Heterostructure Field-Effect Transistors for Millimeter Wave Applications
This thesis deals with the development of low-noise heterostructure field-effect transistors (HFETs) on III-V materials. The emphasis is on devices for high gain and low noise applications. Fabrication processes have been developed and transistors have been fabricated and characterized. Results from pseudomorphic AlGaAs/InGaAs/GaAs, lattice matched InAlAs/InGaAs/InP, pseudomorphic InAlAs/InGaAs/InP, pseudomorphic InGaP/InGaAs/ InP, and metamorphic InAlAs/InGaAs/GaAs materials are presented.
Lattice matched InAlAs/InGaAs/InP materials have been the primary material in our laboratory for some years. This material has been used for both hybrid circuits and MMICs. HFETs on this material with a gate length of 0.15 .my.m have reached an fmax of 380 GHz and an fT of 140 GHz.
Metamorphic GaAs based materials with 80 % In in the channel have been evaluated by characterizing sub-micron HFETs. An fmax of 230 GHz has been achieved on these materials. After insertion of a thin InAs layer in the In0.8Ga0.2As channel, an fT of 100 GHz and a transconductance of 1100 mS/mm were achieved for a 0.3 µm gate length device. These devices exhibit low noise and extremely high gain at low voltages and are very promising for applications where high speed and low power consumption are required. An fmax of 100 GHz was achieved at Vds=0.25 V and Ids=26 mA/mm.
Small-signal modeling of HFET devices is described. By measuring S-parameters of the devices for 1-50 GHz, bias dependent small signal models are extracted and used for both device and material evaluation, as well as in circuit design. We have developed an improved small signal model for millimeter wave frequencies. This model has been validated by measurements at 75-110 GHz.
MMIC circuits have been designed and fabricated, including a 115 GHz single stage amplifier with 5 dB gain and a 110 GHz resistive mixer with 9 dB conversion loss at an LO power of 4 dBm. These circuits were fabricated on lattice matched InAlAs/InGaAs/InP material. Processes developed for fabricating the discrete HFETs as well as the HFET based MMICs reported in this thesis are described in detail.