PHYSICS OF SEMICONDUCTOR DEVICES
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1. When a P-type semiconductor is joined with an N-type semiconductor, a PN junction is formed known as a semiconductor diode. 2. Semiconductor diodes are widely used as rectifiers to convert alternating current (AC) input into direct current (DC) output. 3. In a PN junction, the diffusion of majority carriers across the junction leaves behind charged acceptor and donor ions which form an electric field called the depletion region or space charge region.
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PHYSICS OF SEMICONDUCTOR DEVICES
- 2. Formation of PN – Junction When a P-type Semiconductor is joined together with an N-type Semiconductor a PN junction is formed. And it is also known as a Semiconductor Diode. Semiconductor diodes are widely used in Rectifiers which converts input AC signal into DC output signal.
- 3. P N - - - - - - - - + + + + + + + + Space charge region (OR) Depletion region JunctionJunction Ionized acceptorsIonized acceptors Ionized donorsIonized donors Potential barrier(VPotential barrier(V00)) Potential barrier widthPotential barrier width (W)(W)
- 4. Depletion Region & Space Charge The diffusing majority carriers from the two regions recombine near the junction and disappear. The uncompensated Acceptor and Donor ions set up an Electric field which halts majority carrier Diffusion and causes minority carrier Drift. The two kinds of majority carriers diffusing across the junction meet each other near the junction and undergo recombination's, leaving negative ions on the P-side and positive ions on the N-side of the junction. This distribution of Positive and Negative Charges is called Space charge.
- 5. N P Cathode Anode _ + Diode Symbol
- 6. Fermi level Depletion region PN - junction EFn Valence band Conduction band NP EFp E eVB eVB Ev Ec Ev Ec Energy level diagram
- 7. V - I Characteristics of PN Junction The diode can be operated in two different ways, as Forward and Reverse bias. When positive terminal of the battery is connected to the P- type & negative terminal is to the N-type of the PN-junction diode, known the diode is kept in forward bias. When negative terminal of the battery is connected to the P- type & positive terminal is to the N-type of the PN-junction diode, known the diode is kept in reverse bias.
- 8. Depletion Layer N-regionP-region + + + + Donor ionsAcceptor ions V Potential barrier Forward bias
- 9. Depletion Layer N-regionP-region + + + + Donor ionsAcceptor ions V Potential barrier Reverse bias
- 10. Forward Bias Reverse Bias V I Current Reverse break down current Forward Current Knee Voltage
- 11. The region between knee voltage & breakdown voltage is known as non-ohmic region. Above the knee & breakdown voltage the current increases. Breakdown voltage is due to thermally broken covalent bonds. Diode is conducting in forward bias & non-conducting in reverse bias.
- 12. Light emitting diodes LED’ s are the most visible type of diode, that emits a fairly visible colored light, invisible infra-red or laser type light when a forward current is passed through them. Principle: Basically LED are made from a very thin layer of fairly heavily doped semiconductor material. When the diode is forward biased, electrons from conduction band combine with holes from in valence band, releasing sufficient energy to produce photons of light.
- 14. V Current
- 15. A Rectifier is an electronic circuit which converts alternating current to direct current (OR) unidirectional current. Rectifiers are mainly three types 1.Half wave rectifiers 2.Full wave rectifiers 3.Bridge rectifiers
- 16. An electronic circuit which converts alternating voltage (OR) current for half the period of input cycle hence it is named as half-wave rectifier.
- 17. Half – Wave Rectifier A.C Input Pulsated D .C Output transformer B RL rf
- 18. The ratio of D.C power output to applied A.C power input is known as rectifier efficiency.
- 19. An electronic circuit which converts alternating voltage (OR) current into pulsating voltage (OR) current during both half cycle of input is known as full-wave rectifier.
- 20. Full Wave Rectifier D .C Output A B Center tapped transformer rf rf RL
- 21. The ratio of D.C power output to applied A.C power input is known as rectifier efficiency.
- 22. 2 3 2 2 3 2 ) 2 (2Where )1().........exp( )exp() 2 (2 h kTm N kT EE Nn kT EE h kTm n e C cF C cFe ∗ ∗ = − = − = π π 2 3 2 2 3 2 ) 2 (2Where )2).......(exp( )exp() 2 (2 h kTm N kT EE Np kT EE h kTm p h v FV v FVe ∗ ∗ = − = − = π π Consider Intrinsic Semiconductor Electron Concentration Holes Concentration ion 1 & 2 holds good for both intrinsic and extrinsic semicondu r Thermal equilibrium condition.
- 23. )exp( ,, kT EE Nn EEEEnn cnFn Cn cncFnFn − = === For N type Material )exp( ,, kT EE Nn EEEEnn cpFp Cp cpcFpFp − = === For P type Material Electron Concentration )3.().........exp( )exp( )exp( )exp( )exp( kT eV nn kT eV n n kT EE kT EE kT EE n n B np B p n cncp cpFp cnFn p n − = = − → − − =
- 24. )exp( ,, kT EE Np EEEEpp Fpvp vp vpvFpFp − = === For N type Material )exp( ,, kT EE Np EEEEpp Fnvn vn vnvFnFn − = === For P type Material Hole Concentration )4().........exp( )exp( )exp( )exp( )exp( kT eV pp kT eV p p kT EE kT EE kT EE p p B pn B p n vpvn Fpvp Fnvn p n − = − = − → − − =
- 26. }1{11 −=∆= TK eV ppe B encnci }1{22 −=∆= TK eV nnh B epcpci
- 27. }1){( 21 −+=+= TK eV nphe B epcnciiI }1){( 21 −+=+= − TK eV nphe B epcnciiI
- 28. I0 is called reverse saturation current 021 )( IpcncI np −=+−= 1<< − KT eV e 1)exp(0 − − = KT eV II