ece340Fall03HW17sol - ECE 340 Homework XVII Due Friday Fall...

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ECE 340 Homework XVII Fall 2003 Due: Friday, December 05, 2003 1. A lightly-doped-drain (LDD) structure is incorporated in a MOSFET. To fabricate such a structure, a heavy diffusion or implantation is first used to increase the doping in the drain region. Subsequently, a “drive-in” annealing redistributes the impurities and leaves a finite but lightly doped region immediately next to the inversion channel. Such a structure allows the maximum electric field in the depletion region to be reduced. Prove it by solving the maximum electric field in the following p-n junction configurations, one of which resembles to an LDD structure. Determine the contact potential and the maximum electric field in the depletion region for (a) L=0 (b) L= 800Å and (c) L=8000Å. Solutions: (a), L=0, no LDD, this is similar to what we have done in Chap 5 about n + -p junctions: 0 2 0.0259ln 0.8962 ad i NN VV n == 5 2 1.975 10 p a V Wx c m qN ε ≈= = × 4 0 max 9.078 10 / 2 V E Vc m W =− × (b), There are two junctions here, the p-n junction and the n-n + junction. The contact potential for these two junction are . 01 2 0.0259ln 0.7051 L D D i n , and . 02 0.0259ln 0.1911 dLDD d N N , the total contact potential is 0 0.8962 = . Since the L=800A is a small length, the whole LDD area (x<L) will be depletion region. We could then predict the qualitative distribution of electrical field as the figure below: Approximate Approach: We can make a few reasonable assumptions to make solution easier: N a = 3x10 16 /cm 3 N d = 8x10 18 /cm 3 N d = 5x10 15 /cm 3 x=0 x=L
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This note was uploaded on 12/01/2011 for the course ECE 340 taught by Professor Lebutron during the Fall '03 term at University of Illinois, Urbana Champaign.

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ece340Fall03HW17sol - ECE 340 Homework XVII Due Friday Fall...

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