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Lecture 14
OUTLINE
• pn Junction Diodes (cont’d)
– Transient response: turn-on
– Summary of important concepts
– Diode applications
• Varactor diodes
• Tunnel diodes
• Optoelectronic diodes
Reading: Pierret 9; Hu 4.12-4.15
Turn-On Transient
Consider a p+n diode (Qp >> Qn):
i(t)
Dpn(x)
t
x
vA(t)
xn
dpn
For t > 0:
dx
EE130/230A Fall 2013
x  xn
i

0
qAD p
Lecture 14, Slide 2
t
dQ p
dt
i
Qp
τp
 IF 
Qp
τp
for t  0 
• By separation of variables and integration, we have

Qp (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 
then v A (t ) 
ln 1 
1 e

q  I0

EE130/230A Fall 2013
Lecture 14, 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
EE130/230A Fall 2013
Lecture 14, Slide 4
Summary of Important Concepts
• Under forward bias, minority carriers are injected into
the quasi-neutral regions of the diode.
• The current flowing across the junction is comprised of
hole and electron components.
– If the junction is asymmetrically doped (i.e. it is “one-sided”)
then one of these components will be dominant.
• In a long-base diode, the injected minority carriers
recombine with majority carriers within the quasineutral regions.
EE130/230A Fall 2013
Lecture 14, Slide 5
• The ideal diode equation stipulates the relationship
between JN(-xp) and JP(xn):
Dn L p N D  ni 2 p  side 
 2


J P ( xn )
D p Ln N A  ni n  side 
J N ( x p )
 For example, if holes are forced to flow across a
forward-biased junction, then electrons must also be
injected across the junction.
EE130/230A Fall 2013
Lecture 14, Slide 6
• Under reverse bias, minority carriers are collected
into the quasi-neutral regions of the diode.
– Minority carriers generated within a diffusion length of the
depletion region 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 also contributes negative diode current.
EE130/230A Fall 2013
Lecture 14, Slide 7
pn Junction as a Temperature Sensor
C. C. Hu, Modern Semiconductor Devices for ICs, Figure 4-21
EE130/230A Fall 2013
Lecture 14, Slide 8
Varactor Diode
• Voltage-controlled capacitance
– Used in oscillators and detectors
(e.g. FM demodulation circuits in your radios)
– Response changes by tailoring doping profile:
C j  Vr
n
for
n
1
m 2
Vr  Vbi
EE130/230A Fall 2013
Lecture 14, Slide 9
Optoelectronic Diodes
I  I 0 (e
qVA kT
 1)  I L
I L  qA( LP  W  LN )GL
EE130/230A Fall 2013
Lecture 14, Slide 10
R.F. Pierret, Semiconductor Fundamentals, Figure 9.2
Open Circuit Voltage, VOC
Voc  VA
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I 0


L

W

L
 kTq ln   L p p  L n  GL  1
  t p  pn  n t n  n p

Lecture 14, Slide 11
C. C. Hu, Modern Semiconductor Devices for ICs, Figure 4-25(b)
Solar Cell Structure
Cyferz at en.wikipedia
EE130/230A Fall 2013
Lecture 14, Slide 12
Textured Si surface for reduced reflectance
• Achieved by anisotropic wet etching (e.g. in KOH)
M. A. Green et al., IEEE Trans. Electron Devices, Vol. 37, pp. 331-336, 1990
P. Papet et al., Solar Energy Materials and Solar Cells, Vol. 90, p. 2319, 2006
EE130/230A Fall 2013
Lecture 14, Slide 13
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
EE130/230A Fall 2013
Lecture 14, Slide 14
R.F. Pierret, Semiconductor Fundamentals, Figure 9.5
Light Emitting Diodes (LEDs)
• LEDs are made with compound semiconductors (direct bandgap)
R.F. Pierret, Semiconductor Fundamentals, Figure 9.13
R.F. Pierret, Semiconductor Fundamentals, Figure 9.15
EE130/230A Fall 2013
Lecture 14, Slide 15
Question 1 (re: Slide 12): Why are the contacts to the back
(non-illuminated) side of a solar cell made only at certain
points (rather than across the entire back surface)?
Answer: To increase energy conversion efficiency
i) The absorption depth (average distance a photon travels
before transferring its energy to an electron) for longwavelength photons is greater than the Si thickness.
 The bottom surface oxide and metal layer effectively
form a mirror that reflects light back into the silicon.
ii)There is more recombination in heavily doped contacts than
at a good Si/SiO2 interface; most of the back surface should
be covered by SiO2 so that generated carriers have a high
probability of diffusing to the depletion region before they
recombine.
EE130/230A Fall 2013
Lecture 14, Slide 16
Question 2 (re: Slide 15): What limits the lifetime of an LED?
Answer:
i) LED lifetime is defined to be the duration of operation after
which the light output falls to only 70% of original.
(Even afterwards, the LED will continue to function.)
ii) The power density of an LED can be high (up to 10 W/cm2,
comparable to an electric stove top), causing significant
heating which can degrade the light output through
various mechanisms:
1.Degradation of epoxy package causing partial absorption of light
2.Mechanical stress weakening the wire bond (electrical connection)
3.Formation/growth of crystalline defects, or diffusion of metal into
the semiconductor, resulting in increased recombination via midgap states
EE130/230A Fall 2013
Lecture 14, Slide 17