120 PHILlPS TECHNICAL REVIEW VOLUME 25 PHYSICAL PRINCIPLES OF PHOTOCONDUCTIVITY by L. HEIJNE *). 537.312.5:537.311.4 1. BASIC CONCEPTS; CONTACTS ON SEMICONDUCTORS Interest in photoconductivity has increased considerably in recent years. This is due not only to the numerous technical applications of photoconductive materials but also to the fact that the photoconductive effect has proved to be a useful aid to the study of the properties of semiconduc- tors. A series of three articles in this journal will deal at some length with the general aspects of photoconductivity. It will be shown how the phenomena concerned can be explained in terms of the band theory of solids. Special attention will be paid to the manner in which the photoconduc- tive properties depend on the lifetime of the charge carriers, on the recombination mechanism, on thepresence of various types of impurity centres, on the nature of the contacts and on thepossi- ble occurrence of space charge. The article below, the first of the series, deals with the most important basic concepts, and dis- cusses the influence of fitting contacts OT!- a photoconductor. Introduetion, ical background of various well-known properties As the name implies, photoconductivity is the of photoconductors. Among the subjects considered effect whereby the electrical conduqtjyjty -of a solid will be the sensitivity of photoconductors and their changes under the action of light. This effect, which speed of response, the dependence of the photo- was first observed in 1873 on selenium 1) and which current on the wavelength of the incident light, the subsequently found application in the well-known influence of temperature, and the consequences of selenium cells, has again become increasingly prom- the fact that a piece of photoconducting material inent in the past 15 years. There are two reasons can only be incorporated in a circuit by providing it for this. In the first place, advances in the prepara- with metal contacts. The relation between photocon- tion of very pure substances and the deeper in- ductivity and luminescence will also emerge. No sight into the electronic processes taking place in account, however, will be taken of processes asso- solids have made it possible to manufacture in a ciated with chemical changes in the substance, reproducible way photoconductive cells (photore- such as those occurring in materials used for photo- sistors) of high sensitivity, suitable for use in diverse graphic emulsions 6). spectral regions. In the second place, measurements The present article will be concerned with some of photoconductivity and of phenomena ass'ociated basic concepts and the influence of contacts; the with it have yielded useful information on many second will deal with the influence of impurity cen- important properties of semiconductors or insula- tres on the lifetime of the charge carriers and on the tors, such as the lifetime and mobility of charge speed of response (for cases with one and with more carriers, the "depth" ofimpurity centres, and so on. than one type of impurity centre), and in the third Some practical aspects of photoconductivity have some special effects will be discussed. been dealt with in earlier articles in this journal, Our considerations will be based on the insight in- examples being the application in a television cam- to the properties of semiconductors which has been era tube 2) or in a solid-state image intensifier 3), and gained in recent years from the study of the tran- the preparation and properties of photoconducting sistor materials germanium and silicon. In the series, cells of cadmium sulphide 4) and of indium antimo- particularly in this first article, the ,treatment of the nide 5). various subjects will often begin by considering In three articles, of which this is the first, we shall semiconductors in general, after which they will be examine some general and fundamental aspects of extended or confined to the case where the material photoconduction, a~d attempt to illustrate the phys- also shows photoconductivity. *) Philips Research Laboratories, Eindhoven. 1) W. Smith, Nature 7,303,1873. 6) For a more comprehensive treatment of photoconductivity, 2) Philips tech. Rev. 16, 23, 1954/55 and 24, 57, 1962/63. sec: R. H. Buhe, Photoconductivity of solids, Wiley, New 3) Philips tech. Rév. 19, 1, 1957/58. York 1960, or: T. S. Moss, Photoconductivity in the ele- 4) Philips tech. Rev. 20, 277, 1958/59. ments, Butterworths, London 1952. A review article by 5) Philips tech. Rev. 22, ~17, 1960/61. F. Stöckmann appeared in Z. angew. Phys. 11, 68, 1959. .•1 , ,~L" 1963/64, No. 5 PHOTOCONDUCTIVITY, I 121 Energy levels of the electrons in a solid; the band absolute zero. If the temperature is now increased, scheme electrons are able, by acquiring thermal energy, to Just as electrons in isolated atoms cannot possess rise from the valence band into the conduction band any arbitrary energy, nor can they when the atoms and to remain there for some time. Such a transi- are united to form a crystalline solid. Whereas in an tion can take place more easily the smaller is the isolated atom, however, the energy of an electron width of the forbidden zone, called the energy or can have only a few discrete values, in a solid the band gap, and the higher is the temperature. The possible energy levels are very numerous. Usually' substance which, at absolute zero, was an insulator these levels fall into groups, called the allowed has now become at finite temperature an intrinsic energy bands, which in a perfectly pure crystal are semiconductor, that is to say the electrons contained separated by zones in which there are no energy levels in the conduction band all originate from the at all: the forbidden zones. The differences in energy valence band. The latter band therefore contains an between the neighbouring levels in an allowed ener- equal number of holes. Disregarding small spontane- gy band are extremely small, and such a band may ous fluctuations ("noise"), in thermal equilibrium broadly be regarded as a continuum of possible ener- the density of electrons and 'holes is constant be- gy values. According to Pauli's exclusion principle, cause, on average, as many conduction electrons however, the number of electrons contained in a continuously recombine with holes as are newly band cannot be greater than the number of levels of generated. which the band is made up. If these numbers are The equilibrium densities of the electrons and equal, the band is completely filled. In that case holes are no longer equal if the crystal contains im- there can be no electron movement and therefore no purity centres, e.g. foreign atoms. Some of the va- electric conduction. In an insulator the bands are rious functions; of these impurity centres will be either completely filled or completely empty; a metal dealt with later in this article. Principal among contains one band which is only partially filled. them are the properties of "donating" and "accept- Electrons occupying a level in a partially filled ing" electrons. There are donor centres, i.e. im- band are able, in general, to take up a small amount purity centres that can give up electrons (often of energy from an applied electric field, and thus to an atom containing more valence electrons than move freely. This band is therefore termed the con- the atom of the parent lattice whose place it duction band; the next band below is called the takes), and acceptor centres, which take up valence band. electrons or supply holes (e.g. atoms with fewer In semiconductors the occupation of energy levels valence electrons). An electron bound to such a is nearly the same as in insulators but now either centre may possess an energy which lies in the for- some levels at the bottom of the conduction band bidden zone of the pure crystal. An energy value of are occupied or some at the top of the valence band this kind is generally called an "impurity level" or, are empty (or both conditions occur at the same more specifically, a donor or acceptor level. If the time). In the first case the electron can, as described, energy level of an electron bound to a donor centre move freely under the action of an applied field; this' case is referred to as N-type conductivity. In the other case the conduction can best be described by E considering not the motion of the electrons in the valence band but that of the few vacant places which Î an electron might still have occupied, called holes. This case is referred to as hole conduction or P-type conductivity, since the holes behave as positive charge carriers. The energy states of the charge carriers are con- a b ventionally represented in what is termed a band Fig. I.Energy band scheme of an insulator (a) and of a metal(b). scheme, a diagram in which the energy of the elec- The electron energy E is set out vertically; the top of the ordi- trons is set out vertically. Fig. 1 shows the band nate axis gives the energy of an electron which is just outside the material. The horizontal coordinate has no physical meaning schemes of an insulator and of a metal. .., here, but may be used to represent a space coordinate. Of the As will later be explained, the occupaÏion of the various allowed bands those shown are the highest of the en- tirely filled bands (cross-hatched), called the valence band, and energy levels is not entirely independent of tempera- the conduction band immediately above it. The forbidden zone ture. It is interesting to consider the case of a solid in between, of width LlE, is called the energy or band gap.