Transcript 12.11.2013 - Erwin Sitompul

Semiconductor Device Physics
Lecture 8
Dr.-Ing. Erwin Sitompul
President University
http://zitompul.wordpress.com
2
President University
0
1
3
Erwin Sitompul
SDP 8/1
Semiconductor Device Physics
Chapter 7
pn Junction Diodes: Small-Signal Admittance
President University
Erwin Sitompul
SDP 8/2
Chapter 7
pn Junction Diodes: Small-Signal Admittance
Small-Signal Diode Biasing
 When reversed-biased, a pn junction diode becomes
functionally equivalent to a capacitor, whose capacitance
decreases as the reverse bias increases.
 Biasing additional a.c. signal va can be viewed as a small
oscillation of the depletion width about the steady state value.
Y  G  jC
V0 << VA
RS
C
G
Y
President University
: serial resistance
: capacitance
: conductance
: admittance
Erwin Sitompul
SDP 8/3
Chapter 7
pn Junction Diodes: Small-Signal Admittance
Total pn Junction Capacitance

i
R 1 G
va
C  CJ  CD

CJ  A
Minority
carrier
lifetime
CD 
s
W
 I DC
kT q
Junction / depletion capacitance,
due to variation of depletion charges
Diffusion capacitance,
due to variation of stored minority charges
in the quasineutral regions
• CJ dominates at low forward biases, reverse biases.
• CD dominates at moderate to high forward biases.
President University
Erwin Sitompul
SDP 8/4
Chapter 7
pn Junction Diodes: Small-Signal Admittance
Relation Between CJ and VA
 For asymmetrical step junction,
W
2 s
Vbi  VA 
qN B
NB : bulk semiconductor doping,
NA or ND as appropriate.
 Therefore,
1
W2
2
 2 2 
(Vbi  VA )
2
2
CJ
A s
qN B S A
• A plot of 1/CJ2 versus VA is linear.
• The slope is inversely proportional to NB.
• An extrapolated 1/CJ2 = 0 intercept is equal to Vbi.
President University
Erwin Sitompul
SDP 8/5
Semiconductor Device Physics
Chapter 8
pn Junction Diodes: Transient Response
President University
Erwin Sitompul
SDP 8/6
Chapter 8
pn Junction Diodes: Transient Response
Turn-Off Transient
 In order to turn the diode off, the excess minority carriers must
be removed through net carrier flow out of the quasineutral regions and recombination.
 Carrier flow is limited by the switching circuit.

VF
RF
VR

RR
tr : recovery time
ts : storage delay time
trr : reverse recovery time
Diode switching circuit
President University
Erwin Sitompul
SDP 8/7
Chapter 8
pn Junction Diodes: Transient Response
Turn-Off Transient
 Voltage-time transient
The junction remains forward
biased for 0 < t < ts
vA(t)  0 at t  ts
President University
Erwin Sitompul
SDP 8/8
Chapter 8
pn Junction Diodes: Transient Response
Transient Response of pn Diode
 Suppose a pn diode is forward
biased, then suddenly turned off at
time t = 0.
 The excess minority carrier will be
removed through recombination and
reverse current flow.
 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.
President University
Erwin Sitompul
SDP 8/9
Chapter 8
pn Junction Diodes: Transient Response
Decay of Stored Charge
 Consider a p+n diode:
Dpn(x)
i(t)
IF
Decrease due to
recombination and
reverse current flow
ts
t
IR
vA(t)
pn0
x
xn
 For t > 0:
t
ts
dpn
dx
President University
x  xn
i

0
qADP
Erwin Sitompul
• The current is reversed but
the diode remains forward
biased during 0 < t < ts
SDP 8/10
Chapter 8
pn Junction Diodes: Transient Response
Examples i-t transient
Increase IF
i(t)
ts
ts
t
President University
Decrease p
i(t)
Increase IR
i(t)
ts
t
Erwin Sitompul
t
SDP 8/11
Chapter 8
pn Junction Diodes: Transient Response
Storage Delay Time ts
 ts is the primary quantity used to characterize the transient
response of pn junction diodes

dQP
QP
QP 
i
   IR 


dt
p
p 

0   t  ts
QP : excess hole
charge
 By separation of variables and integration from t = 0+ to t = ts,
noting that
Q (0 ) Q (0 )
IF 
P
p

P
p
 And making the approximation of QP (ts )  0
 We may conclude that
President University
ts   p ln(1  I F I R )
Erwin Sitompul
SDP 8/12
Chapter 8
pn Junction Diodes: Transient Response
Turn-On Transient
 Again, consider a p+n diode:
Dpn(x)
i(t)
A positive current IF is
forced to flow through the
diode beginning at t = 0
I F  I 0 (e qVA
kT
 1)
t
vA(t) VON
pn0
x
xn
 For t > 0:
President University
kT

ln(1  I F I 0 )
q
dpn
dx
x  xn
i

0
qADP
Erwin Sitompul
t
SDP 8/13
Chapter 8
pn Junction Diodes: Transient Response
Turn-On Transient
 Rewriting for turn-on characteristics,
dQP
QP
QP
i
 IF 
dt
p
p
for t  0
 By separation of variables and integration, we have
QP (t )  I F p (1  e
t / p
)
 The stored hole charge in an ideal diode is given by
QP (t )  I DIFF p  I0 p (eqVA / kT 1)
Steady state
 Finally, by assuming that the build-up of stored charge occurs
quasistatically, VA  vA
kT  I F
t / p 
vA (t ) 
ln 1  (1  e
)
q  I0

President University
Erwin Sitompul
SDP 8/14
Semiconductor Device Physics
Chapter 9
Optoelectronic Diodes
President University
Erwin Sitompul
SDP 8/15
Chapter 9
Optoelectronic Diodes
Photodiodes
Reverse current due to
carriers swept by the E-field
Electron-hole pair
generation due to light
I  I dark  I L
I L  qA( LN  W  LP )GL
President University
Erwin Sitompul
SDP 8/16
Chapter 9
Optoelectronic Diodes
I–V Characteristics and Spectral Response
Open circuit
voltage voc
Upper limit
~ highest wavelength
~ lowest frequency
~ lowest energy
I L  GL
Short circuit
current isc
President University
Erwin Sitompul
SDP 8/17
Chapter 9
Optoelectronic Diodes
p-i-n Photodiodes
p-i-n : positive–intrinsic– negative
W ≈ Wi-region
 Most carriers are
generated in the depletion
 Faster response time
(~10 GHz operation)
President University
Reverse biased
• current arises mostly in the totally
•
•
depleted i-region, not in quasineutral
region as in pn diode
generated carriers do not need to
diffuse into the depletion region
before they are swept by the E-field
enhanced frequency response
Erwin Sitompul
SDP 8/18
Chapter 9
Optoelectronic Diodes
Forward bias
Increasing EG
Light Emitting Diodes (LEDs)
 LEDs are typically made of
compound semiconductors
(direct semiconductors with
band-to-band recombination).
 It releases energy by
dissipating light / emitting
photon.
President University
Erwin Sitompul
SDP 8/19
Chapter 9
Optoelectronic Diodes
Homework 6
 1.
(9.50)
Consider a diode with a constant junction capacitance of 18 pF at forward
bias and 4.2 pF at a reverse bias. The minority carrier lifetimes are 10–7 s.
The diode is switched from a forward bias with a current of 2 mA to a
reverse bias voltage of 10 V applied through a 10 kΩ resistor. Estimate the
reverse recovery time (trr). Hint: trr is reached when the magnitude of the
reverse current stay inside the vicinity of 10% of IR.
 2.
Problem 8.2, Pierret’s “Semiconductor Device Fundamentals”.
 Due: 18.11.2013.
President University
Erwin Sitompul
SDP 8/20
(7.2)