4 Diode Circuits

Chapter 4 Diode Circuits

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • Chapter 4Diode CircuitsSo far we have only considered passive circuit elements. Now we will consider our first reactivecircuit element, the diode. Hopefully things will start to get a little more interesting.Along with the diode there is the transistor, which will be discussed in future lectures.These circuit elements are commonly made from a semiconductor basic material. Semicon-ductors along with the passive circuit elements are often integrated onto a single electronicschip. If a hundred or so transistors are on a chip it is referred to as an integrated circuit orIC. Large-scale integrated circuits, LSI, contain thousands of transistors. And today evenvery-large-scale integrated circuits, VLSI, exist, with hundreds of thousands of transistors.Now lets talk physics.4.1Energy LevelsThe physical principles of semiconductor devices can be understood by considering quantumenergy levels in the material. I will just give a sketchy view here and define some terminology.Valance electrons are the electrons outside the closed shells of an atom. In silicon andgermanium there are four valence electrons, arsenic has five and gallium three. The largenumber of valance electrons favour these elements as semiconductors.To sustain an electric current, a material must have charge carriers that are free to move.There is some probability that an atom may eject a valence electron which is then free tomove in the material. The conductivity of a material is thus a function of the number offree charge carriers per unit volume. Based on these probability densities it is common todivide materials into three categories: conductors, semiconductors and insulators.In crystals atoms interact and bind by sharing valence electrons. The wave function isno longer associated with a single atom but extends over the entire crystal. One effect ofthe interaction between the atoms is that the otherwise degenerate energy levels split intoclosely spaced levels. Since the number of atoms is large, it is common to refer to this set oflevels as a continuous energy band.In solid materials there usually exist a valance band which is an energy region wherethe states are filled or partially filled by valence electrons. The conduction band is definedto be the lowest unfilled energy band.So our three materials can be characterized bytheir band structure. An insulator has the valence and conduction band well separated. A64