Fig. 1. A page from one of Gunn’s laboratory notebooks on which he made made the discovery of the Gunn-effect (taken from [1]). The Gunn-diode: Fundamentals and Fabrication Robert van Zyl, Willem Perold, Reinhardt Botha * Department of Electrical and Electronic Engineering, University of Stellenbosch, Stellenbosch, 7600 e-mail: rrvanzyl@firga.sun.ac.za * Department of Physics, University of Port Elizabeth, Port Elizabeth, 6000 e-mail: phajrb@upe.ac.za Abstract — A short tutorial on the Gunn-diode is presented. The principles underlying Gunn-oscillations are discussed briefly and illustrated by relevant simulations. The simulation of a typical Gunn-diode in a cavity is also presented. In conclusion, the fabrication process of low power Gunn-diodes is discussed. Keywords — Gunn-diode, Gunn-effect, transferred electron- effect, GaAs, energy band, Monte Carlo particle simulation. I. INTRODUCTION JB (Ian) Gunn discovered the Gunn-effect on 19 February 1962. He observed random noise-like oscillations when biasing n-type GaAs samples above a certain threshold. He also found that the resistance of the samples dropped at even higher biasing conditions, indicating a region of negative differential resistance. As will be explained later, this leads to small signal current oscillations. In Figure 1 part of the famous page from one of Gunn’s laboratory notebooks is shown with the entry “noisy” on the line for 704 volt. Describing it as the “most important single word” he ever wrote, it laid the foundation for what was to become a major mode of a.c. power generation. Due to their relative simplicity and low cost, Gunn diodes remain popular to this day. It is, however, also true that relatively few electronic engineers understand clearly the principles behind the Gunn-effect. The aim of this paper is to give the reader an overview of the underlying theory of the Gunn–effect and how it is utilised in Gunn-diodes to produce a.c. power [2], [3]. Concepts which will be discussed include the negative differential mobility phenomenon in GaAs, Gunn-domain formation and the basic Gunn-diode structure. A typical simulation of a Gunn- diode in a cavity will also be presented. The University of Stellenbosch, in conjunction with the University of Port Elizabeth, is currently fabricating GaAs Gunn-diodes for research purposes. The aim is to optimize Gunn-diodes for a.c. output at W-band frequencies. A review of this manufacturing process will be given. The simulations in this paper have been performed by a Monte Carlo particle simulator developed at the University of Stellenbosch. A short review of the Monte Carlo simulation of semiconductors is given in [4]. II. THE GUNN-EFFECT IN THE STRICT SENSE A. The Energy Band for GaAs To understand the Gunn-effect it is necessary to have some insight in the behaviour of electrons in a crystal lattice, and most importantly, the allowed energy states electrons can occupy. These are dictated by the energy band structure of a semiconductor which relates an electron’s energy as a function of its wave vector k. The band structure for GaAs is shown in Figure 2. Both the valence (negative electron energy) and conduction (positive electron energy) bands are shown. Only the conduction bands need to be considered for the study of electron dynamics, since electrons in the valence bands are stationary. Energy bands are very complex structures. It is, however, clear from Figure 2 that for realistic electron energies (E < 2 eV) only the lowest conduction band curve need to be taken into account. This curve displays three distinct “valleys” in the crystal spatial orientations labelled ’, L and X. For the purposes of this paper it is sufficient to consider the central ’-valley and satellite L-valley only. For the study of electron transport, the information near the local band minima is important, since electrons are usually located near the bottom of the valleys. For low electron energies, relative to these band minima, the band structure can be approximated by a parabolic E-k relation [5].