diffusion barrier is very stable up to 350°C annealing and can be used for the InAlAs/InGaAs HEMTs and MMICs fabrication.
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