Graduation Year

2007

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Lawrence P. Dunleavy, Ph.D.

Co-Major Professor

Arthur David Snider, Ph.D.

Committee Member

Thomas M. Weller, Ph.D.

Committee Member

Frank Pyrtle, III, Ph.D.

Committee Member

Dennis K. Killinger, Ph.D.

Keywords

microwave, load pull, algorithm, steepest ascent, large-signal, temperature, thermal, trapping, pulsed, static, quiescent, bias, model, thermal resistance, thermal capacitance, GaN, S-parameters, bias tee, infrared

Abstract

Accurate transistor models are important in wireless and microwave circuit design. Large-signal field-effect transistor (FET) models are generally extracted from current-voltage (IV) characteristics, small-signal S-parameters, and large-signal measurements. This dissertation describes improved characterization and measurement validation techniques for FET models that correctly account for thermal and trapping effects.

Demonstration of a customized pulsed-bias, pulsed-RF S-parameter system constructed by the author using a traditional vector network analyzer is presented, along with the design of special bias tees to allow pulsing of the bias voltages. Pulsed IV and pulsed-bias S-parameter measurements can provide results that are electrodynamically accurate; that is, thermal and trapping effects in the measurements are similar to those of radio-frequency or microwave operation at a desired quiescent bias point. The custom pulsed S-parameter system is benchmarked using passive devices and advantages and tradeoffs of pulsed S-parameter measurements are explored. Pulsed- and continuous-bias measurement results for a high-power transistor are used to validate thermal S-parameter correction procedures.

A new implementation of the steepest-ascent search algorithm for load-pull is presented. This algorithm provides for high-resolution determination of the maximum power and associated load impedance using a small number of measured or simulated reflection-coefficient states. To perform a more thorough nonlinear model validation, it is often desired to find the impedance providing maximum output power or efficiency over variations of a parameter such as drain voltage, input power, or process variation. The new algorithm enables this type of validation that is otherwise extremely tedious or impractical with traditional load-pull.

A modified nonlinear FET model is presented in this work that allows characterization of both thermal and trapping effects. New parameters and equation terms providing a trapping-related quiescent-bias dependence have been added to a popular nonlinear ("Angelov") model. A systematic method for fitting the quiescent-dependence parameters, temperature coefficients, and thermal resistance is presented, using a GaN high electron-mobility transistor as an example. The thermal resistance providing a good fit in the modeling procedure is shown to correspond well with infrared measurement results.

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