Monolithic Integration of Thermally Stable Enhancement-Mode and Depletion-Mode Indium Phosphide HEMTs Utilizing Iridium-Gate and Silver-Ohmic Contact Techniques
Zhao, Weifeng
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https://hdl.handle.net/2142/80990
Description
Title
Monolithic Integration of Thermally Stable Enhancement-Mode and Depletion-Mode Indium Phosphide HEMTs Utilizing Iridium-Gate and Silver-Ohmic Contact Techniques
Author(s)
Zhao, Weifeng
Issue Date
2006
Doctoral Committee Chair(s)
Adesida, Ilesanmi
Department of Study
Electrical and Computer Engineering
Discipline
Electrical and Computer Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Excellent ohmic contacts based on Ge/Ag/Ni metallization have been developed for InAlAs/InGaAs/InP high electron mobility transistors (HEMTs) in a temperature range between 385 and 500°C for the first time. A minimum contact resistance of 0.06 O-mm was obtained at an annealing temperature of 425°C for 60 s in a rapid thermal annealing system. Thermal storage tests showed that the Ag-based ohmic contacts had far superior thermal stability than the conventional AuGe/Ni ohmic contacts for InAlAs/InGaAs/InP HEMTs. Auger electron spectroscopy and transmission electron microscopy study for both Ag-based and Au-based ohmic contacts confirmed that metal protrusions created by diffusion of contact metals linked the two-dimensional electron gas layer with the metal contacts to produce excellent ohmic contacts. The formation of liquid AuGe eutectic phase in the AuGe/Ni/Au metallization at relatively low temperature is believed to cause overannealing. The eutectic temperature of Ag-Ge is about 300°C higher than that of the Au-Ge system, leading to a much wider processing window and much better thermal stability for the Ge/Ag/Ni ohmic contacts. After a detailed electrical and microanalysis study of Shottky diodes based on Ir/Ti/Pt/Au metallization, a fabrication process for the realization of thermally stable InAlAs/InGaAs/InP enhancement-mode HEMTs based on annealed Ir-gate and Ge/Ag/Ni-ohmic contacts were developed subsequently. The enhancement-mode InAlAs/InGaAs/InP HEMTs with a gate-length of 0.1 mum demonstrated excellent DC and RF characteristics including: threshold voltage of 198 mV, Gm, max of 945 mS/mm, IDSS of 0.25 mA/mm, and fT of 205 GHz. Thermal stability study of these devices showed that no degradation of the extrinsic transconductance and drain current was observed at storage temperature of 215°C for a storing period of 96 h. At the same time, stable operation of the Ir/Ti/Pt/Au-gate contact was also obtained. Finally, by optimizing the HEMT layer structure and the doping concentration, monolithic integration of depletion-mode and enhancement-mode InAlAs/InGaAs/InP HEMTs were realized through the newly developed fabrication process based on Ge/Ag/Ni-ohmic and Ir-gate contact technologies, which are highly applicable to high speed integrated circuits.
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