Semiconductors

A perfect array of silicon atoms from a silicon lattice is represented in the diagram shown on the right. In reality this perfect arrangement is only found at temperatures very near to absolute zero.
	Heat at room temperature is sufficient to allow some bonds to break, so that a small number of electrons become free to move within the crystal. As the electron breaks free it leaves behind a *hole* which can also appear to move if it is filled with a free electron from a neighboring atom.
This availability of holes and free electrons makes silicon a conductor of electricity although in its 'pure' state it is a relatively poor one. If the silicon is made part of a circuit by connecting a battery across it then free electrons move towards the positive terminal by 'hopping' from hole to hole, making it seem that holes are moving in the opposite direction towards the negative terminal. This behaviour is simulated below. The empty space appears to move from right to left, but this is only because in reality the green spheres are moving from left to right.

If the temperature of the semiconductor is raised further, more bonds break down and more free electrons and holes become available for conduction. The resistance of a semiconductor decreases as the temperature increases. This ability to conduct electric current is known as intrinsic conduction because the charge carriers are provided by the semiconductor itself.
The very weak conductivity of pure silicon or germanium can be greatly increased by the addition of impurities in carefully controlled amounts. Certain impurity atoms are able to form strong bonds with the silicon or germanium atoms even though the number of valency atoms is different. The process of adding impurities under controlled conditions is known as doping and the increased conductivity is known as extrinsic conductivity because the additional charge carriers are provided by atoms introduced from outside.
The diagram on the right shows the result of introducing phosphorous atoms into the silicon crystal. Phosphorous has five valence electrons, four of which form covalent bonds leaving the fifth electron free for carrying charge. Phosphorous is known as a donor impurity because it adds free electrons to the crystal lattice
	The impure silicon is known as an 'n-type' semiconductor because the majority charge carriers are free 'negative' electrons.
This diagram shows the result of adding boron atoms into the silicon crystal lattice. Although boron has just three valence atoms, it accepts an electron from a neighboring silicon atom to complete its covalent bonds. This leaves a hole in the crystal lattice and this hole is free to move and act as a charge carrier. As we saw before it is the valence electrons which actually move but the hole appears to move from atom to atom. Boron is known as an acceptor impurity due to its ability to accept electrons when it enters the crystal lattice.
	The impure silicon in this case is known as 'p-type' because the majority charge carriers are are 'free' positive holes.
it is important to realize that the overall electric charge of both p-type and n-type semiconductors is zero. In each case the total number of electrons is balanced by an identical number of protons in the nuclei of the atoms. The p and n simply refer to the type of majority charge carrier (either holes or electrons). You should also realize that intrinsic conduction still takes place in both types so that some intrinsic conduction takes place via electrons in 'p-type' and some intrinsic conduction takes place via holes in 'n-type' semiconductors. The charge carriers in this instance are known as minority charge carriers.
The p-n Junction Many semiconductor devices rely for their operation on the use of a p-n junction. A p-n junction is the point at which p and n type materials meet in a crystal lattice.
As soon as a p-n junction is created (by doping) free electrons near the junction migrate or diffuse from the n-type material into the p-type material to fill holes. at the same time holes in the p-type material migrate or diffuse into the n-type material where they capture electrons.

The two basic types of transistor are bipolar, which we will look at here and unipolar or field effect transistors which we will look at later.
Bipolar or Junction Transistor This kind of transistor consists of two p-n junctions (hence the name) in the same crystal. A simplified cross section is shown diagrammatically above. There are two arrangements possible n-p-n and p-n-p. The arrow on the symbol of each type shows the direction of conventional current. Remember - electron flow is the reverse of this ! With n-p-n transistors electrons are the majority charge carriers. With p-n-p types holes form the majority carriers.

Bipolar npn transistors		Bipolar pnp transistors
How it works When the base emitter voltage (V_be) is about 0.7V electrons begin to move across the p-n junction into the base region. They are replenished with electrons from the emitter side of the circuit. At the same time holes move from the base towards the emitter, but their number is small compared with the electrons moving in the opposite direction. A small number of electrons combine with holes in the base region but the vast majority continue on to the collector where they are attracted by positive voltage on the collector.
The flow of holes into the base region causes a small base current to flow. This keeps the base-emitter junction forward biased and so maintains the collector current. When the base current is removed the collector current stops. The small base current controls the larger collector current. The bipolar transistor is a current controlled device.
Field Effect Transistors In a circuit a field effect transistor or FET will perform the same function as a bipolar transistor, but it works in a fundamentally different way.
An FET consists of a continuous strip of 'n' or 'p' type semiconductor with a metallic connector at each end called the drain (D) and source (S). The drain and source of an FET, perform the same function as the collector and emitter in a bipolar transistor. A third metal contact called the gate (G) performs the same function as the base lead in a bipolar transistor. Unlike the bipolar transistor however, the gate of an FET is voltage controlled. The gate current is negligible - approx 1pA (1x10^-12A)
How it works If the drain is made *positive* relative to the source, electrons flow from source to drain. If the gate is then made *negative* relative to the source a depletion layer is created. As the gate becomes more negative the depletion layer widens, reducing the drain current I_D. The gate-source cut-off voltage is the voltage (V_GS) at which the drain current ceases to flow.
Summary Bipolar transistors rely on both majority and minority charge carriers (electrons and holes). FET's are Unipolar devices where only majority carriers (electrons) carry charge around the circuit. Bipolar transistors are current controlled devices. FET's are voltage controlled. The input resistance of an FET (the resistance at the gate is very high - hence the tiny gate current).
MOSFET's The diagrams below right show the basic construction and circuit symbol of an n-channel MOSFET.
MOSFET's (metal-oxide-semiconductor FET's) consist of a p-type silicon body or substrate into which two heavily doped n-regions are diffused. These are connected to the drain and source terminals respectively.
A very thin layer of silicon oxide is then grown separating the gate terminal from the n-channel which forms in the p-type substrate when V_GS becomes positive. The drain is normally made positive with respect to the source. If the gate is also made positive with respect to the source holes in the p-type substrate are repelled, leaving behind a narrow channel of n-type silicon which provides a conducting path from source to drain. In essence the channel is originally p-type when V_GS is zero but becomes n-type as V_GS increases positively. The resistance between source and drain then decreases and a drain current will I_D flow. The size of the drain current can be controlled by varying the gate voltage V_GS.
A p-channel MOSFET consists of a very thin layer of silicon oxide separating the gate terminal from the p-channel which forms in the n-type substrate when V_GS is negative. The drain is normally made negative with respect to the source and the drain I_D current is zero until the gate voltage V_GS is made negative with respect to the source.
In an n-channel MOSFET the charge carriers are electrons. In a p-channel MOSFET the charge carriers are holes.
MOSFETS are ideal for constructing integrated circuits, because they can be very compact allowing many thousands of transistors to be packed into a very small area.