UBC Theses and Dissertations
GaAs MESFETS and their applications in digital logic and digital to Analog conversion Abdel-Motaleb, Ibrahim Mohamed
The purpose of the work described in this thesis was to study the use of GaAs MESFETs in digital logic and digital to analog conversion. A part of this work was to test ideas by actually fabricating GaAs MESFET devices in our laboratory by implanting Si into LEC SI GaAs. Initially, the Rockwell "ion implanted planar process" was used. With this process high source and drain resistance result when the magnitude of the threshold voltage is reduced. A recessed gate process was then tried in order to reduce the series resistance, but it was difficult to control in order to achieve an acceptable uniformity. Sadler's self-aligned gate technique was then tried but an acceptable yield was found to be hard to achieve. Based on the experience with the above methods, a Self Aligned Gate Technique Using Polyimide (SAGUPI) was developed. Using this technique it was found to be possible to reduce the series resistance of the transistor and achieve acceptable yields and uniformities. The characteristics of devices fabricated using the ion implanted planar process were measured. The transistor I-V characteristic parameters, Schottky and ohmic contact parameters and implanted region parameters were measured at room temperature and as a function of temperature over the range from -80°C to 80°C. It was found that no large change in the speed of the circuit occurs over this range. A new digital logic (the Common Drain FET Logic, CDFL) was developed for the application in Ultra High Speed Very Large Scale of Integration. The performance of the new logic was compared with that of other GaAs MESFET logic using computer simulation. The simulation results showed that the CDFL has potential for meeting the UHS VLSI requirements. Two 3 bit Digital to Analog Converters (DAC) (one with MESFETS as current sources and the other with saturated resistors) were developed. MESFETs gave better current source characteristics than saturated resistors.
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