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# Lect_03 - ECE442 3. Jose E Schutt-Aine Electrical Computer...

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1 Jose E. Schutt Aine ECE 442 ECE 442 3. PN Junctions and Diodes Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected]

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2 Jose E. Schutt Aine ECE 442 When a p material is connected to an n-type material, a junction is formed – Holes from p-type diffuse to n-type region – Electrons from n-type diffuse to p-type region – Through these diffusion processes, recombination takes place – Some holes disappear from p-type – Some electrons disappear from n-type A depletion region consisting of bound charges is thus formed Charges on both sides cause electric field Î potential = V o PN Junction
3 Jose E. Schutt Aine ECE 442 Potential acts as barrier that must be overcome for holes to diffuse into the n-region and electrons to diffuse into the p-region Open circuit: No external current Junction built-in voltage PN Junction From principle of detailed balance and equilibrium we get: 2 ln AD oT i NN VV n ⎛⎞ = ⎜⎟ ⎝⎠ For Si, V o is typically 0.6V to 0.8V Charge equality in depletion region gives: pA nD qx AN qx AN = A: cross-section of junction x p : width in p side x n : width in n side

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4 Jose E. Schutt Aine ECE 442 PN Junction 2 11 s dep n p o AD Wx x V qN N ε ⎛⎞ =+ = + ⎜⎟ ⎝⎠ :silicon permittivity s 8 11.7 1.04 10 F/m so εε == × n A pD x N xN =
5 Jose E. Schutt Aine ECE 442 When a reverse bias is applied – Increase of space charge region – Diffusion current decreases – Drift current remains constant – Barrier potential is increased – A steady state is reached Under reverse bias the current in the diode is negligible PN Junction under Reverse Bias When Increase in depletion-layer voltage above V o will appear as an external voltage between the diode terminals

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6 Jose E. Schutt Aine ECE 442 Depletion Layer Stored Charge A: cross section area q j : stored charge jN D n qq q N x A == Let W dep = depletion-layer width AD jd e p A D NN A W = + The total voltage across the depletion layer is V o + V R () 2 11 s dep o R WV V qN N ε ⎛⎞ =+ + ⎜⎟ ⎝⎠
7 Jose E. Schutt

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Lect_03 - ECE442 3. Jose E Schutt-Aine Electrical Computer...

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