OUTLINE

1
Lecture 20

• OUTLINE
• pn Junctions (contd)
• small-signal model
• transient response
• turn-off
• Reading Chapters 7, 8

2
Small-Signal Model of the Diode
Small signal equivalent circuit
i

va
C
R1/G
?
Small-signal conductance
3
Review Charge Storage in pn-Diode
4

• 2 types of capacitance associated with a pn
  junction
• CJ depletion capacitance
• (due to variation of depletion charge)
• CD diffusion capacitance
• (due to variation of stored minority charge in
  the quasi-neutral regions)
• For a one-sided pn junction, QP gtgt QN
• so Q QP QN ? QP

5
Depletion Capacitance
What are three ways to reduce Cdep?
6
Total pn-Junction Capacitance
i

va
R1/G
?

• CD dominates at moderate to high forward biases
• Cdep dominates at low forward biases, reverse
  biases

7
CJ-vs.-VA (Reverse Bias)
8
Example
If the slope of the (1/Cdep)2 vs. VA
characteristic is -2x1023 F-2 V-1, the intercept
is 0.84V, and A is 1 mm2, find the lighter and
heavier doping concentrations Nl and Nh.
Solution
9
Summary Small Signal Model
Depletion capacitance
Conductance
Diffusion capacitance
10
Transient Response of pn Diode

• Suppose a pn-diode is forward biased, then
  suddenly turned off at time t 0. Because of
  CD, the voltage across the pn junction depletion
  region cannot be changed instantaneously.
• The delay in switching between
• the ON and OFF states is due
• to the time required to change
• the amount of excess minority
• carriers stored in the
• quasi-neutral regions.

11
Turn-Off Transient

• In order to turn the diode off, the excess
  minority carriers must be removed by net carrier
  flow out of the quasi-neutral regions and/or
  recombination
• Carrier flow is limited by the switching
  circuitry

12
Decay of Stored Charge

• Consider a pn diode (Qp gtgt Qn)

Dpn(x)
i(t)
ts
t
vA(t)
t
For t gt 0
ts
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Examples (qualitative)
Decrease tp
Increase IF
Increase IR
i(t)
i(t)
i(t)
ts
ts
ts
t
t
t
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Storage Delay Time ts

• ts is the primary figure of merit used to
  characterize the transient response of pn
  junction diodes
• By separation of variables and integration from t
  0 to t ts, noting that
• and making the approximation
• We conclude that