Semiconductor Devices and Circuits - Coursework Example

Semiconductor Devices and circuits. Materials are divided into three basic groups ly conductors, Insulators and semi conductors. The difference is centered on their electrical conductivity with conductor material having highest conductivity and insulators the lowest. Semiconductors materials fall between the conductors and insulators and their conductivity affected by factors like light, temperatures, impurities and magnetic field. Covalent bond results when two atoms from different materials share their outermost electrons to achieve a stable balance. Each atom after sharing electrons will attain a full outer shell. Conductors: Conductor materials have large number of free electrons and thus conduct easily. Examples of a good conductor are copper and aluminum and they have, at room temperature, many electrons for conduction. Insulators These are materials which do not conduct. For insulators, there is a very large gap between the two bands i.e. conduction and valence bands. Insulators can only conduct at extremely high temperatures or when they are subjected to very high voltage. Examples of insulators are, wood, glass, paper and mica. Semiconductors: These materials are between conductors and insulators. They are neither conductors nor insulators. They can easily conduct at room temperatures. Examples of these materials are silicon and germanium. Intrinsic semiconductors: These are pure semiconductors since the content of impurity is minimal. These materials are cautiously refined to achieve this form of purity. When intrinsic semiconductors are subjected to room temperature, valence electrons absorb heat energy breaking the covalent bond and move to conduction band. Electrons and holes conduction In intrinsic semiconductors, holes and free electrons are the major charge carriers. Holes and electrons which are generated at room temperatures move in a random motion and can’t conduct any current. When Voltage is applied across the semiconductor, then the electrons and holes assumes one direction therefore causing current flow. The applied voltage moves the free electrons which are in the conduction band. In this, electrons which are negatively charged are repelled from batteries negative terminal and they move towards positive terminal. This allows electric current to flow as a result of electrons movement in the conduction band. The motion of holes in semiconductors is opposite to that of electron. As a result of electrons escape from valence band to conduction band, holes are created in the valence band. When current is applied the electron break and moves to feel the holes present, at the same time this electron leaves another hole and this process continues. The movement of these holes in the valence band leads to current flow referred to as whole current. Extrinsic Semiconductors Small amounts of other materials are added to intrinsic semiconductor to alter the properties and this result to extrinsic semiconductor. This is process of adding an impurity to improve the conductivity of the semiconductor is referred to as doping. The materials used as impurities are called dopant. These semiconductors are the most widely used in practice since they have better conductivity compared to intrinsic semiconductors and are used in manufacture of electronics components such as transistors and diodes. There are two types of extrinsic semiconductors and each depending on the type of impurity used. They are n-type and p-type. N- Type Atoms with five electrons in their outer most shell are referred to as pentavalent atoms. When these pentavalent atoms are used to dope intrinsic semiconductors, they donate a free electron to the semiconductor and they are called donor dopers. Examples of donor materials are phosphorous, arsenic and bismuth. The resulting semiconductor has a large quantity of free electrons and is called n-type semiconductor. Example of n-type semiconductor formation is when arsenic atom with five electrons on the outermost shell combines with silicon with four valence electrons. The four valence electrons of silicon combine with four arsenic electrons to form a covalent bond. The fifth arsenic electron cannot form a covalent bond and this extra electron moves to conduction band resulting to a free electron. The extra electron has negative charge and the resulting semiconductor material is called n-type. In n-type semiconductors, electrons are the major charge carriers. P-type Trivalent atoms which have three valence electrons are used to dope semiconductors and results to acceptor impurity. Adding this impurity creates holes which accept electrons and this doping is referred to as acceptor doping. Examples of acceptor materials are Boron, gallium and indium. The resulting semiconductor will have large quantities of holes and is referred to as p-type semiconductor. Example of p-type semiconductor occurs when silicon (Si) is added to gallium (Ga).Gallium atom with three electrons will combine with silicon crystal to form a covalent bond but with one electron less. There is an incomplete bond in the outer shell and the vacancy is referred to as the hole. Holes are taken as positive charge and the resulting semiconductor is called p-type semiconductor. Conduction in p-type and n-type semiconductor. P-type semiconductor. P- Type material. Holes (Majority) + Electrons –Minority Carriers The above figure shows conduction in p-type material. When an external voltage is applied to the p-type material, the holes will move in a valence band, they are the main charge carriers. The electrons are also present and act as minority charge carriers. Since like charges repel, positive charge from the battery terminal will repel the holes which are then attracted by the negative charge on the batteries negative terminal. N-type semiconductor n- Type material. Holes (Minority) + Free electrons –Majority Carriers The above figure shows conduction in n-type material When n-type semiconductor is connected to battery terminals, the positive terminal will attract the electrons which posses a negative charge. On the other side of the battery terminal, the negative charge will repel electrons towards the positive terminal. This allows free flow of electricity. P-n junction This junction is formed between p-type semiconductor and n-type semiconductor. This junction possesses properties that are greatly used in modern electronics. Both the n-doped and the p-doped semiconductors are relatively conductive but the junction formed between these two semiconductors is non-conductive .In a p-n junction, free electrons will move across from the n-type region and recombine with holes from p-type side. In the same way, holes from p-type semiconductor diffuse to n-type side to combine with majority electrons on n-side. This diffusion of and recombination of electrons and holes on the p-n junction results to a narrow region on both sides of the junction which contains no moving charge and is referred to as the depletion layer. The p-type acquires a small negative charge; similarly the n-type acquires small positive charge. Depletion layer p-type side n-type side Current flow in a p-n junction If p-side is supplied with a positive charge and n side with negative charge, there will be current flow through this junction. Since negative charges are attracted by positive charges, then the free electrons from the n-type are attracted to the holes on p- side. The charges then start drifting across this junction to the other side. Under equilibrium, diffusion current is balances the drift current and the resulting net current across the junction becomes zero i.e. when there is no voltage or current applied from external source, there is an energy barrier that will prevent additional diffusion of charges across the p-n junction. Holes diffusing from p-type will be repelled by the small positive charge in the n-type and likewise electrons moving from n-type will be repelled by slight negative charges that were induced in p-type material. This will result to the junction becoming stable. Forward bias When a p-n junction is connected in forward bias, the p-type of the material is connected to the positive terminal and the negative terminal of the battery is connected to the n-type side. This connection allows the holes in the p-type side and electrons in n-type side to be forced towards the junction. This results to the reduction in width of the depletion area. The positive charge from the battery repels the holes in the p-type material while the negative charge repels the electrons. When the forward bias voltage is increased, the depletion zone collapse and become thinner. As these huge numbers of charge carriers moves across this junction, electric current flow through the junction. P-type N-type Reduced depletion zone Holes movement Electrons movement + - Reverse bias. In reverse bias, the batteries positive terminal is connected to the n-region and the negative terminal to p-region of the p-n junction. Electrons from n-region are attracted by the positive battery terminals and the holes get attracted by the battery negative terminal. As a result of this attraction, the potential barrier widens and increases the barrier potential and thus preventing the flow of current across this junction. The junction thus offers very large resistance to current flow. When reverse voltage is increase above particular value, there will be high reverse current flowing which will eventually damage the diode. This phenomenon is referred to as reverse breakdown of the diode. P-type N-type Enlarged depletion zone Holes movement Electrons movement - + Reverse breakdown of a diode takes will due to avalanche effect and Zener effect. Avalanche effect occurs when the increased reverse voltages in return increases the velocity of minority charge carriers causing collision which breaks the covalent bonds. The huge number of these carriers then crosses the junction and breaks the p-n junction resulting to a very huge reverse current. Zener effect occurs when the depletion zone becomes very thin as a result of heavy doping of a p-n junction. The intense electric field caused by high reverse voltage pulls the electrons from the outer shell. The resulting free electrons will constitute very huge current leading to diode breakdown. Junction diodes Non-conductive p-n layer can be manipulated to make devices such as diodes which can allow electricity to flow in one direction only. Diodes are used to protect electronics devices such as radios and recorders. When the power source for these devices is connected in the wrong way, then no current will flow. Forward current Reverse-bias Forward –bias Diode voltage (VD) Diode breakdown Reverse Current (Id) Transistors: They are semiconductor devices which are used as amplifier and also as electronics switches for electrical power. Basically, the two major types of transistors are the junction transistors and field effect type of a transistor. They have three terminals; base, collector and the emitter. For a p-n-p type, an n –type material is sandwiched between two p-type materials. The emitter and collector terminals are connected to the p layer while the base is connected to the middle n layer. In a p-n-p transistor, the emitter holes cross the emitter base junction and moves towards the base region. p-n-p transistor diagram Collector terminal Emitter Terminal Base terminal NPN In n-p-n transistor, the base voltage should be more positive compared to that of emitter, also the collector voltage must be extra positive, more than the base. There is a flow of electricity in this arrangement as the base will pull the electrons from the emitter as it has much more positive voltage than the emitter. n-p-n transistor diagram Collector terminal Emitter Terminal Base terminal Effect of Heat and light on semiconductor Semiconductors will behave in the same manner as an insulator when at zero temperatures. The temperatures increase when the semiconductor is heated, and due to this thermal energy, the electrons forming the covalent bond break away and move to conduction band .The free electrons and holes generated as a result of electron break enables the current to flow. At higher temperatures, semiconductors will behave like good conductors. Thus an increase in temperature leads to an increase of charge carriers in semiconductors. Similarly, light has the same effect as heat in that the light energy will cause electrons break covalent bond. When semiconductors are illuminated, they pass on light energy which results to electrons breaking and moving to conduction band. Due to these free electrons, the semiconductor when illuminated behaves like a conductor. Concentration of Charge carriers Zero Temperature Work Cited Singh, Jasprit. Electronic and Optoelectronic Properties of Semiconductor Structures. New York: Cambridge University, 2003. Press. Peter, Yu and Manuel, Cardona, Fundamentals of Semiconductors. New York: Springer, 2001. Print. Nye, J. Physical Properties of Crystals: Their Representation by Tensors and Matrices. USA: Oxford University, 1985. Print. Prasad, Sheila. High Speed Electronic and Optoelectronics. New York: Cambridge University, 2009. Press. Pierret, Robert. Semiconductor Device Fundamentals. Springer: Cambridge University press, 2005. Print. Read More