EEE 5 Lecture 19
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Transcript of EEE 5 Lecture 19
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Turn-On Transient
x
Consider a p+
n diode (Q p >> Q n):
t
i (t )
t
v A(t )
0p x x
n
qADi
dxdp
n
For t > 0:
D p n( x )
x n
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By separation of variables and integration, wehave
If we assume that the build-up of storedcharge occurs quasi-statically so that
then
0for
t Q
I Q
idt
dQ
p
p F
p
p p
pt p F p e I t Q
/1)(
pkT qv
pdiffusion p Ae I I t Q 1)(/
0
pt F A e I I
qkT
t v/
0
11ln)(
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Summary of Important Concepts
Under forward bias, minority carriers are injectedinto the quasi-neutral regions of the diode.The current flowing across the junction iscomprised of hole and electron components.
If the junction is asymmetrically doped (i.e. it is one -sided) then one of these components will bedominant.
In a long-base diode, the injected minority carriersrecombine with majority carriers within the quasi-neutral regions.
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The ideal diode equation stipulates therelationship between J N(-xp) and J P (xn):
For example, if holes are forced to flow acrossa forward-biased junction, then electrons mustalso be injected across the junction.
sideni
side pi
An p
D pn
n P
p N
nn
N L D N L D
x J x J 2
2
)()(
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Under reverse bias, minority carriers are collected into the quasi-neutral regions of the diode.
Minority carriers generated within a diffusion length of the depletionregion diffuse into the depletion region and then are swept across the
junction by the electric field.
The negative current flowing in a reverse-biased diode depends on
the rate at which minority carriers are supplied from the quasi-neutral regions.Electron-hole pair generation within the depletion region alsocontributes negative diode current.
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Varactor Diode
Voltage-controlled capacitanceUsed in oscillators and detectors
(e.g. FM demodulation circuits in your radios)
Response changes by tailoring doping profile:
2
1
mn b iV V
for
V C
r
nr j
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Tunnel Diode (contd)
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Optoelectronic Diodes
L N P L
LkT V q
G LW LqA I I e I I
)()1(
A0
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1ln0A Ln L p L
LW L
qkT
I ocGV V
pn
nn
p
p
n p
Open Circuit Voltage, VOC
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p-i-n Photodiodes
W Wi-region, so most carriers are generatedin the depletion region faster response time (~10 GHzoperation)Operate near avalanche to amplify signal
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Light Emitting Diodes (LEDs)
LEDs are typically made of compound semiconductors(direct bandgap)
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Organic LEDs
Some organic materialsexhibit semiconductingproperties
OLEDs are attractive for low-cost, high-quality flexibledisplays