EE216.W2010.Lecture9

EE216.W2010.Lecture9 - 1/30/10 Lecture 8. P-N Junction...

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1/30/10 1 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Lecture 8. P-N Junction Applications PN Junctions as electronic devices – Forward-bias small-signal model – Diffusion capacitance (small signal) – Switching transients (large signal) PN Junction as opto-electronic devices – Photovoltaic (solar) cells: short-circuit current and open-circuit voltage – Light-emitting diodes – Laser diodes – Photodiodes EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe R s = diode series resistance (Ohmic contacts and bulk resistances) g D = diode conductance = 1/diode resistance (forward bias) g D = dI dV = q kT I o e qV A / kT q kT I R g C s D D Circuit Models for the P-N Diode A simple small-signal circuit model for the P-N junction is C j
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1/30/10 2 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe The model behind the depletion capacitance breaks down for forward bias ( V A 600 – 700 mV). A first-order approximation is that 2 / 1 0 0 ) ( 1 1 2 Υ Φ Τ ΢ Σ Ρ Ο Ο Π Ξ Μ Μ Ν Λ + = A bi B A s s j V V N N q K A K C ε The parallel plate capacitance associated with the depletion charge is C j V A 0.7 V K s 0 A 2 K s 0 q 1 N A + 1 N B Λ Ν Μ Ξ Π Ο ( V bi V bi /2) Ρ Σ ΢ Τ Φ Υ 1/ 2 Therefore, C j V A 0.7 V C j V A = 0 V = C jo EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe The parallel capacitance C D typically dominates the junction capacitance in forward bias. Its origin is in the need to supply majority carriers to the quasi-neutral regions in order to maintain quasi-neutrality as the forward bias V A is modulated by a small-signal component v a ( t ). Fig. 6.13 from H&S Note that “A” is “D” … for D iode in the subscript Weird (esp. for EEs) to have a capacitance associate with charge storage is a quasi- neutral region … where are the plates of this capacitor ?
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1/30/10 3 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe This additional capacitance, C D /A = diffusion capacitance (due to minority carrier stored in quasi-neutral regions under forward bias) adds in parallel to C j . For a p+n long-base diode , For a p+n short-base diode , C D = dq P n dv D V D τ p dI F dv A V D = p q kT I o e qV D / kT = p q kT I F = g d p C D = dq P n dv D V D qI F kT W n x n ( ) 2 2 D p Λ Ν Μ Μ Ξ Π Ο Ο = g d Transit The transit time τ Transit can be written as the distance the holes diffuse ( W n x n ) divided by their average velocity 2 D p / ( W n x n ). EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Charge Storage in P-N Junction Diodes What happens inside the diode when you suddenly switch from forward to reverse bias?
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1/30/10 4 EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe PN Junction Diode Switching Transient Surprisingly, the current does not instantaneously go to the low reverse bias value!
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This note was uploaded on 06/05/2010 for the course EE 216 taught by Professor Harris,j during the Fall '09 term at Stanford.

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EE216.W2010.Lecture9 - 1/30/10 Lecture 8. P-N Junction...

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