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| N-type Semiconductor |
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| When the impurity atom is a pentavalent atom say arsenic, phosphorus, these atoms will replace the silicon atom as shown below. |
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| The 4 valence electrons of arsenic are bonded covalently with 4 of silicon atom, while the fifth electron is loosely bound with parent impurity atom and is comparatively free to move. At room temperature, some bonds may get ruptured thereby releasing equal number of free electrons and an equal number of holes. But the total number of holes in N type is relatively low. Hence in N type, electrons are majority charge carriers and holes are minority charge carriers. |
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| It is seen that energies of free electrons in n type are slightly less than that of free electron in lowest energy level in conduction band. The energy 0.05 eV is comparable to the thermal energy of electron and so a very small energy is supplied to excite electrons from donor levels to conduction band. Similarly, in p type holes are to cross 0.08 eV to go to the valence band. |
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| Consider a semiconductor block of length l and area of cross-sectional A. Let ne and nh be the electron density and hole density. If V is the p.d. across the block, then, |
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| I = Ie + Ih |
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| = neAeVe + nhAeVh |
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| where Ve and Vh are the drift velocities of electrons and holes respectively. |
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| In case of low electric fields, semiconductors obey Ohm's law, |
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| where R is the resistance of the semiconductor. |
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| Electric field, |
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| where me and mh are the mobility of electrons and holes and s is conductivity of semiconductor. |
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| As ne, nh increase with rise in temperature, s (conductivity) also increases. |
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