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1 . Lecture #14
OUTLINE
• pn Junctions:
– transient response
• turn-off
• turn-on
• pn diode applications
Reading: Chapters 8 & 9
Spring 2003 EE130 Lecture 14, Slide 1
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.)
Spring 2003 EE130 Lecture 14, Slide 2
1
2 . 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
Spring 2003 EE130 Lecture 14, Slide 3
Decay of Stored Charge
Consider a p+n diode (Qp >> Qn):
∆pn(x) i(t)
ts
t
vA(t)
x
xn
dpn i
For t > 0: =− >0 t
dx x = xn qAD p ts
Spring 2003 EE130 Lecture 14, Slide 4
2
3 . Examples (qualitative)
Increase IF Increase IR Decrease τp
i(t) i(t) i(t)
ts ts ts
t t t
Spring 2003 EE130 Lecture 14, Slide 5
Storage Delay Time ts
• ts is the primary “figure of merit” used to characterize
the transient response of pn junction diodes
dQ p Qp Qp
=i− = − I R + 0+ ≤ t ≤ ts
dt τp τ
p
• By separation of variables and integration from t = 0+
to t = ts, noting that I F = Q p (t = 0) / τ p
and making the approximation Q p (t = t s ) = 0
IF
We conclude that t s ≅ τ p ln1 +
IR
Spring 2003 EE130 Lecture 14, Slide 6
3
4 . Turn-On Transient
Again, consider a p+n diode (Qp >> Qn):
∆pn(x) i(t)
t
vA(t)
x
xn
dpn i
For t > 0: =− <0 t
dx x = xn qAD p
Spring 2003 EE130 Lecture 14, Slide 7
dQ p Qp Qp
=i− = IF − for t ≥ 0 +
dt τp τp
• 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
(
Q p (t ) = I diffusion τ p = I 0 e qv A / kT − 1 τ p )
then v A (t ) =
kT I F
ln 1 +
q I0
1− e p (
−t / τ
)
Spring 2003 EE130 Lecture 14, Slide 8
4
5 . • If τp is large, then the time required to turn on
the diode is approximately ∆Q/IF
where ∆Q = ∆Q p + ∆Q j
Spring 2003 EE130 Lecture 14, Slide 9
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 2003 EE130 Lecture 14, Slide 10
5
6 . Summary of Important Concepts (cont.)
• The ideal diode equation stipulates the relationship
between JN(-xp) and JP(xn)
¾ If holes are forced to flow across a forward-biased
junction, then electrons must also be injected across
the junction
Spring 2003 EE130 Lecture 14, Slide 11
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 2003 EE130 Lecture 14, Slide 12
6
7 . The pn Junction as a Temperature Sensor
I = I 0 (e qV kT
− 1)
Dp D
I 0 = Aqni + n
2
L N
p d Ln N a
What causes the I-V curves to shift to lower V at higher T ?
Spring 2003 EE130 Lecture 14, Slide 13
Zener Diodes
• Diode specifically
designed to
nondestructively break
down at predetermined
voltage
• Commonly referred to
as Zener diodes
(though avalanche is
often the more common
mechanism)
Spring 2003 EE130 Lecture 14, Slide 14
7
8 . Varactor Diodes
• Voltage-controlled capacitor
– Used in oscillators and detectors
(e.g. FM demodulation circuits in your radios)
– Response changes by tailoring doping profile:
−n
C j ∝ Vr
for n= 1
m+2
Vr >> Vbi
Spring 2003 EE130 Lecture 14, Slide 15
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 2003 EE130 Lecture 14, Slide 16
8
9 . Tunnel Diodes (cont.)
Spring 2003 EE130 Lecture 14, Slide 17
Optoelectronic Diodes
I = I 0 ( e qVA kT
− 1) − I op
I op = qAg op ( LP + W + LN ) ~ qAg op ( LP + LN )
Spring 2003 EE130 Lecture 14, Slide 18
9
10 . L +L
Voc = VA I =0
= kT
q ln L p p nL g op + 1
τ p pn + n τ n n p
Spring 2003 EE130 Lecture 14, Slide 19
p-i-n Photodiodes
• W ≅ Wi-region, so most carriers are generated in
the depletion region
Æ faster response time (~10 GHz operation)
• Operate near avalanche to amplify signal
Spring 2003 EE130 Lecture 14, Slide 20
10
11 . Light Emitting Diodes (LEDs)
•LEDs are
typically made of
compound
semiconductors
(direct bandgap)
Spring 2003 EE130 Lecture 14, Slide 21
Organic LEDs
• Some organic materials
exhibit semiconducting
properties
– OLEDs are attractive for
low-cost, high-quality flat-
panel displays
Spring 2003 EE130 Lecture 14, Slide 22
11
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