Diodes
The simplest and most fundamental nonlinear circuit element
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3.1 The ideal Diode
3.1.1 Current-Voltage characteristic
(a) diode circuit symbol
(b) i–v characteristic;
Figure 3.2 The two modes of operation of ideal
diodes and the use of an external circuit to limit the
forward current (a) and the reverse voltage (b).
(c) equivalent circuit
(d) equivalent circuit
in the reverse direction;
in the forward direction.
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3.1.2 A simple application: Rectifier
Figure 3.3 (a) Rectifier circuit.
EXAMPLE 3.1
(b) Input waveform.
12 V
(c) Equivalent circuit when vI  0.
(d) Equivalent circuit when vI < 0.
(a) Find the fraction of each cycle
during which the diode conduct.
(b) Find the peak value of the
diode current.
(c) Find the maximum reverse-bias voltage
that appears across the diode.
Sol) (a) 24 cos = 12
 = 60o , 60  2 / 360  1/ 3
(e) Output waveform.
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(b)
Id 
(c)
VDrp
24  12
 0.12 A
100
 24  12  36 V
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3.1.3 Diode Logic Gate
Find I and V.
Sol) We don’t know whether none, one, or both diodes are
conducting.
Make a plausible assumption, proceed with the analysis, and
then check whether we end up with a consistent solution !
(a) Assume that both diodes are conducting.
Y=A∙B∙C
VB = 0, V = 0
Y=A+B+C
I D2 
10  0
 1 mA
10
Writing a node equation at B, I  I D 2 
VB  (10)
5
0  (10)
, I  1 mA, V =0 V
5
(b) Assume that both diodes are conducting.
10  0
VB = 0, V = 0
I D2 
 2 mA
5
V  (10)
Writing a node equation at B, I  I D 2  B
I 1
(a) OR gate
(b) AND gate (in a positive-logic system).
EXAMPLE 3.2
0  ( 10)
, I  1 mA
10
Assume that D1 is off, and D2 is on.
I2
I D2 
10
Impossible !!
10  (10)
 1.33 mA
15
VB  10  10  1.33  3.3 V
I  0, V  3.3 V
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3.2 Terminal Characteristics of Junction Diodes
We study the characteristics of real diode - specifically, semiconductor junction diode made of silicon.
Figure 3.7 The i–v characteristic of a silicon junction diode.
* The characteristic curve consists of three distinct regions:
Figure 3.8 The diode i–v relationship with some scales
expanded and others compressed in order to reveal details.
1. The forward-bias region, determined by υ > 0
2. The reverse-bias region, determined by υ < 0
3. The breakdown region, determined by υ < -VZR
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3.2.1 The Forward-Bias Region
1. The forward-bias region, determined by υ > 0
i  I s (e / nVT  1)
(3.1)
This equation can be derived from semiconductor theory.
Is = saturation current at a given temperature
scale current
proportional to the cross-sectional area of the diode
order of 10-15 A for low-power small-signal diode
very strong function of temperature (doubles for every 5 oC rise)
VT = kT/q = thermal voltage
k = Boltzmann’s constant =1.38 x 10-23 joules/kelvin
T = the absolute temperature in kelvins =273 + temperature in oC
q = the magnitude of electronic charge = 1.60 x 10-19 coulomb
25 mV at 20 oC
υ>0
n = 1~2 depending on the material and the physical structure of the diode.
1 for diode made using the standard integrated-circuit fabrication process.
(obtained by connecting collector and base, Cha. 5)
2 for discrete diode
When i  I s : i
I s e / nVT
or   nVT ln
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i
IS
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i
I s e / nVT
Threshold voltage
υ>0.7 V, fully conducting
I1  I s e1 / nVT
I2
 e (2 1 )/ nVT
I1
I 2  I s e 2 / nVT
I2
I1
I
V2  V1  2.3nVT log 2
I1
Cut-in voltage
V2  V1  nVT ln
(3.5)
2.3nVT = 60 mV for n=1, 120 mV for n = 2.
Not knowing the exact value of n (which can be obtained from a simple experiment), circuit designers use
the convenient approximate number of 0.1 V/decade for the slope of the diode logarithmic characteristic.
* Threshold Voltage = Voltage drop: 0.7 V @1 mA for small signal diode, 0.7 V@1 A for high power diode.
EXAMPLE 3.3
i  0.7V @1mA
For small signal diode, evaluate IS in the event that n is 1 or 2.
i  I S e / nVT
I S  ie  / nVT
If n  1: I S  103 e 700 / 25  6.9  1016 A, or about 10-5 A
If n  2: I S  103 e 700 / 50  8.3  1016 A, or about 10-9 A
The value of n used can be quite important !!!
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Figure 3.9 Illustrating the temperature dependence of the
diode forward characteristic. At a constant current, the voltage
drop decreases by approximately 2 mV for every 1C increase
in temperature.
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3.2.2 The Reverse-Bias Region
2. The reverse-bias region, determined by υ < 0
i  I s (e / nVT  1)
(3.1)
If υ is negative and a few times greater than VT (25 mV),
i
IS
Real Diode
Reverse current i
>>
order of 1 nA
increases with reverse voltage
mostly due to leakage
doubles for every 10 oC rise
Saturation current I
order of 10-14~10-15.
constant
diffusion
doubles for every 5 oC rise
3.2.3 The Breakdown Region
3. The breakdown region, determined by υ < -VZR
* VZR : The breakdown voltage, knee voltage, Z stands for zener.
* Diode breakdown is normally not destructive provided that the power dissipated in the diode is limited by
external circuitry to a safe value specified in data sheets.
* Voltage regulation is possible in this region. (Sect. 3.5)
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3.3 Modeling the diode Forward Characteristic (Detailed analysis of circuits employing diodes)
ID=? VD=?
Exponential model.
Ideal-Diode model
Piecewise–Linear model
Constant-Voltage-Drop model
Small-Signal model
3.3.1 The Exponential Model – the most accurate description of the diode, but difficult to use
i  I s (e / nVT  1)
When VDD >0.5 V,
(3.1)
I D  I S eVD / nVT
From the circuit, I D 
(3.6)
VDD  VD
R
(3.7)
How to solve this simultaneous equations ?
3.3.2 Graphical Analysis
Use the i- υ curve given in the data sheet, ruler, and your eyes.
3.3.3 Iterative Analysis
Use computer, but you need programming.-ex. 3.4
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3.3.4 The need for Rapid Analysis
* In design process, rapid circuit analysis is necessary,
not in the final conformation process.
* In the final conformation process, SPICE is the best choice.
the piecewise-linear model.
3.3.5 The Piecewise-Linear model
i D  0,
 D  VD 0
i D  ( D  VD 0 ) / rD ,
 D  VD 0
(3.8)
the constant-voltage-drop model
3.3.6 The Constant-Voltage-Drop model
VD  0.7 V
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3.3.7 The Ideal –Diode Model
* This model can be used when input voltage is much greater than the diode voltage drop (0.6~0.8 V).
* We have learned this Sect. 3.1.
* Useful in determining which diodes are on or off.
3.3.8 The Small-signal Model
For dc component, I D  I S eVD / nVT (3.9)
For dc+ac signal  D (t)  VD  d ( t ) (3.10)
Quiescent point
iD (t )  I S e(VD d )/ nVT
iD (t )  I S e VD / nVT ed / nVT
if
d
nVT
iD (t )  I Ded / nVT
1,
using Taylor series and approximation,
Small-signal approximation: iD ( t )
I D (1+
Valid for υd < 10 mV n=2
υd < 5 mV n=1
iD (t )  I D +
(3.12)
ID

nVT d
id 
iD  I D +id
ID

nVT d
d
nVT
) (3.14)
(3.16)
(3.17)
Diode small-signal conductance (Siemens)
diode small-signal resistance = incremental resistance
nVT
rd 
(3.18) :inversely proportinal to the bias current I D
ID
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 i 
rd  1  D 
(3.19)


 D  iD  I D
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EXAMPLE 3.6
V+ = 10 Vdc on which is superimposed υs = 60 Hz sinusoid of 1 V peak (power supply ripple). R = 10 kΩ
Calculate both the dc voltage of the diode and the amplitude of the signal appearing on it.
Assume the diode to have 0.7 V drop at 1 mA and n = 2.
Sol) First, consider dc component only,
ID 
10  0.7
 0.93 mA
10
0.93 mA ≈1 mA, assumed 0.7 V is valid !
rd 
nVT 2  25

 53.8 
ID
0.93

 d (peak)  Vs
3.3.9 Use of the Diode Forward Drop in Voltage Regulation
1
rd
R  rd
0.0538
 5.35 mA
10  0.0538
Voltage regulator: A circuit keeping its output voltage as constant as possible in spite of;
(a) changes in the load current drawn from the regulator output,
(b) changes in the dc power supply voltage that feed the regulator.
EXAMPLE 3.7
n=2
(a) Find the % changes in υO caused by a ±10 % change in the dc power supply voltage.
(b) Find the % changes in υO caused by connection of 1 kΩ load resistance.
nVT 2  25
10  2.1
for each diode, rd 

 6.3 
(a) w/o load, I 
 7.9 mA
I
7.9
1
r
0.0189
2
 37.1 mA
r  3rd  18.9  O  (2  10% of 10V)
rR
0.0189  1
37.1 mV/2
 0.9%
2.1 V
(b) w load, I RL 
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6.2 mV change per diode justifies the small signal model.
2.1
 2.1 mA Then, diode current decreases by 2.1 mA.
1000
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O  2.1  r  39.7 mV
12
13.2 mV change per diode does'nt justifies the small signal model, but exact solution is 35.5 mV.
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3.4 Operation in the Reverse Breakdown Region-Zener Diodes
Voltage regulators are operating in this region.
Special diodes that are manufactured to operate in the breakdown
region are called breakdown diodes or zener diodes.
3.3.9 Specifying and Modeling the Zener Diodes
IZK : knee current (data sheet)
VZ : voltage across the diode at the test current IZT (data sheet)
a few volts ~ a few hundred volts.
V  rz I
rZ : incremental (dynamic) resistance at Q (data sheet)
a few~ a few tens of ohms.
0.5 W (data sheet), 6.8 V can operates safely at 70 mA, maximum.
Model for the zener diode VZ  VZ 0  rz I Z (3.20)
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EXAMPLE 3.8
VZ = 6.8 V @ Iz = 5 mA, rz=20Ω, IZK = 0.2 mA
(a) VO =? w/o load and w/ V+ at its nominal value.
VZ  VZ 0  rz I Z (3.20)
VZ 0  6.7 V
V +  VZ 0
IZ  I 
R  rz

VO  VZ 0  I Z rz
10 - 6.7
 6.35 mA
0.5  0.02
(b) ∆VO =? w/ ±1 V change in
∆VO/
mV/V)
is known as line regulation.
r
20
VO  V  z  1 
 38.5 mA
R  rz
500  20
V+.
∆V+(
Line regulation  38.5 mV/V
(c) ∆ VO =? w/ RL (IL = 1mA). Find load regulation
∆VO/ ∆IL ( mV/mA).
When a load resistance RL is connected that draws a
current IL = 1mA, the zener current will decrease by 1 mA.
VO  rz I Z  20  1  20 mV
Load regulation 
VO
 20 mV/mA
I L
(d) ∆ VO =? w/ RL = 2 kΩ
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 6.7  6.35  0.02  6.83 V
IL ≈ 6.8 V/2 kΩ=3.4 mA VO  rz I Z  20  3.4  68 mV
For more rigorous calculation, you have to analyze
the circuit in Fig. (b). 70 mA
(e) VO =? w/ RL = 0.5 kΩ
6.8 V/ 0.5 kΩ = 13.6 mA. Is this possible ? Impossible!!
Maximum I through R = 6.4 mA!!
The zener diode operates in reverse-bias region !!
The zener diode is off (open circuit).
VZ 0  V 
RL
0.5
 10
5V
R  RL
0.5  0.5
(e) Minimum RL for break down region = ?
Iz = IZK = 0.2 mA at the edge of the breakdown region,
The lowest (worst-case) current through R = (9-6.7)/0.5
= 4.6 mA), then the load current = 4.6 - 0.2 = 4.4 mA
RL  6.7 / 4.4 1.5 k
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3.5 Rectifier Circuit – one of the most important applications of diodes- dc power supplies
Power
transformer
The dc voltage VO is required to be as constant as possible in spite of variations in the ac line voltage and in the
current drawn by the load.
Power transformer : 1. V2=(N2/N1)V1
2. Electrical isolation between the electronic devices and the power-line circuit.
This isolation minimizes the risk of electric shock to the equipment user.
Diode rectifier : convert the input sinusoid υS to a unipolar output.
Filter : reducing the variations in the magnitude of the rectifier output.
Voltage regulator : 1.reducing ripple
2. Stabilizing the magnitude of the dc output voltage of the supply against variations caused by
changes in load current.
3. Circuit using zener diode or IC regulator (7805, 7915)
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3.5.1 The Half-Wave Rectifier.
O 
O  0,
 S <VD 0 (3.21a)
R
R
 S  VD 0
,  S  VD 0 (3.21b)
R  rD
R  rD
Peak Invers Voltage
PIV = Vs
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3.5.2 The Full-Wave Rectifier.
PIV = 2Vs  VD
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3.5.3 The Bridge Rectifier.
 D 3 (revers) = O +  D 2 (forward)
PIV = Vs  2VD  VD  Vs  VD
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3.5.4 The Rectifier with a Filter Capacitor - The Peak Rectifier
Vr =?
iL  O / R
(3.23) iD  iC  iL  C
discharge: O  Vpe  t / CR
e T / CR
1  T / CR,
IL  Vp / R
∆t =?
Vr
(3.24, 3.25)
at the end of discharge, Vp  Vr
Vp
V p cos(t )  V p  Vr
Vpe T / CR
Vp
I
T

 L : ripple voltage (Vr
CR fCR fC
Vp )
(t )2
when t<<1, cos(t )  1 2
 t
Vr
d I
 iL
dt
2Vr / V p
(3.30)
iDav =? during conduction time
Vp
For C, Qsupplied during on time=Qlost during off time
Qsupplied =iCav∆t,
iCav= iDav - IL
Qlost =CVr
 i Dav  I L (1   2Vp / Vr )
i D max  I L (1  2 2Vp / Vr )
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(3.31)
(3.32)
21
EXAMPLE 3.9
Design specification
Input: 60 Hz, sinusoid
Output: Vp=100 V, peak-to-peak ripple 2 V
Load: 10 kΩ
C=?
Vr
Vp
Vp
I
T

 L (Vr
CR fCR fC
C
 t
2Vr / V p (3.30)
Diode on-time/cycle =?
Average diode current =? i Dav  I L (1   2Vp / Vr )
Peak diode current =? i
D max  I L (1  2 2V p / Vr )
* Comparing with the half-wave case, we need half
size capacitor, half diode currents.
Vr fR

Vr  2 V
100
 83.3  F
2  60  10  103
t  2  2 / 100  0.2 rad 0.2/(2*3.14)=3.18%
(3.31)
iDav  10(1   2  100 / 2)  324 mA
(3.32)
iDmax  10(1  2 2  100 / 2)  638 mA
Vr 
Figure 3.30 Waveforms in the full-wave peak rectifier.
Vp
Vp )
Vp
2 fCR
(3.33)
i Dav  I L (1   Vp / 2Vr )
(3.34)
i D max  I L (1  2 Vp / 2Vr )
(3.35)
PIV = 2Vs  VD : for full wave rectifier
PIV  Vs  VD : for bridge rectifier
The most important design parameters !!
This peak-rectifier can be applied to the design of a peak detector and a demodulator
for a amplitude–modulated (AM) signals.
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3.5.5 Precision Half-wave Rectifier-the Super Diode
We need a very high-performance rectifier circuit that has no voltage drop (0.7 V) in some applications !!
1. Initially, υI= υO =0
2. When υI≈ 0>0, υA≈A υI>>0.7 V
3. Diode conducts
4. Voltage follower ! Unity gain
amplifier ! Buffer amplifier !
O   I
I  0
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5. When υI ≈ 0<0, υA≈A υI << 0 V
6. Diode is off.
7. Open loop ! υA will saturate.
8. υO = 0 V
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3.6 Limiting and Clamping Circuits
3.6.1 Limiter Circuits
Double limiter
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3.6.2 The Clamped Capacitor or DC Restorer
O   I + C
The clamped capacitor or dc restorer with a square-wave input and no load.
Peak rectifier
* The output waveform will have its lowest peak clamped to 0 V.
* Reversing diode polarity will provide an output waveform whose highest peak is clamped to 0 V.
RC LPF
demodulator
Pulse width modulation (PWM)
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DC Restorer !
25
3.6.3 The Voltage Doubler or Multiplier
This technique can be extended to provide dc output voltage that are higher multiples of Vp.
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3.7 Physical Operation of Diodes
3.7.1 Basic Semiconductor Concepts
•
•
•
•
•
Electronic materials fall into three categories:
J (A/m2)= σE(V/m), ρ (A-m/V)=1/σ
– Insulators
Resistivity () > 105 -cm
– Semiconductors
10-3 <  < 105 -cm
– Conductors
 < 10-3 -cm
Elemental semiconductors are formed from a single type of atom (IV, Si, Ge)
Compound semiconductors are formed from combinations of column III and V elements
or columns II and VI.
Germanium was used in many early devices.
Silicon quickly replaced silicon due to its higher bandgap energy, lower cost, and is
easily oxidized to form silicon-dioxide insulating layers.
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Bandgap
Energy EG (eV)
Semiconductor
Carbon (diamond)
5.47
Silicon
1.12
Germanium
0.66
Tin
0.082
Gallium arsenide
1.42
Gallium nitride
3.49
Indium phosphide
1.35
Boron nitride
7.50
Silicon carbide
3.26
Cadmium selenide
1.70
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Silicon Covalent Bond Model (cont.)
Near absolute zero, all bonds are
complete. Each Si atom contributes
one electron to each of the four bond
pairs.
Increasing temperature adds
energy to the system and breaks
bonds in the lattice, generating
electron-hole pairs.
n =p= ni.
ni=concentration of carriers in intrinsic semiconductor
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Intrinsic Carrier Concentration
•
The density of carriers in a semiconductor as a function of
temperature and material properties is:
n i2
•
•
•
•
•
•
 EG 
-6
 BT exp 
 cm
 kT 
3
EG = semiconductor bandgap energy in eV (electron volts)
k = Boltzmann’s constant, 8.62 x 10-5 eV/K
T = absolute termperature, K

B = material-dependent parameter, 1.08 x 1031 K-3 cm-6 for Si
Bandgap energy is the minimum energy needed to free an electron
by breaking a covalent bond in the semiconductor crystal.
ni2 ≈ 1.5x1010 cm-3 for Si at 300 K (room temperature)
Diffusion current and Drift current
Diffusion current
dp
dx
(3.37)
dn
dx
(3.38)
J p  qDp
J n  qDn
• Dp and Dn are the hole and electron
diffusivities with units cm2/s.
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Drift current
 drift   p E
(3.39)  p : mobility
J p  drift  qp p E
(3.40a)
J n drift  qnn E
(3.40b)
J drift  q( p p  nn )E
(3.40c)
  1/ q( p p  nn )
(3.41) resistivity
• Diffusivity and mobility are related by Einstein’s relationship:
Dn
n

kT D p

 VT  Thermal voltage (3.42)
q
p
Doped Semiconductor
• Doping is the process of adding very small well controlled amounts of impurities into a
semiconductor.
• Doping enables the control of the resistivity and other properties over a wide range of
values.
• For silicon, impurities are from columns III and V of the periodic table.
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Acceptor Impurities in Silicon
Donor Impurities in Silicon
• Boron (column III
element) has been
added to silicon.
• There is now an
incomplete bond pair,
creating a vacancy
for an electron.
• Little energy is
required to move a
nearby electron into
the vacancy.
• As the ‘hole’
propagates, charge is
moved across the
silicon.
• p type!
• Phosphorous (or other
column V element) atom
replaces silicon atom in
crystal lattice.
• Since phosphorous has five
outer shell electrons, there is
now an ‘extra’ electron in the
structure. Free electron!
• Material is still charge
neutral, but very little energy
is required to free the
electron for conduction since
it is not participating in a
bond.
• n type !
In thermal equilibrium,
concentration of donor atoms
= concentration of free electrons
nn0 N D (3.43)
Thermally generated hole is
decreased. Therefore,
nn 0 pn 0
pn 0
ni2
(3.44)
2
i
p p0
N A (3.46)
p p0
ni2
NA
(3.47)
Free electrons (holes) in n-type silicon are majority carriers and
holes (free electrons) in p-type silicon are minority carriers.
A piece of n-type or p-type silicon is electrically neutral !!
n
(3.45)
ND
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3.7.2 The pn junction Under Open-Circuit Conditions
E field
Diffusion current!
Diffusion
Acceptors (B, III) accept,
recombine with electrons
and become (-).
Donors (P, IV) donate
electrons and become (+).
Drift current!
Minority carriers
ID  IS
This process produces electric potential
and field and stops when coulomb force
equals to diffusion mechanism.
E field
Wdep  xn  x p 
2 s  1
1 


V
q  NA ND  0
(3.50)
0.1~1 μm
There is no free electron and holes in this region.
This region is depleted of free carriers !!
N N 
V0  VT ln  A 2 D  (3.48)
 ni 
Microelectronic Circuits - Fifth Edition
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0.6~0.8 V
33
3.7.3 The pn junction Under Reverse-Bias Conditions
Figure 3.46 The pn junction excited by a constant-current source I
in the reverse direction. To avoid breakdown, I is kept smaller than
IS. Note that the depletion layer widens and the barrier voltage
increases by VR volts, which appears between the terminals as a
reverse voltage.
IS  ID  I
 2   1
1 
Wdep   s  

(3.52)
  V0  VR 
q
N
N

 A
D 
dq
Cj  J
(3.53) depletion (junction) capacitor at Q
dVR V  V
R
Cj 
 sA
Wdep
=
Q
C j0
1
VR
V0
  sq   N A N D   1 
  N  N   V  (3.56)
2

 A
D 
0 
(3.54, 55) C j 0  A 
Cj 
C j0
(3.57)
m

VR 
1

V0 

m : grading coefficient depends on the concentration profile.
Microelectronic Circuits - Fifth Edition
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Copyright  2004 by Oxford University Press, Inc.
34
3.7.5 The pn junction Under Forward-Bias Conditions
pn ( xn )  pn 0 e V/VT
(3.58)
Figure 3.50 Minority-carrier distribution in a forward-biased pn junction. It is
assumed that the p region is more heavily doped than the n region; NA >> ND.
law of juction
pn ( x )  pn 0   pn ( xn )  pn 0  e
 ( x  xn )/L p
(3.59)
Lp diffusion length 1~100 μm
Lp  D p p
(3.60)
τp : excess-minority-carrier-lifetime
Jp  q
Jp  q
Dp
Lp
Dp
Lp
pn 0 (e V/VT  1)e
 ( x  xn ) / L p
pn 0 (e V/VT  1)
D
J n  q n n p 0 (e V/VT  1)
Ln
Microelectronic Circuits - Fifth Edition
(3.61)
 qD p pn0 qDn n p 0  V/V
I  A

 e T  1
 Lp
L
n




 Dp
Dn  V/VT
I  Aqni2 

e
1
 Lp N D Ln N A 



 Dp
Dn 
I S  Aqni2 

 Lp N D Ln N A 




(3.63)
(3.64)

I  I s eV /VT  1
(3.62)
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Diffusion capacitance
Q p  Aq  shaded area under the pn ( x ) exponential
 Aq   pn ( xn )  pn0  Lp
Qn   n I n (3.66) Q   p I p   n I n (3.67)
Q   T I (3.68)
Qp 
L2p
Dp
Ip
Q p   p I p (3.65)
τT : mean transit time
Cd 
dQ C    T  I (3.69)


d
dV
 VT 
Junction (depletion-layer) capacitance
Cj 
C j0
 VR 
1

V0 

m
(3.57)
Microelectronic Circuits - Fifth Edition
Cj 
C j0

V 
1 
 V0 
Sedra/Smith
m
But accuracy is poor. we use C j
2C j 0
Copyright  2004 by Oxford University Press, Inc.
(3.70).
36
Microelectronic Circuits - Fifth Edition
Sedra/Smith
Copyright  2004 by Oxford University Press, Inc.
37
3.8 Special Diode Types
3.8.1 The Schottky-(Barrier) Diode (SBD)
- Metal (anode)-moderately doped n-type semiconductor (cathode) junction.
- Current is conduced by majority carriers (electrons).
No minority-carrier charge-storage effect
Much faster on-off transit time!
- 0.3 ~0.5 V forward voltage drop. GaAs SBD (very fast) voltage drop: ~0.7 V
- Not every metal-semiconductor contact is a diode.
- Ohmic contact for diode lead is metal-heavily doped semiconductor juction.
3.8.2 Varactors
Cj 
C j0
 VR 
1

V0 

m
(3.57)
- Special diode that are fabricated to be used as voltage-variable capacitor.
- Cj is made to be strong function of reverse voltage (m = 3~4)
- These are used for automatic tuning of radio receivers.
3.8.3 Photodiodes
- Special diode that can be used to convert light power to electric
current.
- A photon that has energy greater than the band-gap
energy generate electron-hole pair.
- This electron-hole pair cause the current to flow through
the reversely biased diode.
- The photodiode is an important component of a growing
family of circuits known as optoelectronics or photonics.
- Fiber-optic communication, CD-ROM, automatic door,
automatic flusher…(phototransistors)
- The photodiode without reverse bias is a solar cell (silicon).
- The photodiodes are made of silicon or compound
semiconductor (III and V, such as GaAs, InAlGaAs…)
Microelectronic Circuits - Fifth Edition
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Copyright  2004 by Oxford University Press, Inc.
38
3.8.4 Light-Emitting Diodes (LED) and Laser Diodes (LD)
- LED converts a forward current into light.
- Diffusing minority carriers recombine with majority carriers resulting in emission of photons with energy equal to
bandgap energy.
- This can be done by fabricating the pn junction using a semiconductor of the type known as direct bandgap materials
such as GaAs.
- Diffusing minority carriers recombine with majority carriers resulting in emission of photons with energy equal to
bandgap energy.
- LED display, light source in optical communications (LAN), illumination,…
- Laser diode produces coherent light with a very narrow bandwidth.
- Laser diodes are used in optical communication, CD players, pointers, laser machining, medical instruments…..
- LED + photodiode or phototransistor = optoisolator : complete electrical isolation between electrical circuit that is
connected to the isolator’s input and the circuit that is connected to its output.
- Optoisolator reduces the effect of electrical interference, can be used in the design of medical instruments to reduce
the risk of electrical shock to patients.
Microelectronic Circuits - Fifth Edition
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Copyright  2004 by Oxford University Press, Inc.
39