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EE130 Lecture 21, Slide 1
Spring 2007
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
EE130 Lecture 21, Slide 2
Spring 2007
x
Turn-On TransientAgain, consider a p+n diode (Qp >> Qn):
t
i(t)
t
vA(t)
0 pxx
n
qAD
i
dx
dp
n
For t > 0:
pn(x)
xn
EE130 Lecture 21, Slide 3
Spring 2007
• By separation of variables and integration, we have
• If we assume that the build-up of stored charge occurs quasi-statically so that
then
0for ττ
tQ
IQ
idt
dQ
p
pF
p
pp
ptpFp eItQ τ/1τ)(
pkTqv
pdiffusionpAeIItQ τ1τ)( /
0
ptF
A eI
I
q
kTtv τ/
0
11ln)(
EE130 Lecture 21, Slide 4
Spring 2007
• If p is large, then the time required to turn on the diode is approximately Q/IF
jp QQQ where
EE130 Lecture 21, Slide 5
Spring 2007
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
EE130 Lecture 21, Slide 6
Spring 2007
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
EE130 Lecture 21, Slide 7
Spring 2007
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
EE130 Lecture 21, Slide 8
Spring 2007
Varactor Diodes
• Voltage-controlled capacitor– Used in oscillators and detectors
(e.g. FM demodulation circuits in your radios)
– Response changes by tailoring doping profile:
21mn
biVV
for
VC
r
nrj
EE130 Lecture 21, Slide 9
Spring 2007
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
EE130 Lecture 21, Slide 10
Spring 2007
Tunnel Diodes (cont.)
EE130 Lecture 21, Slide 11
Spring 2007
)(~)(
)1( A0
NPopNPopop
opkTVq
LLqAgLWLqAgI
IeII
Optoelectronic Diodes
EE130 Lecture 21, Slide 12
Spring 2007
1ln
0A opnLp
L
LL
qkT
Ioc gVVp
n
nn
p
p
np
EE130 Lecture 21, Slide 13
Spring 2007
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
EE130 Lecture 21, Slide 14
Spring 2007
Light Emitting Diodes (LEDs)
•LEDs are typically made of compound semiconductors
(direct bandgap)
EE130 Lecture 21, Slide 15
Spring 2007
Organic LEDs
• Some organic materials exhibit semiconducting properties– OLEDs are attractive for
low-cost, high-quality flat-panel displays