THEORY OF TRANSISTORS AND SEMICONDUCTORS

a. General

. In order to examine and discuss the theory ofsemiconductors, it is necessary to consider the nature of crystals. Mostsolids, except those that have a biological structure of cells such as theleaves of trees and bones, have a crystal structure. In fact, many substancescan　be　seen　to　have　a　specific　crystal　pattern　when　viewed　through　a　microscope.They can be identified by the angles and planes of their surface areas: somecrystal　materials　form　as　cubes;　and　some　form　as　long　needles　and　variations　ofhexagonal　structures.　When　a　semiconductor　device　is　examined　it　is　not　possibleto see the crystal. Therefore, the atomic structure of selected crystals willbe　discussed　in　order　to　better　understand　how　semiconductors　work.

b. Atomic Structure and Valence Electrons.

(1) Atomic structure. To begin the discussion of semiconductorsit　is　necessary　to　look　at　the　atomic　structure　of　germanium,　silicon,　aluminum,and　phosphorous.

NOTE In this discussion only the interaction of electrons and protons will becovered. Other particles within the atom are of no importance in thisexplanation.

(a) Germanium atom. Part A of figure 21 shows that thenucleus or core of the atom contains 32 protons or positive particles; it alsohas　41　neutrons　or　neutral　particles.　Notice,　also,　that　there　are　32　electronsor　negative　particles　that　travel　around　the　nucleus　in　four　rings　(shells).

1. In its normal state the atom is electricallyneutral; the number of negative particles (electrons) equals the number ofpositive particles (protons). The inner three rings are complete with a totalof 28 tightly bound electrons. The outer ring is incomplete and has only fourelectrons.　These　electrons　in　the　outer　ring　are　called　valence　electrons;　theyare　free　to　move　around　within　the　crystal　structure.
2. The outer ring of a single germanium atom isincomplete　with　only　four　electrons.　Its　outer　ring　would　be　complete　with　eightelectrons. The problem therefore, is to increase the number of electrons in theouter　ring.
3. The solution is to allow the single atom to shareits valence electrons with other adjacent atoms. Fortunately, the valenceelectrons attempt to pair off with those of another atom to complete the outershell. When this happens and atoms are bound together through their valenceelectrons, it is called an electronpair bond. A more common term for atomssharing　electrons　in　this　manner　is　covalent　bonding.

　 (b) Silicon atom. The silicon atom contains 14 electrons(B, figure 21). The similarity between a germanium atom and a silicon atom isthat they both have four valence electrons. Semiconductor devices andtransistors　may　be　made　from　either　germanium　or　silicon　crystals. (c) Aluminum atoms. Notice that there are 13 electrons inorbit　(C,　figure　21).　This　atom　contains　three　valence　electrons. (d) Phosphorous　atoms　(D,　figure　21).　Similar　to　the　otheratoms that have been covered, its outer ring is also incomplete. A phosphorousatom　contains　5　valence　electrons. (2) Valence electrons. The valence electrons in each atom of agood conductor, such as copper, are loosely bound to the nucleus. Under theinfluence of an electric field, they move easily through the conductor.However,　valence　electrons　that　are　part　of　a　covalent　bond　do　not　readily　breakaway from their bonds. That is why crystal materials such as germanium andsilicon　are　poor　conductors　under　normal　conditions.c. Electron Flow in Semiconductors. (1) At room temperature there is enough heat (thermal energy) tocause the semiconductor crystals to vibrate and shake loose some electrons fromtheir covalent bonds. The few electrons set loose are free to move or driftaimlessly　throughout　the　semiconductor　crystal;　they　are　called　free　electrons. (2) After　an　electron　breaks　away　from　its　covalent　bond,　the　atomit leaves is then missing one electron. The result is that when the atom losesone electron, it contains more protons (positive charges) than electrons(negative charges). That particular atom is positively charged. But rememberthat the free electron is still within the crystal material so that the totalnumber　of　protons　and　electrons　in　the　crystal　still　balance　each　other.　As　faras the overall piece of semiconductor material is concerned, it is stillelectrically　neutral.So far, only the atomic structureof semiconductors that are not influenced by any outside force such as heat orlight,　have　been　analyzed.　Without　heat　or　light,　all　of　the　electrons　are　heldin their orbits they are not able to break away to move within the crystalmaterial. Therefore, semiconductors at low temperatures are not exposed to anyother　outside　force　are　actually　nonconductors.