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