15.1 Aim

This chapter introduces the reader to semiconductors and semiconducting devices that are important to radiography. This is a very large field, and the chapter concentrates on the barrier layer rectifier, which has made a major impact on radiographic science. After an overview of the development and manufacture of integrated circuits. the chapter concludes with an overview of the used of semiconductor devices in radiography.

15.2 Introduction

The use of semiconductor materials and the associated technology play an increasingly important part in our everyday lives as well as in radiographic science. Today it is possible to produce millions of electronic circuits on a small silicon chip using ultra-large-scale integration (ULSI) techniques. This has enabled the production of microprocessors that are capable of being programmed to perform specific tasks. Such devices are cheap and easy to program. They can perform a wide range of functions and are very reliable. Because of this, microprocessors are found in devices from wristwatches to aircraft flight systems and it is not surprising to learn that they are used in many devices in diagnostic imaging and radiotherapy departments.

However, the microprocessor is a complicated solid-state device and a detailed description of its operation is beyond the scope of this text.

Other, simpler, solid-state devices are widely used in X-ray circuitry and are suitable for inclusion in this chapter after a general description of the properties of semiconductor materials.

15.3 Intrinsic semiconductors

An intrinsic semiconductor is a chemically pure semiconductor, which is also assumed to have perfect regularity of atoms within its crystalline structure or lattice. The concept of semiconducting materials was briefly introduced in Section 7.3.2 where the properties of conductors, insulators and semiconductors were compared in terms of the energy band model for the orbiting electrons. This was illustrated in Figure 7.2 and this diagram is reproduced here as Figure 15.1 (below). As shown in Figure 15.1B, one of the characteristics of semiconductors is that there is a small energy gap (up to a few eV) between the top of the valence band and the bottom of the conduction band. At very low temperatures, all the outer electrons have energies near the bottom of the valence band, and no electrons are able to take part in electrical conduction, as there are no free electrons in the conduction band. As mentioned in Chapter 7, increasing the temperature of a semiconductor increases its conductivity. At normal room temperatures, many electrons are able to gain sufficient energy (because of the increased kinetic energy of the atoms) to jump up to the conduction band and so take part in electrical conduction.

Figure 15.1 Energy level bands for (A) a conductor, (B) a semiconductor and (C) an insulator.

15.3.1 Positive holes

Associated with each electron which is able to jump up to the conduction band is a ‘vacancy’ in the valence band, referred to as a positive hole or just hole. This hole may be filled by an electron from the valence band of a neighbouring atom, but in doing so the electron leaves a hole in the valence band of that atom. In this way, a hole may appear to move around the crystal lattice of a semiconductor (behaving like a positive charge) until eventually an electron drops down from the conduction band to fill the hole and remove it from the valence band. This process is referred to as recombination. At any one moment in time, all three of the above processes are occurring:

1. Electrons are being excited into the conduction band creating holes in the valence band.

2. There is movement of electrons in the conduction band and holes in the valence band.

3. There is a recombination of electrons in the conduction band with holes in the valence band.

The overall conductivity of such an intrinsic semiconductor is the sum of the effects of the movements of the electrons in the conduction band and the holes in the valence band.

Insight

Consider the situation depicted in part A of the diagram below, where there is a row of ball bearings at the base of a box, with no available space between them for sideways movement to take place. If we now lift ball bearing C onto the lid of the box, it is possible for ball bearing D to move to the left to fill the space once occupied by C. In doing so, D has now created a space to its right, i.e. the hole may be considered as moving in the opposite direction to the ball.

Now consider the above situation as it refers to the valence and conduction bands of a semiconductor. No net movement is initially possible in the valence band because it is full of electrons. When an electron is raised to the conduction band, movement within the valence band is possible. Thus, if an electron moves from right to left within the band, it leaves a hole in its starting position, i.e. the hole appears to move from left to right. If an electron is removed from the valence band of an atom, then the atom is positively charged since the protons in the nucleus now outnumber the orbiting electrons; the hole is referred to as a positive hole. If a semiconductor is passing a current such that the electron flow in the conduction band is from left to right, there will also be an effective flow of positive charge associated with the movement of positive holes in the valence band in the opposite direction.

15.3.2 Silicon

Silicon is currently the most widely used general semiconductor material. It has an atomic number of 14 and has 14 protons in its nucleus and 14 electrons orbiting that nucleus (see Ch. 18). This means that the two inner shells (K– and L-shells) are completely full and contain two and eight electrons respectively. The next shell out from the nucleus is the M-shell and this exhibits a stable configuration when it contains either eight or 18 electrons (see Ch. 18). In this case, it contains four electrons and so may be regarded as an incomplete shell in the solitary silicon atom. However, in the silicon crystal there is a regular arrangement of atoms in which each silicon atom shares its outer electrons with four neighboring atoms so that each atom appears to have eight electrons in its M-shell and thus stability (see Fig. 15.2). Such electron bonds are known as covalent bonds and the electrons are termed valence electrons and inhabit the valence energy band of the atom. The covalent bonds give the crystal its regularity by inhibiting the movement of any particular atom. At room temperature, these bonds are being continuously broken and reformed as some of the valence electrons are gaining sufficient energy to reach the conduction band (bond broken) and electrons from the conduction band fall back into the valence band (bond reformed). The eight-electron configuration of the M-shell behaves like a full shell and so the valence band is effectively full until an electron moves up to the conduction band. As previously explained, when this happens, electron flow in the conduction band and positive-hole flow in the valence band are both possible.

Figure 15.2 Pure silicon as an example of an intrinsic semiconductor. (A) A silicon atom showing the four electrons in its valence shell; (B) the covalent bonds formed in a pure silicon crystal by each silicon atom sharing electrons with four neighbouring silicon atoms.

Intrinsic semiconductors, which we just considered, have very limited practical use, due to their low conductivity. If small amounts of specific impurities are added to them (by a process called doping), they are then known as extrinsic semiconductors and have properties which allow us to use them as rectifiers and integrated circuits (ICs), all of which are found in most X-ray generators. Extrinsic semiconductors will now be discussed.

15.4 Extrinsic semiconductors

The addition of small amounts of specific impurities to silicon or germanium is the basis on which most extrinsic semiconductors are produced. The doping may be heavy or light, depending on the component being produced. A typical concentration is one part of impurity to 10 million parts of pure silicon. The electrical conductivity of the extrinsic semiconductor is much greater than that of an intrinsic semiconductor and the level of conductivity can be controlled by altering the ratio of doping material to pure material. The impurity atoms within the silicon crystal lattice are the source of this greatly increased electrical conductivity. This is because the type of impurity is chosen either to enhance electron flow in the conduction band (this gives an N-type semiconductor) or to enhance the flow of positive holes in the valence band (a P-type semiconductor). These two types of extrinsic semiconductor will now be considered.

15.4.1 N-type semiconductors

As we have seen, single atoms of intrinsic semiconductors have four valence electrons. To produce an N-type extrinsic semiconductor, a pentavalent impurity (one with five valence electrons) is used as the doping material. Arsenic, antimony and phosphorus are examples of pentavalent elements that are suitable. Figure 15.3 (See page 96) illustrates the effect of introducing an atom of phosphorus into the crystalline structure of silicon. Four of the valence electrons in the phosphorus form covalent bonds and the fifth electron is unbonded. This electron has an energy level which is just below the bottom of the conduction band (see Fig. 15.3B). At normal room temperatures, it is therefore virtually a free electron since it is easily lifted into the conduction band and can take part in electrical conduction if a potential difference is applied across the crystal.