Lecture3 - 10/23/2010 10/23/2010 2...

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10/23/2010 1 LECTURE 3: Carrier Action; Definition of Current and Its components EECS 170A 10/23/2010 1 10/23/2010 2 CURRENT CONDUCTION AND RESISTIVITY In previous lectures we answered on how current was created where we started with definition of current After all current means #of charges passing per second. Hence, in order to conduct electrical current we need to transport carriers from one location to other. Carriers can move around by the effect of two different mechanisms: Diffusion or electromagnetic field , that generate Diffusion and Drift currents () F qE q v B  Diffusion subject will be covered after the review of drift current . Let’s remember on Drift Current induced by electromagnetic field. We said that under constant electric field, E, and magnetic field B, the charged particle will expose to force F given by 10/23/2010 3 CURRENT CONDUCTION AND RESISTIVITY (2) Electrons are active carriers which move around randomly. However, the net displacement is 0 when there is no external force is present. External forces provide net displacement >0 In free space the motion of electron is uninterrupted In lattice the motion of electrons are interrupted by collisions E B * ** No magnetic field hol ) es ( where , are effective mass vales for electrons and n np F qE dv Fq m v E d q t B mm   Random walk E=0 , 0 displacement Directional movement E 0 CURRENT CONDUCTION AND RESISTIVITY (3) What are these values at room temperature (it is temperature dependent!) 10/23/2010 4 00 0 1.18 0.81 0.55 0.36 0.066 0.52 Si m m Ge m m GaAs m
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10/23/2010 2 CURRENT CONDUCTION AND RESISTIVITY (4) If we assume that we apply a constant electric field across a semiconductor with uniform carrier distribution and charges are drifted by a nominal drift velocity v d due to these collisions We can write the total current due for holes and electrons = total charge passing through an arbitrary unit area per unit time 10/23/2010 5 + V - Reference plane A = Area L = Length Hole Current Density . . Electron Current Density . .( ) they move to opposite direction pd p nd n Jq p v n v    CURRENT CONDUCTION AND RESISTIVITY (5) We can associate the drift velocity to mobility that measures how carriers respond to the external field Since electrons and holes respond in different fashion to the applied electric field they different mobilities, μ n , μ p At low electric fields (E<10 4 V/cm) we can simplify our current equation as 10/23/2010 6 By assuming 1 pp EE J qpv qp q pE These are simplified equations for silicon ,, 1/2 2 1 1 By assuming 1( ) For Electrons with electron density n and charge -q By assuming 1 ) d p p sat p sat nn dn n sat n sat v E v n vq n q n E v E v      CURRENT CONDUCTION AND RESISTIVITY (6) Mobility is a parameter which is temperature dependent and also impurity concentration dependent 10/23/2010 7 CURRENT CONDUCTION AND RESISTIVITY (7) So the total current density, knowing the fact that electrons and holes are
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This note was uploaded on 12/13/2010 for the course ELECTRICAL EECS 170A taught by Professor Ozdalboyraz during the Fall '10 term at UC Irvine.

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Lecture3 - 10/23/2010 10/23/2010 2...

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