Dr Satyanarayan Dhal Lecture4 Intrinsic Carrier Concentration Objective We want to find the concentration of intrinsic carriers in terms of bandgap Some expressions The energy of an electron in the conduction band ID: 1030782
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1. Minority carrier impact Dr. Satyanarayan DhalLecture-4
2. Intrinsic Carrier ConcentrationObjective :We want to find the concentration of intrinsic carriers in terms of bandgap.Some expressions :The energy of an electron in the conduction band : Here Ec is the energy at the conduction band edgeMe is the effective mass of an electron.
3. Density of StatesThe concentration of electrons in the conduction band :N = Density of States X Fermi Probability Distribution Function
4. Energy scale for statistical calculationsThe Fermi distribution function is shown on the same scale, for a temperature kT << Eg.The Fermi level is taken to lie well within the bandgap, as for an intrinsic semiconductorIf E = µThen F = ??
5. A very important relation !!!No involvement of Fermi Level.The same assumption is made also with the impurities as well.
6. Intrinsic MobilityThe mobility is the magnitude of the drift velocity per unit electric field.Both positive for electrons and holesThe electrical conductivity is the sum of the electron and hole contribution.
7. Conductivity due to Impurity !!!Deficit Semiconductors : There are certain compound semiconductors, deficiency of one constituent will act as an impurity.Doping : The measured addition of impurities to a semiconductor.7Donor Ionization EnergyBohr Radius of DonorCharges with As in Si
8. Conductivity due to Impurity !!!Deficit Semiconductors : There are certain compound semiconductors, deficiency of one constituent will act as an impurity.Doping : The measured addition of impurities to a semiconductor.8Charges with B in Si
9. Method for Measuring Minority & Majority Carrier Mobilities in Solar Cells9Describe what minority carrier diffusion length is, and calculate its impact on Jsc, Voc. Describe how minority carrier diffusion length is affected by minority carrier lifetime and minority carrier mobility. Describe how minority carrier diffusion length is measured. Lifetime:- Describe basic recombination mechanisms in semiconductor materials.Calculate excess carrier concentration as a function of carrier lifetime and generation rate. Compare to background (intrinsic + dopant) carrier concentrations. Mobility:- Describe common mobility-limiting mechanisms (dopants, temperature, ionic semiconductors).
10. Method for Measuring Minority & Majority Carrier Mobilities in Solar Cells10Minority Carrier Diffusion Length :The average distance a minority carrier moves before recombining.Importance to a Solar Cell: Photoexcited carriers must be able to move from their point of generation to where they can be collected.Longer diffusion lengths generally result in better performance.
11. Impurity photovoltaic (IPV) effectused to enhance solar cell infrared response and therefore enhance cell conversion efficiency.Involves on the insertion of deep defects in the solar cell. Creates a multistep absorption mechanism for sub-band gap photons to create new electron-hole pairs.11Ref : Ghania Azzouzi and Wahiba Tazibt / Energy Procedia 41 ( 2013 ) 40 – 49