INVESTIGATIONS ON THE TRAPATT DIODE

Abstract

ollowing the discovery of a high—efficiency mode of operation in silicon avalanche diodes by Prager, Chang and Weisbrod in 1967 [1], [2], there has been a great deal of research interest in this mode. This mode, commonly referred to in the literature as the TRAPATT mode, is now known to occur over a wide frequency range in diodes of different materials and with different impurity profiles. This thesis reports the results of investigations carried out on this important mode of oscillation. The main experimental arrangements used and results obtained in the study of TRAPATT oscillations, the types of diodes in which the TRAPATT phenomenon is known to occur, the results on the use of the TRAPATT diode as an amplifier, the mechanism of TRAPATT operation, the computer techniques used in its analysis, and the circuit design for TRAPATT oscillations are reviewed in brief. The physics of the device is studied, where the main theories propounded to explain the TRAPATT operation are given in outline together with the assumptions niade in the analysis and the conclusions drawn. Particular attention is given to the more cori'1only accepted explanation given for TRAPATT oscillations by DeLoach and Scharfetter [3] and by Clorfeine, Ikola and i1apoli [L]a Doth these theories make rather stringent assumptions, In particular, DeLoach and Scharfetter assume the ionization coefficients for electrons and holes to be equal, 2 while Clorfeine et al, neglect ionization by holes in comparison with ionization by electrons. A new and more general theory in which the ionization coefficients for electrons and holes are allowed to have different values is developed and reported here. The structure analyzed is a step junction n+pp+ silicon diode. The carrier densities in the different zones of the diode when operating in the TRAPATT mode are evaluated and the field distribution is studied.. Different stages in the operation of the device are analyzed which project a coherent picture of the TRAPATT cycle. The actual field distribution inside the diode is computed from which the terminal voltage is calculated. The resulting voltage-current waveforms are evaluated and plotted. Their Fourier components are then determined which yield actual numerical values for the complex impedance of the diode, the a-c power output and the efficiency of operation at the fundamental frequency. The results deduced from the theory are found to be in excellent accord with the available experimental data. Some directions in which further work on the subject can be carried out are indicated. An up-to-date list of references on .the TRAPATT diode is appended.