Lecture 4 pn Junctions (Diodes) pn junctions 1-1 Wednesday 27/9/2017
Lecture 4
pn Junctions (Diodes)
pn junctions 1-1 Wednesday 27/9/2017
Agenda Continue pn junctions
Equilibrium (zero bias) • Depletion rejoins • Built-in potential
Reverse and forward bias
I-V characteristics • Bias voltage • Forward and reverse currents • Reverse breakdown
Special Purpose Diodes
pn junctions 1-2
Test Yourself
Choose the correct answer for the following two statements: i. The resistance of …………. increase with rise in temperature degrees.
1. Insulators 2. Conductors 3. Semiconductors 4. Dielectrics
ii. Charge carrier motion which is caused by an electric field due to an externally applied voltage is known as:
1. Carrier Drift 2. Thermal Motion 3. Carrier Diffusion 4. Brownian motion
pn junctions 1-3
Last Lecture
Thermal equilibrium: balance between drift and diffusion
We can divide
semiconductor into
three regions:
• Two quasi-neutral n and p regions (QNR’s)
• One space charge region (SCR)
pn junctions 1-4
depletion region
Equilibrium (zero bias)
pn junctions 1-5
The Depletion Approximation Assume the QNR’s are perfectly charge neutral
Assume the SCR is depleted of carriers (depletion region)
pn junctions 1-6
Space charge density
pn junctions 1-7
Electric Field
The electric field is proportional to the integral of the space charge density and will be zero in the (charge) neutral regions outside of the depletion region
pn junctions 1-8
Carrier Profiles: in thermal equilibrium (zero bias)
pn junctions 1-9
In equilibrium: dynamic balance between drift and diffusion for electrons and holes inside SCR
Built-in Potential (zero bias)
Because of the electric field across the junction, there exists a built-in potential. Its derivation is shown above
pn junctions 1-10
,
,
2
1
p
n
p
p
p
x
x
p
pp
p
dpDdV
dx
dpqDpEq
n
p
p
p
pp
p
pDxVxV
dx
dpD
dx
dVp
ln)()( 12
200
ln , ln
i
DA
n
p
n
NN
q
kTV
p
p
q
kTV
ndiffndrift
pdiffpdrift
II
II
,,
,,
pn junction under bias
Upon application of voltage
Electrostatics upset: depletion region
widens or shrinks
Current flows With rectifying
behavior
Carrier charge storage
pn junctions 1-11
pn junction in reverse bias
pn junctions 1-12
When the n-type region of a diode is connected to a higher potential than the p-type region, the diode is under reverse bias, which results in wider depletion region and larger built-in electric field across the junction
Carrier Profiles: under reverse bias
At reverse bias voltage depletion region
widens
Net drift current
in SCR ⇒ minority
carrier extraction
from QNRs
Carrier flow is small
because there are few
minority carriers
extracted from QNRs
from the minority side
pn junctions 1-13
- ve
terminal
+ ve
terminal Holes
(majority)
attraction
Electrons (majority)
attraction
holes
electrons
p n
pn junction in forward bias
pn junctions 1-14
• When the n-type region of a diode is at a lower potential than the p-type region, the diode is in forward bias
• The depletion width is shortened and the built-in electric field decreased
Carrier Profiles: under forward bias
At forward bias voltage depletion region shrinks
Net diffusion current in
SCR ⇒ minority carrier
injection into QNRs
Carrier flow can be high
because lots of minority
carriers are injected
into QNRs from the
majority side
pn junctions 1-15
+ ve
terminal
- ve
terminal
Holes
(majority)
pushing
Electrons (majority)
pushing
holes
electrons
p n
IV characteristics of pn juntions
Non-linear device
pn junction (Diode) current equation:
pn junctions 1-16
)1( e T
D
V
V
SD II
p n
VD
ID
Physics of forward bias:
pn junctions 1-17
• Potential drop across SCR reduced by V • minority carrier injection in QNRs
• Minority carrier diffusion through QNRs • Minority carrier recombination at contacts to the QNRs
(and surfaces) • Large supply of carriers injected into the QNRs
e T
D
V
V
DI
Physics of reverse bias
pn junctions 1-18
• Potential drop across SCR increased by V • minority carrier extraction from QNRs
• Minority carrier diffusion through QNRs • Minority carrier generation at surfaces & contacts of
QNRs • Very small supply of carriers available for extraction I saturates to small value
pn junctions 1-19
pn junctions 1-20
pn junctions 1-21
Reverse Breakdown
When a large reverse bias voltage is applied, breakdown occurs and an enormous current flows through the diode
pn junctions 1-22
Special Purpose Diodes
pn junctions 1-23
Zener Light-emitting Photo
Schottky
PIN Tunnel
Laser
Current-regulator
Varactor
Special Purpose Diodes
Light Emitting Diode (LED) In a forward-biased p-n junction,
recombination of the holes and electrons requires energy possessed by the unbound free electrons
In Si and Ge, most of the energy is dissipated in the form of heat and photons
But in other material such as GaAs, the energy generate light but it is invisible for the eye to see (infrared)
Other materials that emit light during forward-bias operation
pn junctions 1-24
Color Construction Forward Voltage
Green
Orange
Red
GaP
GaAsP
GaAsP
2.2
2.0
1.8
Special Purpose Diodes
Photodiode a special light sensitive diode with a clear
window to the pn junction
operated with reverse bias
Reverse current increases with greater incident light
pn junctions 1-25 Irradiance, H
Rev
erse
cu
rren
t, (
)I l
0
Dark current
Special Purpose Diodes
Schottky diode a metal-to-semiconductor contact diode that is used
primarily in high frequency and fast-switching applications
has a low forward voltage drop and high efficiency but rather low reverse voltage rating.
pn junctions 1-26
Cathode Anoden
n region Metal region
Metal-silicon junction
Special Purpose Diodes
PIN diode a three layer diode consisting of p and n layers
separated by a narrow intrinsic layer
In microwave applications, the pin diode acts as a voltage-controlled resistor
Certain types are used as photodetectors in fiber optic systems
pn junctions 1-27
Anode Cathode
n region p regionintrinsicregion
p i n
A K
Special Purpose Diodes
Tunnel diode has a characteristic curve that shows a negative
resistance reading between B and C with a small forward voltage
The negative resistance region is unstable. Taking advantage of this characteristic, the tunnel diode can be used in an oscillator circuit at microwave frequencies
pn junctions 1-28
Tunnelingcurrent
Negative-resistanceregion
IF
VFA
B
C
0
pn junctions 1-29
Lecture Summary Covered material Continue pn junctions
Depletion rejoins Built-in potential (zero bias) Forward and reverse bias I-V characteristics
• Bias voltage • Forward and reverse currents • Reverse breakdown
Special purpose diodes (LED)
Material to be covered next lecture Diode equivalent circuits
DC and AC analysis