Current Flow in
Semiconductors
1
1. Drift Current
There are two distinctly different mechanisms for the movement of
charge carriers and hence for current flow in semiconductors: drift and
diffusion.
• Q: What happens when an electrical field (E) is applied to a
semiconductor crystal?
• A: Holes are accelerated in the direction of E, free
electrons are attracted.
• Q: How is the velocity of these carriers defined?
p =hole mobilityPpp
n =electron mobilityPpp
v p − driftv p−=drift =p Ep E
(eq3.8)
vn − drift v=n−−
 =E− n E
(eq3.9)
drift n
E =electric fieldPpp
E =electric fieldPpp
2
1. Drift Current
note that electrons move with velocity 2.5 times higher
than holes
.E (volts / cm)
.p (cm2/Vs) = 480 for silicon
.n (cm2/Vs) = 1350 for silicon
3
An electric field E established in a bar of silicon causes the holes to
3.3.1. Drift
drift inCurrent
the direction of E and the free electrons to drift in the
opposite direction. Both the hole and electron drift currents are in
the direction of E.
• Q: What happens when an electrical field (E) is applied to
a semiconductor crystal?
• A: Holes are accelerated in the direction of E, free
electrons are repelled.
HOLES
• Q: How is the velocity of these holes
defined?
ELECTRONS
 p =hole mobility
n =electron mobility
v p−drift =  p E
vn−drift = − n E
E =electric field
E =electric field
4
1. Drift Current
5
Example 6: Drift current
A uniform bar of n-type silicon of 2 μm length has a voltage of 1 V applied
across it. If 𝑁𝐷 = 106 /cm3 and 𝜇𝑛 = 1350cm2 /V.s, find (a) the electron drift
velocity, (b) the time it takes an electron to cross the 2-μm length, (c) the driftcurrent density, and (d) the drift current in the case the silicon bar has a cross
sectional area of 0.25μm2 .
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Lecture 03
6
Example 6: Drift current, contd.
b. Time taken to cross 2μm length
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
2 × 10−6
=
=
= 30ps
𝑣𝑑𝑟𝑖𝑓𝑡
6.75 × 104
c. The current density 𝐽𝑛 is given by
𝐽𝑛 = 𝑞𝑛𝜇𝑛 𝐸
= 1.6 × 10−19 × 1016 × 1350 ×
1
= 1.08 × 104 A/cm2
−4
2 × 10
d. Drift current 𝐼𝑛 = 𝐽𝑛 𝐴
𝐼𝑛 = 0.25 × 10−8 × 1.08 × 104 = 27μA
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7
2. Diffusion Current
• Take the following example…
• inject holes – By some
unspecified process, one injects
holes in to the left side of a silicon
bar.
• concentration profile arises –
Because of this continuous hole
inject, a concentration profile
arises.
• diffusion occurs – Because of this
concentration gradient, holes will
flow from left to right.
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inject
holes
Lecture 03
diffusion occurs
concentration
profile arises
8
4. The pn Junction with Open-Circuit Terminals
4.1. Physical Structure
pn junction structure
• p-type semiconductor
• n-type semiconductor
• metal contact for connection
Simplified physical structure of the pn junction. As the pn junction implements the
junction diode, its terminals are labeled anode and cathode.
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9
The pn Junction with Open-Circuit Terminals
1. Physical Structure
pn junction structure
• p-type semiconductor
• n-type semiconductor
• metal contact for connection
Simplified physical structure of the pn junction. As the pn junction implements the
junction diode, its terminals are labeled anode and cathode.
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10
2. pn Junction
Operation with Open-Circuit Terminals
• Q: What is state of pn junction with open-circuit
terminals?
• A: Read the below…
• p-type material contains majority of holes
• these holes are neutralized by equal amount of bound
negative charge
• n-type material contains majority of free electrons
• these electrons are neutralized by equal amount of bound
positive charge
11
2. pn Junction
Operation with Open-Circuit Terminals
bound charge
• charge of opposite polarity to free electrons / holes of a given
material
• neutralizes the electrical charge of these majority carriers
• does not affect concentration gradients
free electrons
free holes
positive bound
charges
negative bound
charges
p-type
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
12
4.2. Operation with Open-Circuit Terminals
• Q: What happens when a pn-junction is newly formed – aka. when
the p-type and n-type semiconductors first touch one another?
• A: See following slides…
Lecture 03
13
Step #1: The p-type and n-type semiconductors are
joined at the junction.
p-type semiconductor
filled with holes
junction
n-type semiconductor
filled with free electrons
Figure: The pn junction with no applied voltage (open-circuited terminals).
14
Step #2: Diffusion begins. Those free electrons and holes which
are closest to the junction will recombine and, essentially,
eliminate one another.
p-type
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
15
Step #3: The depletion region begins to form – as diffusion
occurs and free electrons recombine with holes.
The depletion region is filled with “uncovered” bound charges – who
have lost the majority carriers to which they were linked.
p-type
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
16
Step #4: The “uncovered” bound charges affect a voltage
differential across the depletion region. The magnitude of this
barrier voltage (V0) differential grows, as diffusion continues.
voltage potential
No voltage differential exists across regions of the pn-junction
outside of the depletion region because of the neutralizing effect of
positive and negative bound charges.
barrier voltage
(Vo)
p-type
n-type
location (x)
17
Step #5: The barrier voltage (V0) is an electric field whose
polarity opposes the direction of diffusion current (ID). As the
magnitude of V0 increases, the magnitude of ID decreases.
diffusion
current drift
(ID)
p-type
current
(IS)
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
18
Step #6: Equilibrium is reached, and diffusion ceases, once the
magnitudes of diffusion and drift currents equal one another –
resulting in no net flow.
Once equilibrium
is achieved,
no netdrift
current current
flow exists (Inet = ID – IS)
diffusion
current
within the pn-junction
condition.
(I ) while under open-circuit
(I )
D
p-type
S
depletion
region
n-type
19
Step #5: The barrier voltage (V0) is an electric field whose
polarity opposes the direction of diffusion current (ID). As the
magnitude of V0 increases, the magnitude of ID decreases.
diffusion
current drift
(ID)
p-type
current
(IS)
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
20
Step #6: Equilibrium is reached, and diffusion ceases, once the
magnitudes of diffusion and drift currents equal one another –
resulting in no net flow.
Once equilibrium
is achieved,
no netdrift
current current
flow exists (Inet = ID – IS)
diffusion
current
within the pn-junction
condition.
(I ) while under open-circuit
(I )
D
p-type
S
depletion
region
n-type
21
Step #4: The “uncovered” bound charges affect a voltage
differential across the depletion region. The magnitude of this
barrier voltage (V0) differential grows, as diffusion continues.
voltage potential
No voltage differential exists across regions of the pn-junction
outside of the depletion region because of the neutralizing effect of
positive and negative bound charges.
barrier voltage
(Vo)
p-type
n-type
location (x)
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22
4.2. Operation with Open-Circuit Terminals
• Q: What happens when a pn-junction is newly formed – aka. when
the p-type and n-type semiconductors first touch one another?
• A: See following slides…
Lecture 03
23
Step #1: The p-type and n-type semiconductors are
joined at the junction.
p-type semiconductor
filled with holes
junction
n-type semiconductor
filled with free electrons
Figure: The pn junction with no applied voltage (open-circuited terminals).
24
Step #2: Diffusion begins. Those free electrons and holes which
are closest to the junction will recombine and, essentially,
eliminate one another.
p-type
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
25
Step #3: The depletion region begins to form – as diffusion
occurs and free electrons recombine with holes.
The depletion region is filled with “uncovered” bound charges – who
have lost the majority carriers to which they were linked.
p-type
n-type
Figure: The pn junction with no applied voltage (open-circuited terminals).
26
Step #4: The “uncovered” bound charges affect a voltage
differential across the depletion region. The magnitude of this
barrier voltage (V0) differential grows, as diffusion continues.
voltage potential
No voltage differential exists across regions of the pn-junction
outside of the depletion region because of the neutralizing effect of
positive and negative bound charges.
barrier voltage
(Vo)
p-type
n-type
location (x)
27
5. The pn Junction with an Applied Voltage
5.1. Qualitative Description of Junction Operation
• Figure to right shows pnjunction under three
conditions:
• (a) open-circuit – where
a barrier voltage V0
exists.
• (b) reverse bias – where
a dc voltage VR is
applied.
• (c) forward bias – where
a dc voltage VF is
applied.
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Lecture 03
Figure 11: The pn junction in: (a)
equilibrium; (b) reverse bias; (c)
forward bias.
28
1) no voltage
applied
1) negative voltage
applied
1) positive voltage
applied
2) voltage differential
across depletion zone
is V0
2) voltage differential
across depletion zone
is V0 + VR
2) voltage differential
across depletion zone
is V0 - VF
• Figure to right shows pn-junction
3) ID < IS
3) ID = Iconditions:
under three
S
• (a) open-circuit – where a barrier
voltage V0 exists.
• (b) reverse bias – where a dc
voltage VR is applied.
• (c) forward bias – where a dc
voltage VF is applied.
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3) ID > IS
Figure 3.11: The pn junction in:
(a) equilibrium; (b) reverse bias;
(c) forward bias.
Lecture 03
29
Reverse Biased Diode’s Application: VoltageDependent Capacitor
The PN junction can be viewed as a capacitor. By varying VR, the
depletion width changes, changing its capacitance value; therefore,
the PN junction is actually a voltage-dependent capacitor.
30
Example: How does a Voltage
dependent capacitor work ?
5.2. The Current-Voltage Relationship of the
Junction
(eq3.40) I = IS (eV / VT − 1)
• saturation current (IS) –
is the maximum reverse
current which will flow
through pn-junction.
• It is proportional to
cross-section of
junction (A).
• Typical value is 1018A.
Figure 13: The pn junction I–V
characteristic.
32
Example: calculate the current flowing in a p-n
junction subjected to a forward voltage of 0.4V
given that saturation current Is=10-18 A and
thermal voltage VT= 0.026 V
5.3 Reverse Breakdown
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34
Zener breakdown
• The electric field in the depletion layer increases to cause breaking
covalent bonds and generating electron-hole pairs.
• The electrons generated in this way will be swept by the electric
field into the n side and the holes into the p side. Thus these
electrons and holes constitute a reverse current across the junction.
• Once the zener effect starts (VR=5V), a large number of carriers can
be generated, with a negligible increase in the junction voltage. Thus
the reverse current in the breakdown region will be large and its
value must be determined by the external circuit.
• the reverse voltage appearing between the diode terminals will
remain close to the specified breakdown voltage VZ.
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Avalanche breakdown
• The minority carriers that cross the depletion region under the
influence of the electric field gain sufficient kinetic energy to be
able to break covalent bonds in atoms with which they collide.
• The carriers liberated by this process may have sufficiently high
energy to be able to cause other carriers to be liberated in
another ionizing collision.
• This process keeps repeating in the fashion of an avalanche, with
the result that many carriers are created that are able to support
any value of reverse current, as determined by the external
circuit, with a negligible change in the voltage drop across the
junction.
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36
5.3 Reverse Breakdown
• The maximum reverse-bias potential that can be applied before
entering the breakdown region is called the peak inverse voltage
(referred to simply as the PIV rating) or the peak reverse voltage
(denoted the PRV rating).
37
6. Capacitive Effects in the pn Junction
1. Depletion or Junction Capacitance
When a pn junction is reverse biased
Where
2. Diffusion Capacitance
When a pn junction is forward biased (‫)المعادالت أعاله كانت موجودة هنا و تم إصالح الخطأ‬
𝜏 𝑇 is the mean transit time of the junction.
I is the forward-bias current.
38
6. Capacitive Effects in the pn Junction
• junction capacitance:
✓ due to the dipole in the transition region (associated with the charge
stored in the depletion region).
✓ Also called transition region capacitance or depletion layer capacitance.
✓ Dominates under reverse bias conditions.
• Charge storage (Diffusion) capacitance:
✓ associated with the minority carrier charge stored in the n and p materials
as a result of the concentration profiles established by carrier injection.
✓ Also referred to as diffusion capacitance.
✓ Dominant when the junction is forward biased.
39
Summary (1)
• Today’s microelectronics technology is almost
entirely based on the semiconductor silicon. If a
circuit is to be fabricated as a monolithic integrated
circuit (IC), it is made using a single silicon crystal,
no matter how large the circuit is.
• In a crystal of intrinsic or pure silicon, the atoms
are held in position by covalent bonds. At very low
temperatures, all the bonds are intact; No charge
carriers are available to conduct current. As such,
at these low temperatures, silicone acts as an
insulator.
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41
Summary (2)
• At room temperature, thermal energy causes some
of the covalent bonds to break, thus generating
free electrons and holes that become available to
conduct electricity.
• Current in semiconductors is carried by free
electrons and holes. Their numbers are equal and
relatively small in intrinsic silicon.
• The conductivity of silicon may be increased
drastically by introducing small amounts of
appropriate impurity materials into the silicon
crystal – via process called doping.
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Summary (3)
• There are two kinds of doped semiconductor: ntype in which electrons are abundant, p-type in
which holes are abundant.
• There are two mechanisms for the transport of
charge carriers in a semiconductor: drift and
diffusion.
• Carrier drift results when an electric field (E) is
applied across a piece of silicon. The electric field
accelerates the holes in the direction of E and
electrons oppositely. These two currents sum to
produce drift current in the direction of E.
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Summary (4)
• Carrier diffusion occurs when the concentration of
charge carriers is made higher in one part of a
silicon crystal than others. To establish a steadystate diffusion current, a carrier concentration
must be maintained in the silicon crystal.
• A basic semiconductor structure is the pn-junction.
It is fabricated in a silicon crystal by creating a pregion in proximity to an n-region. The pn-junction
is a diode and plays a dominant role in the
structure and operation of transistors.
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44
Summary (5)
• When the terminals of the pn-junction are left open,
no current flows externally. However, two equal and
opposite currents (ID and IS) flow across the junction.
Equilibrium is maintained by a built-in voltage (V0).
Note, however, that the voltage across an open
junction is 0V, since V0 is cancelled by potentials
appearing at the metal-to-semiconductor
connection interfaces.
• The voltage V0 appears across the depletion region,
which extends on both sides of the junction.
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Summary (6)
• The drift current IS is carried by thermally generated
minority electrons in the p-material that are swept
across the depletion region into the n-side. The
opposite occurs in the n-material. IS flows from n to
p, in the reverse direction of the junction. Its value
is a strong function of temperature, but
independent of V0.
• Forward biasing of the pn-junction, that is applying
an external voltage that makes p more positive than
n, reduces the barrier voltage to V0 - V and results in
an exponential increase in ID (while IS remains
unchanged).
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46
Semiconductor Diode Notation
Various types of junction diodes
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47
Diode Testing
• Diode Checking Function
• Ohmmeter Testing
• Curve Tracer
48
Introduction
• In this Lecture we will learn
❑ application of the diode in the design of rectifier circuits, which
convert ac voltages to dc as needed for powering electronic
equipment.
❑ a number of other practical and important applications: limiting and
clamping circuits.
❑ Special diode types: LED, Photo diode, Schottky diode, Varactor
diode, Zener diode.
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49
4.5. Rectifier Circuits
• One important application of
diode is the rectifier –
• Electrical device
which converts
alternating current
(AC) to direct
current (DC)
• One important application of
rectifier is dc power supply.
Figure 4.20: Block diagram of a dc
power supply
step #1: Decrease RMS magnitude of AC wave
via power transformer
step #2: convert full-wave AC signal to full-wave
rectified signal (still time-varying and periodic)
step #3: employ low-pass filter to reduce wave
amplitude by > 90%
step #4: employ voltage regulator to eliminate
ripple
step #5: supply dc load
.
Oxford University Publishing
Figure 4.20:Microelectronic
Block Circuits
diagram
ofandaKenneth
dcC.power
supply
by Adel S. Sedra
Smith
(0195323033)
4.5.1. The Half-Wave Rectifier
• half-wave rectifier
– utilizes only
alternate half-cycles
of the input
sinusoid
• Constant voltage
drop diode model is
employed.
Figure 4.21: (a) Half-wave rectifier (b) Transfer characteristic of the rectifier circuit (c)
Input and output waveforms
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52
4.5.1. The Half-Wave Rectifier
In selecting diodes for rectifier design, two important parameters
must be specified:
• current-handling capability – what is maximum forward current
diode is expected to conduct?
• peak inverse voltage (PIV) – what is maximum reverse voltage it
is expected to block w/o breakdown?
It is usually prudent to select a diode that has a reverse breakdown
voltage at least 50% greater than the expected PIV.
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53
4.5.2. The
Full-Wave Rectifier
• Q: How does fullwave rectifier differ
from half-wave?
• A: It utilizes both
halves of the input
• One potential is
shown to right.
Figure 4.22: Full-wave rectifier utilizing a
transformer with a center-tapped secondary
winding.
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54
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Figure 4.22: full-wave rectifier utilizing a transformer with a centertapped secondary winding: (a) circuit; (b) transfer characteristic
assuming a constant-voltage-drop model for the diodes; (c) input
and output waveforms.
55
4.5.2. The Full-Wave Rectifier
• Q: What are most important observation(s) from this
operation?
• A: The direction of current flowing across load never
changes (both halves of AC wave are rectified). The fullwave rectifier produces a more “energetic” waveform
than half-wave.
• PIV for full-wave = 2VS – VD
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4.5.3. The Bridge Rectifier
• An alternative
implementation of
the full-wave
rectifier is bridge
rectifier.
• Shown to right.
Figure 4.23: The bridge rectifier circuit.
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57
when instantaneous source voltage is positive, D1
and D2 conduct while D3 and D4 block
Figure 4.23: The bridge rectifier circuit.
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58
when instantaneous source voltage is positive, D1
and D2 conduct while D3 and D4 block
Figure 4.23: The bridge rectifier circuit.
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59
4.5.3: The Bridge Rectifier (BR)
• Q: What is the main advantage of BR?
• A: No need for center-tapped transformer.
• Q: What is main disadvantage?
• A: Series connection of TWO diodes will reduce output
voltage.
• PIV = VS – VD
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4.5.4. The Rectifier
with a Filter Capacitor
• step #1: source voltage is
positive, diode is forward
biased, capacitor charges.
• step #2: source voltage is
reverse, diode is reversebiased (blocking),
capacitor cannot
discharge.
• step #3: source voltage is
positive, diode is forward
biased, capacitor charges
(maintains voltage).
Figure 4.24 (a) A simple circuit used to illustrate the effect…
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61
4.5.4. The Rectifier
with a Filter Capacitor
• Q: Why is this example unrealistic?
• A: Because for any practical application, the converter
would supply a load (which in turn provides a path for
capacitor discharging).
62
4.5.4. The Rectifier
with a Filter Capacitor
• Q: What happens
when load resistor
is placed in series
with capacitor?
• A: One must now
consider the
discharging of
capacitor across
load.
63
4.5.4. The Rectifier
with a Filter Capacitor
circuit state #1
output voltage for state #1
vO ( t ) = v I ( t ) − v D
vO ( t ) = Vpeak e
−
t
RC
output voltage for state #2
circuit state #2
64
output voltage for state #1
vO ( t ) = v I ( t )
vO ( t ) = Vpeak e
−
t
RC
output voltage for state #2
Figure 4.25: Voltage and Current Waveforms in the Peak Rectifier
Circuit WITH RC >> T. The diode is assumed ideal.
65
4.6.2. The Clamped
Capacitor or DC Restorer
• Q: What is a dc restorer?
• A: Circuit which removes the
dc component of an AC wave.
• Q: Why is this ability important?
• A: Average value of this output
is effective way to measure
duty cycle
Figure 4.32: The clamped
capacitor or dc restorer with a
square-wave input and no load
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4.6.3: The Voltage Doubler
• Q: What is a voltage doubler?
• A: One which multiplies the
amplitude of a wave or signal
by two.
Figure 4.34: Voltage doubler: (a) circuit;
(b) waveform of the voltage across D1.
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Special Diode Types
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68
Optical Diodes
There are two popular types of optoelectronic devices:
light-emitting diode (LED) and photodiode.
The Light-Emitting Diode (LED)
LED is diode that emits light when biased in the forward direction
of p-n junction.
Anode
Cathode
The schematic symbol and construction features.
The Light-Emitting Diode (LED)
LED that are produced in an array of shapes and sizes.
LED characteristics:
characteristic curves are very similar to those for p-n junction diodes
higher forward voltage (VF)
lower reverse breakdown voltage (VBR).
The Light-Emitting Diode (LED)
Application
The seven segment display is an example of LEDs use for display of decimal
digits.
The 7-segment LED display.
LED Displays
LED displays are packages of many LEDs arranged in a pattern, the most
familiar pattern being the 7-segment displays for showing numbers (digits
0-9).
The Light-Emitting Diode (LED)
Light Spectrum
Red, green and blue LEDs
The Light-Emitting Diode (LED)
When a light-emitting diode is forward
biased, electrons are able to recombine
with holes within the device, releasing
energy in the form of photons.
This effect is called electroluminescence
and the color of the light (corresponding
to the energy of the photon) is
determined by the energy gap of the
semiconductor.
Fabricating the pn junction using a
semiconductor of the type known as
direct-bandgap materials.
LED - Light Emitting Diodes
UV – AlGaN
Blue – GaN, InGaN
Red, green – GaP
Red, yellow – GaAsP
IR- GaAs
Calculating an LED resistor value
An LED must have a resistor connected in series to
limit the current through the LED. The resistor value,
R is given by:
R = (VS - VL) / I
VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted
If the calculated value is not available, choose the nearest standard resistor
value which is greater, to limit the current. Even greater resistor value will
increase the battery life but this will make the LED less bright.
For example
If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a
current
I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest greater standard value).
The Photodiode
Photodiode is a p-n junction that can convert light energy
into electrical energy.
It operates in reverse bias voltage (VR), as shown in Figure,
where Iλ is the reverse light current.
It has a small transparent window that allows light to strike
the p-n junction.
The resistance of a photodiode is calculated by the formula
as follows:
Iλ
RR =
VR
I
Alarm System using Photodiode
Photodiode Alarm Circuit
IR Transmitter Circuit
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The Schottky Diode
❑ The Schottky diode’s significant characteristic is its fast switching
speed.
❑ This is useful for high frequencies and digital applications.
❑ It is not a typical diode in that it does not have a p-n junction.
❑ Instead, it consists of a doped semiconductor (usually n-type) and
metal bound together.
Schottky diode (a) symbol and (b) basic internal construction
Zener Diode
Zener diode is a p-n junction diode
that is designed to operate in the
reverse breakdown region.
+
V
Z
Two things happen when the
reverse breakdown voltage (VBR) is
reached:
The diode current increases
drastically.
The reverse voltage (VR) across
the diode remains relatively
constant.
In other words, the voltage across a
zener diode operated in this region
is relatively constant over a range
of reverse current and nearly equal
to its zener voltage (VZ) rating.
K
Cathode (K)
I
−
Anode (A)
Z
A
VB
R
Zener diode voltage-curent (V-I) characteristic.
Zener Diode
Ideal-and-Practical Zener Equivalent Circuits
I
F
VR
V
VF
Z
IR
Ideal model and characteristic curve of a
zener diode in reverse breakdown.
The constant voltage drop =
the nominal zener voltage.
Practical model and characteristic curve of a zener diode, where
the zener impedance (resistance), ZZ is included.
A change in zener current (ΔIZ) produces a
small change in zener voltage (ΔVZ).
Varactor (Varicap Diode)
Varactor is a type of p-n junction diode that
operates in reverse bias. The capacitance of
the junction is controlled by the amount of
reverse bias.
Varactor diodes are also referred to as
varicaps or tuning diodes and they are
commonly used in communication systems.
Varactor diode symbol
Basic Operation
The capacitance of a reverse-biased
varactor junction is found as:
C=
A
d
C = the total junction capacitance.
A = the plate area.
ε = the dielectric constant (permittivity).
d = the width of the depletion region
(plate separation).
Reverse-biased varactor diode acts
as a variable capacitor.
Varactor (Varicap Diode)
When the junction diode is
reverse biased, the insulating
barrier widens reducing diode
capacitance.
The barrier forms the dielectric,
of variable width, of a capacitor.
The N and P type cathode and anode are the two plates of the capacitor.
In the diagram, the diode and coil form a resonant circuit.
The capacitance of the diode, and thereby the resonant frequency, is
varied by means of the potentiometer controlling the reverse voltage
across the varicap.
The capacitor prevents the coil shorting out the voltage across the
potentiometer.
Summary (1)
• Rectifiers convert ac voltage into unipolar
voltages. Half-wave rectifiers do this by
passing the voltage in half of each cycle and
blocking the opposite-polarity voltage in the
other half of the cycle.
• The bridge-rectifier circuit is the preferred
full-wave rectifier configuration.
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Summary (2)
• The variation of the output waveform of the
rectifier is reduced considerably by connecting a
capacitor C across the output load resistance R.
The resulting circuit is the peak rectifier. The
output waveform then consists of a dc voltage
almost equal to the peak of the input sine wave, Vp,
on which is superimposed a ripple component of
frequency 2f (in the full-wave case) and of peak-topeak amplitude Vr = Vp/2fRC.
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Summary (3)
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• Combination of diodes, resistors, and possible
reference voltage can be used to design voltage
limiters that prevent one or both extremities of
the output waveform from going beyond
predetermined values – the limiting levels.
• Applying a time-varying waveform to a circuit
consisting of a capacitor in series with a diode
and taking the output across the diode provides
a clamping function.
• By cascading a clamping circuit with a peak-
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Summary (4)
• Beyond a certain value of reverse voltage
(that depends on the diode itself), breakdown
occurs and current increases rapidly with a
small corresponding increase in voltage.
• Diodes designed to operate in the breakdown
region are called zener diodes. They are
employed in the design of voltage regulators
whose function is to provide a constant dc
voltage that varies little with variations in
power supply voltage and / or load current.
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