EE216.W2010.Lecture7

EE216.W2010.Lecture7 - Lecture 7 P-N Junction under Bias...

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1 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Lecture 7. P-N Junction under Bias P-N junctions in reverse bias Carrier concentrations Charge storage and the small-signal depletion capacitance P-N junctions in forward bias Minority carrier boundary conditions: the law of the junction Carrier transport: qualitative understanding – Quasi-neutrality: established after dielectric relaxation EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe A built in potential, V bi , provides a barrier to majority carrier current. If an external bias is applied to the junction with the N side +ve, P side -ve, the barrier to majority carrier flow is increased. P N E f E i e holes P-N Junction in Thermal Equilibrium V bi V bi
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2 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Applied voltage appears essentially entirely across the depletion region. The total potential drop across the junction is , V bi -V A . p N + + + + + + + + V A Ohmic contact V A dropped here Negligible voltage dropped here + Biased PN Junction + - + - V A EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Reverse Biased Junction ( V A < 0) V A e holes e holes Drift Drift Diffusion Diffusion E fn E fp
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3 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe E field across depletion region increases. W increases, mobile carriers pulled further away from junction. Minority carrier current flow is no longer zero, but it is very small (leakage) For step junctions, from Eq. (10) and (11): (14) i.e., V bi is replaced by ( V bi - V A ) (Note V A is negative) ( ) 2 / 1 1 1 2 Υ੟ Φ੯ Τ੏ ΢ਯ Σਿ Ρਟ Ο৿ Ο৿ Πਏ Ξ৯ Μ৏ Μ৏ Νয় Λি + = + = A bi D A o s n p V V N N q K x x W ε ( ) W V V A bi = 2 (15) Reverse Biased Junction ( V A < 0) E MAX
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4 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe The small signal capacitance of the junction is given by (16) where A = cross-sectional area This result holds for any arbitrary doping profile. Given any profile, if W is determined, the small signal capacitance C can be calculated. W depends on the dopant profile and the applied bias W AK dV dQ C o s A ε = = PN Junction Capacitance EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Qualitative Charge-Voltage Plot Compare with Q = C V
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5 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Charge, Field and Potential for Reverse Bias V D < 0 V EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Charge Storage in pn Junction Circuit element:
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6 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Incremental Charge q j EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe
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