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1、Technical English,2020/8/5,Introduction to Semiconductors,New words,conductor导体 insulator绝缘体 crystalline晶体的 valency化合价 bond结合 covalent共价的 DNA 脱氧核糖核酸 deoxyribonucleic acid,2,本节中用到的几种化学元素（物质）名称：,germanium 锗 silicon硅 tin 锡 boron 硼 selenium 硒 arsenic 砷 tellurium 碲 antimony 锑 arsenide 砷化物 gallium镓,4,Text
2、 part 1,Semiconductors are materials that exhibit properties that are between a metal and a non-metal. Their electrical properties are part way between a conductor and an insulatorthus they are called “semi-conductors”. (Actually they are much closer to insulators than conductors.) A number of mater
3、ials have these properties: The crystalline forms of most elements of Group IV (valency 4) (silicon1, germanium, tin).,5, Some forms of specific elementsboron (Group III), arsenic and antimony (Group V), selenium and tellurium (Group VI). Crystalline alloys of elements in Group III (valency 3) with
4、elements of group V (valency 5). Gallium arsenide (a compound or alloy of gallium and arsenic) is a good example here. Some compounds of elements of Group VI with elements of Group II.,6,Probably the most versatile elements are those with four electrons in their outer shell (Group IV: carbon, silico
5、n, germanium, tin, lead). Because of the fact that they can have four bonds to four other elements, covalent compounds of endless complexity can be formed. (Indeed DNA is a single carbon-based molecule.) All of these elements can exist in multiple physical forms. Carbon can take many forms such as g
6、raphite (a form a bit like a classical metal) which conducts electricity very well or diamond (a crystalline form) which is an insulator.,7,Almost anyone who has ever soldered an electrical connection knows the difference between a good soldered joint (where the metal solidifies in a polymorphic for
7、m) and a “dry joint” where the solder crystallizes.2 Polymorphic solder is a very good electrical conductor. Solder (an alloy of lead and tin) in its crystalline form is a semiconductor!,8,In fact carbon is not classified by chemists as a semiconductor even though diamond does semiconduct at relativ
8、ely high temperatures (well above room temperature). The technical reason is that carbon has very strong molecular bonds which do not break down very easily (the energy “gap” between the valence band and the conduction band is greater than that provided by ambient heat at room temperature.) It would
9、 be somewhat difficult in practice to use carbon as a basis for semiconductor electronics since it is hard to grow large, extremely pure diamonds to cut up and use as chips!,9,A major difference between semiconductors and conductors is their electrical behavior at different temperatures. In general,
10、 when you heat up a semiconductor its resistance to the flow of electricity decreases (or its conductance increases). In metals the opposite happens. As the tempe- rature of a metal increases then its resistance also increases (or conductance decreases).,10,Today silicon is the most commonly used se
11、miconductor although germanium was used almost exclusively in early (1950s) solid state devices. Silicon (like most elements) can take a number of physical forms. It resembles a metal in its electrical conductivity properties. Crystalline silicon is the base for almost all semiconductors today.,11,1
12、2,Figure 1 Silicon crystal lattice,A silicon crystal lattice is illustrated in Figure 1. Each silicon atom is bonded to four other silicon atoms with covalent bonds. Note that although the picture is two-dimensional the crystal structure is (of course) three dimensional. A perfect silicon crystal (a
13、t a temperature near absolute zero) has no free electrons and thus cannot conduct electricity. A pure silicon crystal at any tempe- rature above absolute zero will have some bonds broken by the random action of heat and so there will be some free electrons in the structure and you will get some cond
14、uction.,13,In addition to using pure silicon or germanium you can also use crystals made of almost any alloy of elements in Group III (with three electrons in their outer shell) with elements in Group V (with five electrons in their outer shell). The best known material used in this way is gallium a
15、rsenide although there are many others.,14,Electrical Conduction in an Intrinsic Semiconductor An “intrinsic” semiconductor is a uniform material that can act as a semiconductor without the need to introduce anomalies in the structure by doping. Pure crystals of silicon, germanium and gallium arseni
16、de are intrinsic semiconductors at room temperature.,15,Dopants,Semiconductors dont get very interesting (or useful) until you introduce dopants. A dopant is a very small (indeed minuscule) amount of a controlled impurity introduced into the crystalline structure. The presence of the dopant is responsible for the properties of the semiconductor. Figure 2(left) shows a silicon crystal lattice doped with boron. Boron has three electrons in its outer shell. The regular crystalline structu
