president universityerwin sitompulsdp 8/1 lecture 8 semiconductor device physics dr.-ing. erwin...
TRANSCRIPT
President University Erwin Sitompul SDP 8/1
Lecture 8
Semiconductor Device Physics
Dr.-Ing. Erwin SitompulPresident University
http://zitompul.wordpress.com
2 0 1 3
President University Erwin Sitompul SDP 8/2
Chapter 7pn Junction Diodes: Small-Signal Admittance
Semiconductor Device Physics
President University Erwin Sitompul SDP 8/3
V0 << VA
RS : serial resistance
C : capacitance
G : conductance
Y : admittance
Small-Signal Diode BiasingChapter 7 pn Junction Diodes: Small-Signal Admittance
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
President University Erwin Sitompul SDP 8/4
DCD
IC
kT q
sJC A
W
Junction / depletion capacitance,
due to variation of depletion charges
i
J DC C C av
1R G
Diffusion capacitance,due to variation of stored minority charges in the quasineutral regions
Minority carrier lifetime
Total pn Junction CapacitanceChapter 7 pn Junction Diodes: Small-Signal Admittance
• CJ dominates at low forward biases, reverse biases.• CD dominates at moderate to high forward biases.
President University Erwin Sitompul SDP 8/5
2
bi A2 2 2 2J s B S
1 2( )
WV V
C A qN A
sbi A
B
2W V V
qN
NB : bulk semiconductor doping,
NA or ND as appropriate.
Relation Between CJ and VA
Chapter 7 pn Junction Diodes: Small-Signal Admittance
For asymmetrical step junction,
Therefore,
• A plot of 1/CJ2 versus VA is linear.
• The slope is inversely proportional to NB.• An extrapolated 1/CJ
2 = 0 intercept is equal to Vbi.
President University Erwin Sitompul SDP 8/6
Chapter 8pn Junction Diodes: Transient Response
Semiconductor Device Physics
President University Erwin Sitompul SDP 8/7
R
R
V
R
F
F
V
R
tr : recovery timets : storage delay time
trr : reverse recovery time
Diode switching circuit
Turn-Off TransientChapter 8 pn Junction Diodes: Transient Response
In order to turn the diode off, the excess minority carriers must be removed through net carrier flow out of the quasi-neutral regions and recombination. Carrier flow is limited by the switching circuit.
President University Erwin Sitompul SDP 8/8
The junction remains forward biased for 0 < t < ts
vA(t) = 0 at t = ts
Turn-Off TransientChapter 8 pn Junction Diodes: Transient Response
Voltage-time transient
President University Erwin Sitompul SDP 8/9
Transient Response of pn DiodeChapter 8 pn Junction Diodes: Transient Response
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 8/10
n
n
P
0x x
dp i
dx qAD
t
vA(t)
ts
Dpn(x)
Decrease due to recombination and
reverse current flow t
i(t)
ts
FI
RI
xpn0
xn
• The current is reversed but the diode remains forward biased during 0 < t < ts
Decay of Stored ChargeChapter 8 pn Junction Diodes: Transient Response
Consider a p+n diode:
For t > 0:
President University Erwin Sitompul SDP 8/11
t
i(t)
ts
Increase IF Increase IR Decrease tp
t
i(t)
ts
t
i(t)
ts
Examples i-t transientChapter 8 pn Junction Diodes: Transient Response
President University Erwin Sitompul SDP 8/12
P P PR s
p p
0dQ Q Q
i I t tdt
P PF
p p
(0 ) (0 )Q QI
P s( ) 0Q t
s p F Rln(1 )t I I
QP : excess hole charge
Storage Delay Time tsChapter 8 pn Junction Diodes: Transient Response
ts is the primary quantity used to characterize the transient response of pn junction diodes
By separation of variables and integration from t = 0+ to t = ts, noting that
And making the approximation of
We may conclude that
President University Erwin Sitompul SDP 8/13
Dpn(x)
t
i(t)
vA(t)
xn
n
n
P
0x x
dp i
dx qAD
x
t
AF 0 ( 1)qV kTI I e
ON F 0ln(1 )kT
V I Iq
pn0
A positive current IF is forced to flow through the
diode beginning at t = 0
Turn-On TransientChapter 8 pn Junction Diodes: Transient Response
Again, consider a p+n diode:
For t > 0:
President University Erwin Sitompul SDP 8/14
P P PF
p p
for 0dQ Q Q
i I tdt
p/P F p( ) (1 )tQ t I e
A /P DIFF p 0 p( ) ( 1)qV kTQ t I I e
p/FA
0
( ) ln 1 (1 )tIkTv t e
q I
Steady state
Turn-On TransientChapter 8 pn Junction Diodes: Transient Response
Rewriting for turn-on characteristics,
By separation of variables and integration, we have
The stored hole charge in an ideal diode is given by
Finally, by assuming that the build-up of stored charge occurs quasistatically, VA vA
President University Erwin Sitompul SDP 8/15
Chapter 9Optoelectronic Diodes
Semiconductor Device Physics
President University Erwin Sitompul SDP 8/16
dark LI I I
Electron-hole pair generation due to light
Reverse current due to carriers swept by the E-field
L N P L( )I qA L W L G
PhotodiodesChapter 9 Optoelectronic Diodes
President University Erwin Sitompul SDP 8/17
L LI G
Open circuit voltage voc
Short circuit current isc
Upper limit ~ highest wavelength
~ lowest frequency ~ lowest energy
I–V Characteristics and Spectral Response Chapter 9 Optoelectronic Diodes
President University Erwin Sitompul SDP 8/18
W ≈ Wi-region Most carriers are
generated in the depletion Faster response time
(~10 GHz operation)
• 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
p-i-n : positive–intrinsic– negative Reverse biased
p-i-n PhotodiodesChapter 9 Optoelectronic Diodes
President University Erwin Sitompul SDP 8/19
Incr
easi
ng
EG
Forward bias
Light Emitting Diodes (LEDs)Chapter 9 Optoelectronic Diodes
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 8/20
Homework 6
Due: 18.11.2013.
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. (7.2)Problem 8.2, Pierret’s “Semiconductor Device Fundamentals”.
Chapter 9 Optoelectronic Diodes