Lecture11

In equilibrium ef is constant therefore the band

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Unformatted text preview: oncentrations, which is determined by the dopant concentrations. •  In equilibrium, EF is constant; therefore, the band energies very with position: adsf EE 332 Spring 2013 Drift and Diffusion Directions EE 332 Spring 2013 No Net Current in Equilibrium •  In equilibrium, there is no net flow of electrons or holes JN = 0 and JP=0 The drift and diffusion current components must balance each other exactly. (A built-in electric field exists, such that the drift current exactly cancels out the diffusion current due to the concentration gradient.) dn JN = qnµnE + qDN =0 dx EE 332 Spring 2013 Carrier Concentration Gradient Under Electric Field Carrier Concentration Gradient Under Electric Field Consider a piece of non-uniformly doped semiconductor EE 332 Spring 2013 Potential Difference Due to Carrier Concentration Gradient Potential Difference Due to Carrier Concentration Gradient The ratio of carrier densities (n,p) at two points depends exponentially on the potential difference between these points: EE 332 Spring 2013 nstein Relation Between andand μ Einstein Relation Between D D μ Under equilibrium conditions, JN=0 and JP =0 EE 332 Spring 2013 Example: Diffusion Constant Example: Diffusion Constant What is the hole diffusion constant in a sample of silicon with μp = 410 cm2/Vs? EE 332 Spring 2013 Summary Summary Summary •  Electron/hole concentration gradient è༎ diffusion •  Current flowing in a semiconductor is comprosed of drift & diffusion components for electrons and holes EE 332 Spring 2013...
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