Semiconductors: The substances whose electrical conductivity lies between conductors and insulators are called Semiconductors.
e.g.: Silicon, Germanium, Carbon, Cadmium Sulphide.
* The conductivity of semiconductor increases with increasing temperature.
* Semiconductors have negative temperature coefficient of resistance.
* In semiconductors, electrical conduction is due to electrons and holes.
* The resistance of semiconductors decreases due to the addition of impurities.
* In a semiconductor there is a small energy gap (≈1 ev) between valence band and conduction band.
* At absolute zero temperature conduction band is completely empty and semiconductors behaves as an insulator.
* Semi conducting elements are tetravalent i.e., there are four electrons in their outer most orbit.
* Their lattice is face centred cubic (F.C.C.)

Intrinsic Semiconductors:
* A semiconductor in pureform is called intrinsic semiconductor.
* In intrinsic semiconductor no. of free electrons = no. of holes.
* For intrinsic semiconductor the Fermi energy level is at the centre of the conduction band and valence band.
* When electric field is applied across intrinsic semiconductor electrons and holes are move in opposite direction.
Extrinsic Semiconductors:
* Semiconductor to which impurities are added is called extrinsic semiconductor.
* Extrinsic semiconductor the Fermi level is not exactly at the middle of the conduction band and valence band.

Types of Extrinsic Semiconductors:
n-Type Semiconductors: * When a small amount of pentavalent impurity (e.g.: Arsenic) is added to an intrinsic semiconductor.
* In the n-type semiconductors, the no. of electrons in the conduction band is more than the no. of holes in the valency band.
* In n-type semiconductor the fermi level shifts towards the conduction band.
* There is no charge on n-type semiconductor because it is formed by the combination of neutral atom.
p-type semiconductors:
* When a small amount of trivalent impurity (e.g.: Gallium) is added to intrinsic semiconductor.
* In p-type semiconductor 'holes' are majority charge carriers, while electrons are minority charge carriers.
* In p-type semiconductor the fermi level shifts towards the valence band.
* There is no charge on p-type semi conductor because it is formed by the combination of neutral atom.
p - n Junction Diode
* If a p-type semi-conductor suitably joined to a n-type semiconductor the junction is called p-n junction and the device so formed is called p-n junction diode.

* At the junction on both sides a region is formed which is depleted of charge carries [electrons and holes]. This region is called depletion region.
* It has two electrodes, hence it is called diode.    

Symbolic representation of diode

* The direction of current flow is represented by the arrow head.    
* An electric field is developed across the junction, which is in a direction to oppose the further diffusion of electrons from n-side.
* The potential developed across the barrier layer is called barrier potential. It is 0.7 volt for Silicon diode and 0.3 volt for Germanium diode.
* When no external source is connected to diode, it is called unbiased.
Forward-Bias:
* When p-side is connected to positive terminal and n-side to negative terminal of battery, the diode is said to be forward biased.
* When battery voltage exceeds the barrier potential majority charge carriers cross the junction.

* The electrons from n side drift towards the junction and crosses it and holes in the opposite direction.
* The barrier potential decreases.
* The width of the depletion layer decreases.
* Thus forward biased diode conducts.
Reverse - Bias:
* When p-side is connected to negative terminal and n side to positive terminal of battery the diode is said to be reverse biased.
* The battery acts as reverse bias to a majority charge carrier. It acts as a forward bias to minority carrier.
* The minority charge carriers move across the junction. This constitute reverse saturation current which is ≈ 1 µ A.
* The barrier potential increases.
* The width of depletion region increases.

Break down:
* If the applied voltage is increased, the covalent bond in semiconductor are broken and a large no. of charge carriers [electron-hole] are generated. So, current increases very sharply. This is called "break down".
* The voltage at which break down takes place is called "break down" voltage.
* As the break down occurs, the diode conducts very heavy, i.e., it offers very low resistance.
p-n junction diode as a rectifier:
* A diode conducts in forward bias and does not conduct in reverse bias. This unidirectional property leads to application of diode in rectifier.
* Rectifier: It is a device used to convert A.C. to D.C.
Half Wave Rectifier:
* Half wave rectifier make use of single diode.
* During (+ve) half of input (diode D is forward biased) diode conducts.
* During (-ve) half of input (diode D is reverse biased) diode does not conduct.
* So output is obtained only during positive half cycle of input.

       Where rf is diode forward resistance and RL is load resistance.
* In half wave rectification, a maximum of 40.6% of a.c. power is converted into d.c. power.
Full-wave rectifier:
* A full wave rectifier make use of two diodes D₁ and D₂ and centretapped transformer.
* During positive half (D₁ is forward biased, D₂ is reverse biased) diode D₁ conducts.
* During negative half (D₂ is forward biased and D₁ is reverse biased) diode D₂ conducts.
* Thus we obtain output during both positive and negative half cycles of input, alternatively by D₁ and D₂.
* Efficiency of full wave rectifier η =  
* In a full wave rectifier, a maximum of 81.2% of ac power is converted to dc power.

Zener Diode:
* It is a heavily doped reversed biased p-n junction diode which is operated in the break down region. Its symbol is 
* Zener diode is always used in reverse bias.
* The resistance of Zener diode becomes Zero.
* Zener diode is used as voltage regulator. The circuit diagram is
In this:       I = IL + IZ    
               Vin = VZ + i R
              Vout = VZ
Transistor:
* Transistor is a three layer semiconductor device consisting two p-n junction diodes.
* A transsistor transfers a current signal from a path of low resistance to a path of high resistance.
* TRANSFER + RESISTOR = TRANSISTOR.

* Transistor consists of three regions called, Emitter (E), Base (B) and Collector (C).
* The Emitter base junction is forward biased and the collector base junction is reverse biased.

* Transistors are mainly used as amplifiers and also as oscillators and switches.
* Transistor can be connected into the circuit in three different ways.
      i) Common - Base configuration. (CB)
     ii) Common - Emitter configuration. (CE)
     iii) Common - Collector configuration. (CC)
* The current amplification factor (or) current gain (α) of common base configuration is given by

                                    
     The value of 'α' is less than unity and it ranges from 0.9 to 0.99.
* The current amplification (or) current gain (β) of common emitter configuration is given by 

                                     
     Its value is typically in the range 20 - 200.
* Relation between α and β

     As 'α' approaches unity, β approaches infinity.
* A transistor has very high current gain in C.E. configuration. Because of this reaction C.E. configuration is used.
Relation between transistor currents:

      The three transistor currents always bear the following ratio.   IE : IB : IC = 1 : (1− α) : α
* Input resistance of transistor in C.E. configuration is given by

* Output resistance of transistor in C.E. configuration is given by

                 
* Voltage gain =  current gain × resistance gain