Electron-Hole Theory

　
Electron-Hole Theory
(1) As electrons travel from one atom to another, they fill theholes in some atoms and leave holes in other atoms. Basically, two things areoccurring: electrons are moving in one direction and holes are moving in theopposite　direction.

(2) In A, figure 23, an electron leaves atom "A", making it apositively charged atom. An electron from atom "E" breaks away and fills thefirst　hole　leaving　the　second　atom　with　a　hole.
(3) In B, figure 23, the hole has moved from atom "A" to atom"E".　An　electron　leaving　atom　"B"　will　fill　the　hole　in　atom　"E".
(4) In　C,　figure　23,　an　electron　from　atom　"C"　will　fill　the　holein　atom　"B",　neutralizing　that　atom　and　leaving　a　hole　in　atom　"C".
(5) D and E, figure 23, show additional hole and electronmovement　between　atom　"C"　and　atom　"H".
(6) Notice　that　the　movement　of　holes　is　opposite　to　the　directionof the movement of the electrons. The crystal material is still electricallyneutral, and there is one free electron at one end of the crystal and one holeat　the　other　end　(F,　figure　23)..　Under　certain　conditions,　then,　electrons　maybreak away from their orbits. The loss of an electron in the outer ring of anatom　leaves　a　hole　in　the　ring　and　makes　them　positively　charged.　This　positiveatom may now attract an electron from another atom. When an electron fromanother atom fills the hole of the positively charged atom, we can say that thefirst　atom　is　now　neutral　and　the　second　atom　is　now　a　positively　charged　atom.e. Impurities.(1) General. It is possible for atoms of elements, other thangermanium and silicon, to join the crystal structure. These elements are addedintentionally during the processing of germanium or silicon and are referred toas　impurities.
　(2) Donor and acceptor impurities. Two groups of elements can beadded to the crystal structure of either germanium or silicon. The elements inone　group　are　called　donors;　in　the　second　group　they　are　called　acceptors.
(a) A　donor　element　or　atom　is　classified　as　such　because　ithas five valence electrons. Common donor elements are arsenic, phosphorous, andantimony (A, figure 24). When a donor atom is compared with a germanium orsilicon　atom,　an　additional　valence　electron　is　found　(A　and　B,　figure　24).
(b) An acceptor element contains only three valenceelectrons. Aluminum, boron, gallium and indium are types of acceptor atoms (C,figure 24). When the acceptor atom is compared with either germanium orsilicon, one less valence atom is found. This missing atom is then referred toas　a　hole　(B　and　C,　figure　24).

　type Semiconductor Materials use Donor Impurities.(1) Figure 25 shows a semiconductor crystal in which one of thesemiconductor atoms has been replaced by a donor atom. The donor impuritycontains　five　valence　electrons.　Note　that　four　of　the　valence　electrons　form　acovalent bond with electrons of four of its neighbors. The electrons of thesemiconductor　atoms　and　donor　atoms　that　enter　into　the　covalent　bonds　form　verystable　structures　and　are　not　readily　displaced.
(2) The fifth valence electron of the donor atom cannot form acovalent bond and the nucleus of the donor atom has a very weak influence overthis　excess　electron.　The　excess　electron,　therefore,　is　mobile　and　is　called　afree electron. At normal room temperature, enough thermal or heat energy ispresent to cause this excess electron to break away from the donor and driftthrough the crystal structure. The result is that for each donor atom, weobtain　what　may　be　considered　one　free　electron.
(3) Germanium or silicon material containing donor impurities isreferred to as Ntype material. The letter N refers to the negative charge ofthe excess or free electrons. Impurities are added to Ntype semiconductormaterial in the proportion of one atom to several million semiconductor(germanium,　silicon)　atoms.