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Small -signal and noise characterization of discrete millimeterwave high electron mobility transistors

Ajay Aravind Prabhu, University of Massachusetts Amherst


High Electron Mobility Transistors (HEMT's) have emerged as the device of choice for high frequency and low-noise amplifier applications. To design superior low-noise amplifier circuits at high frequencies, accurate small signal and noise model of the device must be extracted.^ Traditionally, test-amplifiers and wafer probe measurements have been used to perform HEMT modeling. A critical study of the two techniques resulted in the design and construction of a new test fixture that attempts to combine the advantages of these two techniques. In this new fixture, a device is probed by a matched test amplifier circuit. It was found that the calibration accuracy of such a fixture was limited due to coupling between the probes and the difficulty in fabricating appropriate calibration standards. A new type of calibration standards made of micro-coaxial lines was used to calibrate the fixture. To correct for the coupling between probes, a form of a new technique called 11-term calibration was used.^ The effect of coupling between conventional wafer-probes on the accuracy of S-parameter measurement of a discrete HEMT was studied. A second form of the 11-term calibration technique was used to calibrate the coupled wafer-probes. This procedure uses calibration standards that are readily available on a commercial wafer-probe calibration-substrate.^ A critical study of HotFET/ColdFET equivalent-circuit extraction procedure revealed certain errors that can adversely affect high frequency HEMT modeling. A modified HotFET/ColdFET technique has been demonstrated, which eliminates this error, and in fact, simplifies the procedure. The new procedure uses just one forward bias measurement at low gate-current (as opposed to several high gate-current measurements required in the original procedure).^ A low noise HEMT modeling is incomplete without extracting its noise model. The Pospieszalski noise model, which involves the determination of equivalent temperature of the drain-to-source resistance in the device circuit model, simplifies the task of noise model extraction. However, it is important to accurately measure the noise produced by the device; and to know the exact source impedance presented to the device at the time of noise measurement. These requirements make noise modeling of ultra-low-noise devices a challenging task. These issues are described in detail in the last chapter. ^

Subject Area

Electrical engineering

Recommended Citation

Prabhu, Ajay Aravind, "Small -signal and noise characterization of discrete millimeterwave high electron mobility transistors" (1997). Doctoral Dissertations Available from Proquest. AAI9823766.