OVERVIEW OF RECENT DEVELOPMENT OF HEMTS IN THE MM-WAVE RANGE

Recent expansion in the demand for high frequency applications requires new transistors with better performance than the MESFETs, especially in the upper-microwave and mm-wave frequency ranges. To realize better transistors than the MESFET by utilizing features of the heterostructure, high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) are being developed. GaAs pseudomorphic HEMTs (PHEMTs) are currently the main type of low noise transistors being used in various microwave and mm-wave systems. Very low noise figures of 1.5 dB at 60 GHz and 2.1 dB at 94 GHz are achieved with GaAs PHEMTs. Furthermore, InP HEMTs, with better noise performance than GaAs PHEMTs, are being developed to replace the latter. State of the art noise figures for InP HEMTs are, for example, 0.8 dB at 60 GHz and 1.2 dB at 94 GHz. GaAs based power HEMTs show higher efficiencies than competing MESFETs for frequencies over 10 GHz, and are comparable to them below 10 GHz. In the higher frequency range, the state of the art in the output power of HEMTs gives a -6 dB/octave line connecting 4 W at 20 GHz and 0.1 W at 100 GHz. As there are still reliability problems yet to be solved with the low noise InP HEMTs and the power HEMTs, further study of degradation mechanisms in AlInAs/InGaAs/InP systems, r.f. operation testing of really high power HEMTs, systematic reliable data, etc. would be necessary for them to be accepted as having been established to be reliable devices.