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TOPIC: Semiconductor Theory. Introduction There are three basic types of materials that we are concerned with in electronics. These are conductors, semiconductors and insulators. Materials that have very low electrical resistivity (in the order of 1 x 10-6 ohm-metres) are called conductors. Materials that have very high electrical resistivity (in the order of 1 x 1013 ohm-metres) are called insulators. Semiconductors are materials that have resistivity values in between those of conductors and insulators, they are neither good conductors nor good insulators. Examples of conductors: Copper Aluminium Silver Gold Examples of insulators: Rubber PVC Paper Mica Examples of Semiconductors: Silicon Germanium Semiconductor materials are used to make a range of devices that are used in modern electronic circuits. In order to understand how these devices work we must first gain an understanding of the electrical properties of naturally occurring (intrinsic) semiconductors. We then need to learn about the electrical properties of extrinsic semiconductors. Extrinsic semiconductor material is just a naturally occurring pure semiconductor material that has been modified by a manufacturing process. Intrinsic semiconductor materials The naturally occurring semiconductor materials that are used to manufacture electronic devices are Silicon and Germanium (Germanium is an older choice of material which is less used today). Pure Silicon First a very pure crystal of silicon must be produced. The atomic structure of the silicon can be represented by the diagram below. Silicon like all semiconductors is a group 4 element and its atoms have only four electrons in the outer shell ( 4 valence electrons). It takes eight electrons to fill the outer shell and make it stable. The atoms share their valence electrons with neighbouring atoms so that each atom effectively contains eight electrons in the outer shell. This sharing of valence electrons with neighbouring atoms forms covalent bonds. It is these covalent bonds that bind the atoms together. The silicon atoms form a square lattice Each silicon nucleus has four electrons in its outer shell These electrons are paired with the corresponding electrons in adjacent atoms. These are called covalent bonds. Covalent bonds are what binds the material together The net result is that each nuclei (along with the electrons in the inner shells) are surrounded by eight outer electrons tightly bound in the atomic structure. Note this is a simplified diagram showing a 2 dimensional representation of the structure of silicon. Obviously silicon has a 3 dimensional structure and the covalent bonds do not really lie in a single plane as shown in the diagram. The actual arrangement of covalent bonds forms a shape called a tetrahedron. This diagram does give a good representation of how the electrons are bound to the atoms. This reflects the fact that there are no free electrons to produce an electrical current if a voltage is applied to the material. The Doping of Semiconductors The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing ntype and p-type semiconductors. Pentavalent impurities Impurity atom with 5 valence electrons produce n-type semiconductors by contributing extra electrons. Trivalent impurities Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a "hole" or electron deficiency. Semiconductor Current Both electrons and holes contribute to current flow in an semiconductor. CURRENT FLOW in an N-TYPE MATERIAL is similar to conduction in a copper wire. That is, with voltage applied across the material, electrons will move through the crystal toward the positive terminal just like current flows in a copper wire. CURRENT FLOW in a P-TYPE MATERIAL is by positive holes, instead of negative electrons. Unlike the electron, the hole moves from the positive terminal of the P material to the negative terminal.