Elements of Electronics

P—N junctions play a crucial role in many modern electronic devices including the most important, the transistor. In order to explain the electrical properties of P—N junctions, it is first essential to discuss the physics of the constituent semiconducting materials from which such junctions are fabricated. Present understanding of electrical conduction processes in solids is based on what is known as energy-band theory. One approach to this theory is to consider what happens to the energy states of electrons as the separation of initially isolated atoms is progressively reduced to the interatomic distances found in solids. It is well known that the energy levels available to electrons in isolated atoms, such as those of a gas, are discrete. This is a consequence of the quantisation principle and there is a multitude of experimental evidence for discrete levels, for example discrete optical and X-ray atomic spectra. Thus starting with N atoms at a large separation, there is initially a corresponding N-fold degeneracy (or coincidence) of the discrete electronic energy levels. As the separation of the atoms is decreased towards the equilibrium spacing in solids, the electronic charge distributions of adjacent atoms eventually begin to overlap. When this happens, the energy of an electron is determined not only by forces due to its own nucleus and electrons, but by forces exerted by the nucleus and electrons of adjacent atoms. As a result the N-fold degeneracy is lifted and the energy levels spread into bands as illustrated in figure 1.1.

Resistors, capacitors and inductors are considered to be linear components because the amplitude of the alternating current through them is proportional to the amplitude of the alternating voltage across them. Components for which this proportionality does not hold are correspondingly called nonlinear devices. A diverse range of properties is available among these devices and they are widely used in electronic circuits for many different purposes. P—N junctions with ohmic contacts to the P and N regions belong to the class of nonlinear devices whose members are properly known as diode rectifiers although they are commonly referred to as just diodes or rectifiers for brevity. The diode nomenclature was introduced originally to describe the two-electrode thermionic valve, which has similar circuit properties to the P—N junction (see section 12.5). The term rectifier arises from the property of presenting a very low resistance when voltage of one polarity is applied, but a very high resistance when voltage of the opposite polarity is applied. Thus when such a device is included in an a.c. circuit only a unidirectional current flows; the alternating current is said to have been rectified. While other kinds of nonlinear two-electrode or diode devices exist with properties and applications that are unrelated to rectification (see, for example, section 14.1), it is the rectifying type of diode that finds by far the most widespread use in electronics. Appropriately, this chapter concentrates on the circuit applications of diode rectifiers.

The basic ingredient of junction transistors is the P—N junction, which has already been considered extensively in chapter 1. A junction transistor consists of two such junctions formed back-to-back in the same single crystal and separated by much less than a minority-carrier diffusion length. The two junctions must be formed within one continuous crystal; two separate junctions with their surfaces stuck together in some way or electrically connected will not suffice! Two distinct types of junction transistor are possible, NPN (see figure 3.1a) or PNP depending on the conductivity type of the central region. Their operation is conceptually identical except for the roles of electrons and holes being interchanged and the polarities of the bias potentials reversed. Thus it will suffice to discuss the NPN transistor. One junction, called the emitter junction, is biased in the forward direction while the other, called the collector junction, is biased in the reverse direction. Pursuing the nomenclature further, the central region between the junctions is called the base, while the outer regions adjacent to the emitter and collector junctions are respectively known as the emitter and collector. Each region is provided with an ohmic contact to permit external connections. In its simplest mode of operation the transistor is connected to a circuit essentially as shown in figure 3.1a. This particular arrangement is referred to as the common-base configuration because the base lead is common to bath the input and output circuits.

Many electronic systems may be regarded as having a pair of input terminals and a pair of output terminals, between which a signal is processed (see figure 4.1). For example, a signal might be amplified between the input and output terminals. This approach is valid for devices such as transistors that have only three terminals because one terminal is common to both the input and output. The electronic system responds to external input circuits or drives external load circuits by means of the terminal quantities V _i,I _i , V _o and I _o and its behaviour is specified by the interdependences of these variables. Once these dependences are established, the response of the system to input and output connections can be deduced independently of any knowledge of the detailed internal action or circuitry. Relationships between V _i,I_i, V _o and I _o can be represented graphically. Plots of V _i,I _i, V _o and I _o against one another are called static characteristics or just characteristics for brevity. Particularly useful are V _i versus I _i, V _o versus I _o and I _o versus I _i. These are appropriately called input, output and transfer characteristics, respectively. Notice that the usual convention is adopted in which I _i and I _o are defined as positive when they flow into the input and output terminals (see figure 4.1).

The three variants of the single-stage transistor amplifier, namely the common-emitter, the common-base and the common-collector have been analysed and discussed in chapters 4 and 5. However, a single stage by itself may not provide enough gain or the input and output resistances may be unsuitable for a particular application. In these situations, the required performance can be obtained by coupling two or more single stages together to form a multistage amplifier. Stage configurations incorporated in the complete amplifier should be selected on their merits for the particular job concerned. Table 4.1 (page 94) which lists the small-signal properties of the configurations, is a useful guide. For example, the design of an amplifier with low input and output resistances could involve a common-base input stage and a common-collector output stage. Often a common-collector stage is used as the first or last stage of a multistage amplifier to achieve either a high input resistance or a low output resistance. A common-emitter stage followed by a common-base stage has a good high-frequency response. This is because the common-base stage provides a very low load resistance for the common-emitter stage and has itself an inherently good high-frequency performance.

The term feedback when used in conjunction with amplifiers describes the technique of taking a fraction of the signal at some point, usually the output, and applying it to an earlier point, usually the input. In the general model for feedback illustrated in figure 7.1, an input signal θ_i is combined with a fraction β of the signal θ_o from the output of an amplifier of gain A such that the input to that amplifier is A symbol θ has been used to represent signals under discussion here because they may be either currents or voltages. Since θ _o Aθ*

In the earlier chapters, the treatment of the subject of amplification was restricted to the manipulation of a.c. signals only. This was convenient because it allowed the introduction of the essential concepts connected with amplification but kept the circuits relatively straightforward. While the amplification of just a.c. signals covers a wide variety of amplifier uses, there are a considerable number of applications in which the amplification of the d.c. component of a signal is also required.

It was shown in section 7.5 that positive feedback applied to an amplifier augments any spurious input signal and if the magnitude of the loop gain is greater than or equal to unity, that is |β ^∗ A ^∗| ⩾1, oscillation ensues. This means that an oscillator can be made by applying positive feedback round an amplifier provided its gain is sufficient to overcome any losses arising from the feedback network. The amplifier should be adequately decoupled from the supply and ideally its output impedance should be zero and its input impedance infinite. The last two requirements ensure that the amplifier action is not affected by loading and that the feedback network itself is not loaded by the amplifier input. For many applications an operational amplifier is particularly suitable. The frequency of oscillation may be controlled by choosing a suitable frequency-selective feedback network.

Electronic circuits can be broadly classified as either analogue or digital. In an analogue circuit the input and output signals are continuously variable within certain physical limits set by the devices and circuitry and often bear a simple functional relationship to each other. Linear amplifiers and sine-wave oscillators are examples of analogue arrangements. In contrast digital circuits operate with signals that can take only discrete values, usually few in number. Intermediate values cannot be sustained and occur only momentarily as the signal is changed from one discrete state to another.

Two types of field-effect transistor exist, the junction gate field-effect transistor and the metal-oxide field-effect transistor. The abbreviation FET is commonly used to refer to either type while terms JFET (sometimes JUGFET) and MOSFET conveniently serve to distinguish the two variants. Compared with the bipolar transistor, both types of FET exhibit very high input resistance which leads to very high current and power gain. In this chapter the topic of FETs is broached by discussing the JFET. After dealing with both JFETs and MOSFETs and a cross-section of their circuit applications, a brief treatment of thermionic valves is included since amplifying types possess very similar properties to FETs.

As pointed out in section 1.6, when the impurity concentrations on both the P and N sides of a P—N junction are sufficiently large (> 10²⁰ cm⁻³), the Fermi levels lie within the appropriate energy bands and the materials are said to be degenerate. This is the situation in the tunnel diode, sometimes called the Esaki diode after its Japanese inventor. In the absence of an applied voltage, the energy-level diagram is as shown in figure 14.1a. When a small forward bias is applied, besides the usual current that arises from carriers climbing the energy barrier at the junction, a current flows owing to a process graphically known as tunnelling. In general, if an energy barrier exists between states of the same energy at different positions, there is always a finite probability of particles traversing the barrier by tunnelling through it rather than climbing over it. The tunnelling process can be understood in terms of quantum theory and the tunnelling probability calculated using wave mechanics. Because of the heavy doping, the depletion region and hence the energy barrier in a tunnel diode is extremely narrow and the tunnelling probability for current carriers is high. Consequently appreciable numbers of electrons can tunnel from the conduction band states on the N side to any vacant states (holes) of the same energy on the P side. The electron tunnel current clearly reaches a maximum when the forward bias matches the levels of the conduction electrons on the N side with those of the holes on the P side as depicted in figure 14.1 b.

The invention of the maser represented a major landmark in the evolution of electronics and soon led to the development of the laser. Both the maser and laser find important application in the field of electronic communication, the latter having enormous communication potential in view of the vast bandwidth available at optical frequencies. Actually, the laser is just one important device among many that are useful in the exciting and rapidly developing branch of electronics known as optoelectronics. This broad subsection may be described as being concerned with electrically actuated optical signalling, ranging from the simple indication of an event to the transmission of extremely complex information over vast distances.

This book has been concerned with the basic building blocks of electronics rather than complete systems. However, before concluding it will be shown how the circuits that have been discussed in the preceding chapters can be used to build a few representative systems. The systems chosen for consideration are very widely used in practice and employ a broad spectrum of electronic circuitry.

For a proper appreciation of electronics it is essential to gain some practical experience of the subject. With this in mind, the following group of laboratory exercises has been devised to complement and substantiate the previous theoretical treatment. It is intended that the exercises be carried out in parallel with reading the main text. A time is given with each exercise, which, in the authors’ experience, is sufficient to enable an average student to complete it.

Many commercial oscillators have an output resistance of a few hundred ohms. If distortion of the signal V _i applied to the input of the circuit shown in figure 17.2 is to be avoided when R is small, the reactance 1/ωC must be much larger than this output resistance. In fact, unless ωC is small enough, the output resistance of the oscillator in conjunction with the capacitance C tends to integrate in front of the C—R circuit being investigated. Hence C and ω should not be increased above the values suggested to get τ ≫ T. As instructed, this must be achieved by making R large.