✤ SEMICONDUCTOR : Those solids which have higher resistivity and lower density of electrons

                           1. They have negative temp coefficient.

                           2. with temp their resistivity decreases rapidly.

                             Because large no of content and breakdown.

                                                    Semiconductor

          Intrinsic (unidirectional)                                                  Extrinsic

      Pure form of semi conductor                                  Impure form of Semi conductor

   (impurity less than 1 part in 1010) 

                    (ne=np)                                                          Ex: GaAs, Cds etc.

                  Ex - Si, Ge etc.

✤ DOPING -> To increase the conductivity of semiconductor some desirable impurity is added in them, this process is called doping and added substance                     is called dopant.

     1. The concentration of dopant atom should be small about 1 Part per million.

   ❆ More doping → will become conductor

   ❆ Less doping → become as insulator

     2. The dopant atom should nearly replace the semiconductor atom.

     3. The semiconductor material should be of v.high purity 99.999

     4. Size of dopant atom should be almost same.

✤ Type of dopant: 1. Pentavalent -- They have 5 valence electrons, e.g., P

                               2. Trivalent -- They have 3 valence 'e'

                                                       e.g. - In, B, Al

✤ Method of doping:  1. By molten semiconductor

                                    2. By heating

                                    3. By bombarding

        1. Obtained by adding impurity                                      1. Obtained by adding

             of trivalent dopant.                                                          impurity of pentavalent

             e.g., In, B etc.                                                                  e.g., As, P etc.

        2. These are called acceptors.                                      2. They are called donor.

        3. Hole density is much higher                                      3. Electron density is

             than free electrons.                                                      much higher than hole.

                  np >> ne                                                                              ne >> np

            # holes are "vacancy of                                              They are electrically neutral

             electrons" valence band.                                      

                           Difference b/w Intrinsic and extrinsic

                Intrinsic                                                          Extrinsic

1. Pure form of semiconductor          1. Impure form of semiconductor

2. Conductivity is low                         2. conductivity is high

3. Conductivity depend                       3. Conductivity depend on temp

     on temp. and dopant.

        #   1. In intrensic semiconductor ne=ng

             2. In n-type semiconductor ne >> na

             3. In p-type semiconductor na >> ne

             4. Minimum energy required to create a hole-electron pair, Eg = hVmin

                    Eg = hc/λmin      h = Plank's constant = 6.6x10-34 Js

                                                 c = 3x108 m/s.

#      1. Mobility of charge carrier, u = V/E -> electric field

         2. Electric current in semiconductor

                                I = eA(neVe+naVa)

              Here A = area, e = 1.6 x 10-19C, Ve = velocity of e

                  Vh = velocity of "hole", µe = mobility of e, µa = mobility of "hole".

         3. Electrical conductivity

                                σ = e(neµe+naµa)

         4. Resistivity

                                P = 1 / e (neµe+nhµh)

                           unit of P -> Ωm Unit of μ -> m2V-1s-1

✤ P-n junction: when p-type semiconductor is mixed with n-type semiconductor then it is called p-n junction.

                                                            

✤ Some terms related to P-n junction:

✧ Depletion region => It is the small part in the vicinity of the junction which is depleted of free charge carrier. It has only immobile ions.

✧ Barrier Potential => It is the minimum Potential require to overcome depletion region. (VB)

               §. VB depend upon (i) nature of Semiconductor (ii) temp (iii) doping

✧ Diffusion current => diffusion of majority charge carrier across the junction gives the electric current from p→n is called diffusion current.

✧ Drift current => The current is setup by the barrier field from n→p is called drift current.

                                §. drift current and diffusion current are in opposite direction.

                                §. in equilibrium drift current = diffusion current.

                                § in semi conductor VB = 0.5 Volt

                                       width of depletion = μ

                                       electric field E = 5 X 105 Vm-1

                            Symbol of P-n junction

                            

✤ Forward biasing: If +ve terminal of a battery is connected to p-type and -ve terminal to the n-side then the p-n junction is said to be forward biased

                                                  V → applied voltage

                                              VB → barrier potential

                                        

✤ Reverse biasing : If +ve terminal of a battery is connected with n-type and -ve with p-type then p-n junction is said to be  reverse biased.

                                                

                

✤ Rectifier => The process of converting alternating current into direct current is called rectification, and the device is called rectifier.

                        • Type of rectifier => ① half wave rectifier

                                                          ② full wave rectifier.

                  1) HALF WAVE RECTIFIER

                                    

                  2) FULL WAVE RECTIFIER

                                        

✤ Zener diode: =>It is a specially designed diode to operate in reverse breakdown region.

✤ Principle => It is based on the fact that when Zener is used in reversed breakdown region the Voltage across it remains Practically constant for a large                         change in the reverse current.

                    

❆ Condition for using :              1. Zener is used in reverse biased

                                                    2. Zener should have voltage more than freq.

                                                    3. Current is less than the down voltage(V2) maximum zener current.

✧ use :      1. As a voltage regulator.

✤ Photodiode => It is a p-n junction fabricated from a photosensitive semiconductor and provided with a transparent window so as to allow the light fall

                            on its junction.

                                

✤ Condition for using

        uses: 1. in light operating switches

                  2. In detection of optical signals.

                  3. In demodulation of signals.

❆ Light emitting diode: It is a heavily-doped forward biased p-n junction which convert electrical-optical energy into infrared or visible light.

                                           Symbol                                                                Circuit:

                                

           1- The colour of emitted light depends on band gap energy.

           2- Intensity of emitted light depend upon current.

Use               1. LED's use as indicator lamps.

     2. In optical communication.

     3. In digital clock etc.

✤ Solar cell: It is a junction diode which converts solar energy into electrical energy.

                        • Principle: It is based on photo electric effect circuit: (image of a circuit diagram)

                            § Three basic process in solar cell.

                                         1) generation              2) separation         3) collection.

                           uses: 1) in space crafts         2) in wrist watches             3) in solar                                                                                                                         batteries.

  Note:          1. solar cells are prepared from Si (Eg=1eV)

                    2. Solar cell can supply current only if the incident light of energy (E=hv) is                         greater than the energy gap(Eg) of junction diode.

✤ Germanium preferred over silicon because energy gap for Ge (Eg = 0.7 eV) is smaller then the Si (Eg = 1.1 eV)

     Note: 1. Fermi energy: It is the maximum possible energy Possessed by free electrons at 0K.

                2. In pure semiconductor, the fermi level is about half way in the  energy gap.

                3. with rise in temperature, the fermi level moves towards the centre of energy gap.

                4. n-type semiconductor is better than p-type as electrons has more mobility than hole. Conduction current in n-type is more than p-type.

                            

 Note: with rise in temp

                1) resistance of semiconductor decreases

                2) conductivity of semiconductor" " increases

 Note: The pn junction can be considered to be equivalent to a capacitor with P and n- regions acting as the plates of capacitor and depletion region as the             dielectric medium.

 Note: In germanium, the ratio of reverse to forward resistance is 40000:1 for silicon is 10⁶:1

                                                    

          2. In pn junction, there is a diffusion of majority carriers across the junction in forward biasing and drifting of charge carriers in reverse biasing.

 Note: An ideal junction diode act as a perfect conductor when it is in forward biased. And perfect insulator in reverse biased.

         2. The drifting of electron from n to P side and drifting of holes from P to n side will make conventional current.

     1. The output obtained from p-n junction is unidirectional, intermittent and pulsating d.c.

     2. efficiency is small (30%) to 40.6%

     3. frequency of output = frequency of input

     4. output dc = Idc x R1.

     1. The out put from full wave is unidirectional, continuous with ripple contents which are not perfect d.c.

     2. Effeciency is high. (60-80%)

     3. frequency of out put voltage/current is twice the input.

     4. Ripple factor = Value of a.c / Value of d.c = I a.c / I d.c.

     5. effeciency of rectifier = (output d.c / input a.c) * 100

 1. Semiconductors prepared by chemical method leave impurities at the part for thousand (PPT) level. But in commercial it should be purified at PPb         (part for billion).

 2. Energy gap for     1) insulator >3ev.     2) for silicon, 1.1 ev         3) germanium, 0.7ev.

            1. resistance of semiconductor decreases.

            2. conductivity increases.

           1. diffusion      2. drift.

 10. p-n junction can be considered to be equivalent to a capacitor with p and n region acting as plates of capacitor and depletion region as dielectric         medium.

Q)         Calculate Vo and I if be and Si Conduct

                                                         1) 0.3V and 0.7 volt.

                                                         2) Find total current in --

                                                    

             3) Find current in 1ohm.