Lecture #21
ANNOUNCEMENTS
• No coffee hour today 
• Quiz #3 on Friday (March 9)
– Material of HW #5 & #6 (Lectures 13-17)
– Closed book, no calculators; 3 pages of notes allowed
OUTLINE
• pn Junctions:
– transient response: turn-on
• Summary of important pn-diode concepts
• pn diode applications
Reading: Chapters 8 & 9
Spring 2007
EE130 Lecture 21, Slide 1
Turn-On Transient
Again, consider a p+n diode (Qp >> Qn):
Dpn(x)
i(t)
t
x
vA(t)
xn
dpn
For t > 0:
dx
Spring 2007
x  xn
i

0
qAD p
EE130 Lecture 21, Slide 2
t
dQ p
dt
i
Qp
τp
 IF 
Qp
τp
for t  0 
• By separation of variables and integration, we have

Q p (t )  I F τ p 1  e
t / τ p

• If we assume that the build-up of stored charge
occurs quasi-statically so that


Qp (t )  I diffusionτ p  I 0 eqvA / kT  1 τ p


kT  I F
t / τ p 
ln 1 
1 e
then v A (t ) 

q  I0

Spring 2007
EE130 Lecture 21, Slide 3
• If tp is large, then the time required to turn on
the diode is approximately DQ/IF
where DQ  DQ p  DQ j
Spring 2007
EE130 Lecture 21, Slide 4
Summary of Important Concepts
• Under forward bias, minority carriers are injected
into the quasi-neutral regions of the diode
• Current flowing across junction is comprised of hole
and electron components
• In order for one of these components to be dominant,
the junction must be asymmetrically doped
Spring 2007
EE130 Lecture 21, Slide 5
Summary of Important Concepts (cont.)
• The ideal diode equation stipulates the relationship
between JN(-xp) and JP(xn)
 If holes are forced to flow across a forward-biased
junction, then electrons must also be injected across
the junction
Spring 2007
EE130 Lecture 21, Slide 6
Summary of Important Concepts (cont.)
• Under reverse bias, minority carriers are collected
into the quasi-neutral regions of the diode
• Minority carriers within a diffusion length of the
depletion region will diffuse into the depletion region
and then be swept across the junction by the electric
field.
 Current flowing in a reverse-biased diode depends
on the rate at which minority carriers are supplied in
the quasi-neutral regions
Spring 2007
EE130 Lecture 21, Slide 7
Varactor Diodes
• Voltage-controlled capacitor
– Used in oscillators and detectors
(e.g. FM demodulation circuits in your radios)
– Response changes by tailoring doping profile:
C j  Vr
n
for
Vr  Vbi
Spring 2007
n
1
m 2
EE130 Lecture 21, Slide 8
Tunnel Diodes
• Degenerately doped
such that EFp < Ev and
EFn > Ec
• Can achieve negative
differential resistance
– useful in high-speed
circuits and perhaps
static memories
Spring 2007
EE130 Lecture 21, Slide 9
Tunnel Diodes (cont.)
Spring 2007
EE130 Lecture 21, Slide 10
Optoelectronic Diodes
I  I 0 (e
qVA kT
 1)  I op
I op  qAgop ( LP  W  LN ) ~ qAgop ( LP  LN )
Spring 2007
EE130 Lecture 21, Slide 11
Voc  VA
Spring 2007
I 0


L

L
 kTq ln   L p  p  nL  g op  1
  t p  pn  n t n  n p

EE130 Lecture 21, Slide 12
p-i-n Photodiodes
• W  Wi-region, so most carriers are generated in
the depletion region
 faster response time (~10 GHz operation)
• Operate near avalanche to amplify signal
Spring 2007
EE130 Lecture 21, Slide 13
Light Emitting Diodes (LEDs)
•LEDs are
typically made of
compound
semiconductors
(direct bandgap)
Spring 2007
EE130 Lecture 21, Slide 14
Organic LEDs
• Some organic materials
exhibit semiconducting
properties
– OLEDs are attractive for
low-cost, high-quality flatpanel displays
Spring 2007
EE130 Lecture 21, Slide 15