Saturday, September 4, 2010

p-n diode

A P-N diode
The most frequently encountered  semiconductor electronic device  is the diode. It is a two terminal device consisting of a P-N junction formed either in germanium or silicon crystal. P-N junction is also sometimes called as crystal diode as the P-N junction is directly grown out of a crystal . A diode is a unidirectional device  it permits a easy flow to the current passing from one side only and offers a high resistance when the direction of the current is opposite.
  It is because , on the formation of the p-n junction some of the holes from the P-type material tend to diffuse across the boundary into N-type material and some of the free electrons similarly diffuse into the P-type material as shown  where blue circles are the holes and green circles are the free electron

this happens because of the concentration of holes is higher on P-type than on N-type and theconcentration of free electrons is higher on N-type than at P type . this process is called diffusion , the region across the junction is completely immobilized which leads to the formation of a depletion layer , transition layer or potential barrier.
The thickness of the depletion region is of the order of 1 micron (millionth of a meter ,10^(-6) meters )  .

  • Forward biasing- When a positive voltage is applied across the P-type semiconductor and negative terminal connected to the N-type semiconductor , then the diode is said to be in forward bias . the holes of P-type being positive charge repel away from the supplied positive charge and move towards the junction and similarly the free electrons from N-type also are driven towards the junction , In other words the forward potential establishes an electric field opposite to the potential barrier and hence the width of the depletion layer is reduced , and current could pass through it(diode) easily.

  • Reverse biasing - When the negative terminal is connected to the P-type semiconductor and the positive terminal to the N-type , the diode is in reverse bias . Because the applied reverse voltage establishes a electric field which acts in the same direction as that of potential barrier , thus making the depletion layer wider and offering high resistance to the current flowing through it .
Diodes are mainly used in rectifiers(AC to DC converters), clippers and as a safety device (to block the flow of current in reverse order), 

Sunday, August 15, 2010


Almost every electronics devices (mostly active components) ranging from transistor to ICs are all made of semiconductor. Semiconductor is one of the most important Electronics engineering material ever, which revolutionized  the modern world of Electronics. semiconductor devices have found wide application because of their compactness,reliability, power efficiency and low cost . They are also used in power devices ,optical sensors , light emitters(LED) and even solid state lasers.
        Resistivity of metals like iron and copper is 0.1 x  10-6 Ω·m and  1.7 x   10-8 Ω·m . The resistivity of  insulators like glass is 9 x 10-11 Ω·m , whereas the resistivity of semiconductor substance  are between 10-4   to 0.5  Ω·m . Thus those substance which have resistivity lower than insulators and higher than conductors are the semiconductors. 

Pure silicon crystal
(source :
      Anyway, Resistivity is not the only factor which decide whether the substance is a semiconductor or not . There are other factors(properties) too which make a semiconductor crystal unique and quite useful in the field of electronics . like ...

  • The resistivity of the semiconductor decreases with temperature an vice versa (semiconductor materials behaves as insulator at absolute zero (zero Fahrenheit))
  • The second and the most important property is that if  suitable impurity are added (doped) to a semiconductor block like arsenic,gallium , phosphorous the current conducting property increases , giving rise to a new improved semiconductor block with high conductivity  "Extrinsic semiconductor "  

Extrinsic semiconductor are mainly of two kinds "the P-type" and the "the N-type" . The P-type semiconductor are created by doping impurities with valency 3 (trivalent) like boron,aluminium etc and N- type semiconductors are created by doping impurities with valency 5 (pentavalent) impurity like arsenic and phosphorous .

                       like in a silicon crystal with valency 4 when a trivalent impurity (impurity for p-type semiconductor) is doped  the 3 outer electron (valence electron) of silicon crystal form a covalent bond with the 3 electron of the trivalent impurity . But in the fourth covalent bond only silicon atom contributes one valence electron and there is a deficiency of electron which is called a hole .Holes are positively charged and are the prime charge carrier in P-type semiconductor.
                               Whereas when a pentavalent(valency 5) impurity are doped the four atoms of silicon forms a covalent bond with the 4 valence electron of impurity leaving behind one free electron . These free  Electrons are the prime charge carrier of N-type semiconductor.

N-type semiconductor is more preferred then P-type as the mobility (drift velocity to applied electric field across material) of electrons ( prime charge carrier of N-type) is greater than that of holes (charge carrier of P-type).  

Thursday, August 12, 2010

Introduction to Electonics

The world Electronics means "electron mechanics" , It is a science which deals with the motion of electrons under the influence of electric or magnetic field , The ability to control electron flow is usually applied to information handling or device control ,thus giving rise to two field of engineering disciple Electronics & Telecommunication engineering.

All electronic circuits however complicated contains a large number of some basic electronics components
these components are either brought individually and are assembled together on a circuit board using soldering or through the process of micro-miniaturisation (reduction in size) are reduced to  extremely small dimension to make an integrated circuit (IC) , IC  revolutionized the world of electronics today .As many circuits which once used to be impossible to make once (late in 1948s) as it contains many thousands of components, can be made(fabricated) today on a single semiconductor chip. 
Electronics components are mainly divided into two parts Active components and passive components 

  • Active components - those devices which are capable of amplifying or processing an electrical signal .    eg Transistor, silicon controlled rectifier (SCR)
  • Passive components - Those devices which have neither of the feature of active components and are generally incapable of power gain , But can dissipate or store energy. eg Resistor , capacitor , inductor