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Lecture%2019 - EEE 352: Lecture 19 Electrical Properties of...

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1 EEE 352: Lecture 19 Electrical Properties of the Electrons and Holes * Majority and minority carriers * Motion of minority carriers Diffusion Recombination and Lifetime of excess carriers * Work function Electron affinity Thermionic emission Majority and Minority Carriers Previously, we found that ( ) T k E N N n np B G v c i / exp 2 = = When we add e.g. donors, n increases! However, the above relation remains valid in equilibrium. Hence, if n ~ N d , then n T k E N N N N n n n p B G d v c d i i << = = exp ~ 2 2 minority carrier majority carrier This relationship remains valid as long as the semiconductor is in EQUILIBRIUM . When we create EXCESS carriers , we drive the semiconductor OUT OF EQUILIBRIUM, where 2 i n np > Excess Carriers J x , E x z y When we shine light upon the semiconductor, we create EXCESS CARRIERS , and p n n p n n p p n n np i i = = > + + = 2 0 0 2 0 0 ) )( ( In optical GENERATION of excess carriers, the number of extra electrons is equal to the number of extra holes: new electron new hole photon induced transition The photo-generated density decays away from the surface—the number of photons decreases into the material as they are absorbed to create electron-hole pairs . y n The light creates more carriers at the surface. Since the light is attenuated as it moves into the semiconductor, fewer carriers are created away from the surface. The real world does not like this inhomogeneous pile of carriers (like trying to create a pile of water), and they flow toward regions of lower concentration. n, p move away from the region of high density (the surface in this case) [ Web ]
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2 Excess Carriers and Diffusion y n The spatial variation of the excess density leads to a current flow— DIFFUSION CURRENT . electrons: dy n d D n dy n d e = v ~ v holes: dy p d D p dy p d h = v ~ v dy n d peD dy n d neD pe ne J h e h e = + = v v Excess Carriers and Diffusion y n If no current flows in the y-direction, then a voltage must build between the front and back surfaces, and we can describe the density as T k y eV E E p B F v ) ( exp ~ Then x B B E T k pe dy y dV T k pe dy p d = = ) ( Comparing the currents: This is the EINSTEIN RELATION . e T k D e T k D B e e B h h µ = = Excess Minority Carriers and Diffusion Excess Carriers and Recombination In fact, the EXCESS CARRIERS have to go away at some point. If the light is turned OFF, then n 0. This occurs by RECOMBINATION , in which an electron and a hole find each other and reverse the creation process.
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This note was uploaded on 09/19/2009 for the course EEE 352 taught by Professor Ferry during the Spring '08 term at ASU.

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Lecture%2019 - EEE 352: Lecture 19 Electrical Properties of...

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