12-12-2012, 06:26 PM
P-N JUNCTION DIODE
P-N JUNCTION DIODE.ppt (Size: 2.46 MB / Downloads: 93)
OBJECTIVE
1. describe the electrical properties of semiconductors and distinguish between p-type and n-type material;
2. explain the formation of a depletion layer at a p-n junction;
3. discuss the flow of current when the p-n junction diode is forward-biased or reverse-biased;
4. discuss the I-V characteristic of the p-n junction diode.
5. use the diode for half-wave rectification;
6. use the bridge rectifier (4 diodes) for full-wave rectification;
7. represent half-wave and full-wave rectification graphically;
8. discuss the use of a capacitor for smoothing a rectified ac wave;
9. answer questions and solve problems regarding the topics mentioned above.
INTRODUCTION
In the modern world no other technology permeates every nook and cranny of our existence as does electronics. The p-n junction is at the heart of this technology. Most electronics is silicon based, that is, the devices are made of silicon. Silicon wafers are subjected to special procedures which result in what is called p-type silicon material and n-type silicon material. Where these two types of materials meet we have a p-n junction. The physical characteristics of this junction are responsible for all the electronic wizardry we have become accustomed to. Televisions, radios, stereo equipment, computers, scanners, electronic control systems (in cars for example), all these have silicon based technology as there foundation.
SEMICONDUCTORS AND ELECTRONICS
Semiconductors are materials whose electrical conductivities are higher than those of insulators but lower that those of conductors.
Silicon, Germanium, Gallium, Arsenide, Indium, Antimonide and cadmium sulphide are some commonly used semiconductors.
Semiconductors have negative temperature coefficients of resistance, i.e. as temperature increases resistivity deceases.
INTRINSIC SEMICONDUCTOR
Both silicon and germanium are tetravalent, i.e. each has four electrons (valence electrons) in their outermost shell.
Both elements crystallize with a diamond-like structure, i.e. in such a way that each atom in the crystal is inside a tetrahedron formed by the four atoms which are closest to it.
Each atom shares its four valence electrons with its four immediate neighbours, so that each atom is involved in four covalent bonds.
EXTRINSIC CONDUCTION
A pure or intrinsic conductor has thermally generated holes and electrons. However these are relatively few in number. An enormous increase in the number of charge carriers can by achieved by introducing impurities into the semiconductor in a controlled manner. The result is the formation of an extrinsic semiconductor. This process is referred to as doping. There are basically two types of impurities: donor impurities and acceptor impurities. Donor impurities are made up of atoms (arsenic for example) which have five valence electrons. Acceptor impurities are made up of atoms (gallium for example) which have three valence electrons.