2_1_class slides 11 - EE2 Fall 2007 Class 11 slides Outline...

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EE2 Fall 2007 Class 11 slides
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Outline 1. Review of current flow in semiconductors 2. Drift current 3. Mobility 4. Conductivity and resistivity 5. Temperature dependence of mobility 6. Diffusion 7. Einstein relation 8. Preliminary discussion on generation and recombination
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Electric Current Flow Electric current flows in a solid when electrons move from one end of the solid to the other and each electron carries a charge of –q coulomb resulting in a flow of electric charge through the solid. If Q coulomb flows through the solid per second we say an electric current of Q amperes flows in the solid. Typically, the symbol for electric current is I’ Electric current can flow in semiconductors by two mechanisms: one called the drift current which is due to the flow of electrons caused by the electric field due to the applied electrostatic potential difference. This is the electric current that we normally associate with current flow in metals and other conductors. In semiconductors there is another mechanism of electric current flow caused by spatial variations in the concentrations of electrons. This mechanism is called diffusion and causes a current flow due toelectrons flowing from regions of high concentration to regions of low concentration,
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Drift Current For the drift current to flow there should be an applied electric field. From our earlier studies we know electric field is the negative gradient of the
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