Direct optical control of a microwave phase shifter using GaAs field-effect transistors
The design and analysis of a novel optical-to-microwave transducer based upon direct optical control of microwave gallium arsenide (GaAs) field-effect transistor (FET) switches is the subject of this thesis. The switch is activated by illuminating the gate depletion region of the FET device with laser light having a photon energy and wavelength appropriate to the generation of free carriers (electron-hole pairs) within GaAs. The effects of light on the DC and microwave properties of the GaAs FET are explored and analyzed to permit the characterization of the switching performance and transient response of a reflective microwave switch. The switch is novel in that it utilizes direct optical control, whereby the optically controlled GaAs FET is directly in the path of the microwave signal and therefore relies on optically-induced variations in the microwave characteristics of the switch. This contrasts with previous forms of optically controlled switches which rely on indirect methods with the optical stimulus inducing variations in the DC characteristics of the GaAs FET, such that there is no direct interaction between the optically illuminated GaAs FET and the microwave signal. Measured and simulated results relating to the switching performance and transient response of the direct optically controlled microwave switch have been obtained and published as a result of this work. For the first time, good agreement is achieved between the measured and simulated results for the rise and fall times associated with the transient response of the gate photovoltaic effect in optically controlled GaAs FET switches. This confirms that the GaAs FET, when used as an optically controlled microwave switch, has a transient response of the order of several micro-seconds. An enhanced model of the GaAs FET switch has been developed, which represents a more versatile approach and leads to improved accuracy in predicting switching performance. This approach has been shown to be valid for both optical and electrical control of the GaAs FET. This approach can be used to model GaAs FET switches in discrete or packaged forms and predicts accurately the occurrence of resonances which may degrade the switch performance in both switching states. A novel method for tuning these resonances out of the switch operating band has been developed and published. This allows the switch to be configured to operate over the frequency range 1 to 20 GRz. The agreement between the models and measured data has been shown to hold for two very different GaAs FET structures. The results of the direct optically controlled microwave GaAs FET switch have been used as the basis for the design of a novel direct optically controlled microwave phase shifter circuit; Measured and simulated results are in good agreement and verify that the performance of the optically controlled phase shifter is comparable with previously published results for electrically controlled versions of the phase shifter. The 10 GRz phase shifter was optically controlled over a 1 GRz frequency range and exhibited a mid-band insertion loss of 0.15 dB. The outcome of the work provides the basis for directly controlling the phase of a microwave signal using the output of an optical sensor, with the GaAs FET acting as an optical-to-microwave transducer through a monolithic interface.