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Fundamental experimental study of the operation of gunn effect diodes in resonant circuits Gergis, Isdoris Sobhi

Abstract

This work is concerned with a fundamental experimental study of the operation of Gunn effect diodes in microwave cavities. Diodes with different parameters were fabricated from single GaAs crystals. The diodes were tested in resistive circuits for measuring some of their characteristics and for assessing their performance. The effect of the ohmic contacts on the performance of the diodes was demonstrated. Determination of the carrier concentration and mobility for the GaAs crystal was made using the Hall effect and conductivity measurements. Microwave coaxial cavities were designed and built for the operation of the diodes. The cavities had wide tuning range and provided variable impedance at the diode by changing the coupling to the output line. The results obtained showed that the upper limit of the frequency of oscillation varied from about 1.8 f to 2.5 f as the nl product of the diode varied from 0.4 to 3x10¹² cm⁻². At frequencies smaller than the transit time frequency the operation was in two modes; one was the delayed domain mode at lower values of bias. The other was observed at higher bias and was considered to be amultiple domain mode in which the domain reaches the anode before the voltage swings below threshold, and a new domain forms and then gets quenched before a new cycle begins. For the delayed domain mode, a theory has been developed which was in fair agreement with the experimental results. It was found that the range of bias in which the diode could oscillate coherently decreased as both the frequency and the nl product were decreased. The results obtained in the LSA mode showed that the lower limit of oscillation decreased as the carrier concentration decreased, but increased when the bias increased.

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