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Semiconductor —I
• Materials that permit flow of electrons are called
conductors (e.g., gold, silver, copper, etc.).
• Materials that block flow of electrons are called
insulators (e.g., rubber, glass, Teflon, mica, etc.).
• Materials whose conductivity falls between those
of conductors and insulators are called
semiconductors.
• Semiconductors are “part-time” conductors
whose conductivity can be controlled.
Semiconductor —II
• Silicon is the most common material used to build semiconductor devices.
• Si is the main ingredient of sand and it is estimated that a cubic mile of seawater
contains 15,000 tons of Si.
• Si is spun and grown into a crystalline structure and cut into wafers to make electronics devices.
Semiconductor —III
• Atoms in a pure silicon wafer contains four electrons in outer orbit (called valence
electrons).
– Germanium is another semiconductor material with four valence electrons.
• In the crystalline lattice structure of Si, the valence electrons of every Si atom are
locked up in covalent bonds with the valence electrons of four neighboring Si atoms.
– In pure form, Si wafer does not contain any free charge carriers.
– An applied voltage across pure Si wafer does not yield electron flow through the wafer.
– A pure Si wafer is said to act as an insulator.
• In order to make useful semiconductor devices, materials such as phosphorus (P) and
boron (B) are added to Si to change Si’s conductivity.
N-Type Silicon
• Pentavalent impurities such as phosphorus, arsenic, antimony, and bismuth have 5
valence electrons.
• When phosphorus impurity is added to Si, every phosphorus atom’s four valence
electrons are locked up in covalent bond with valence electrons of four neighboring
Si atoms. However, the 5th valence electron of phosphorus atom does not find a
binding electron and thus remains free to float. When a voltage is applied across the
silicon-phosphorus mixture, free electrons migrate toward the positive voltage end.
• When phosphorus is added to Si to yield the above effect, we say that Si is doped
with phosphorus. The resulting mixture is called N-type silicon (N: negative charge
carrier silicon).
• The pentavalent impurities are referred to as donor impurities.
P-Type Silicon —I
• Trivalent impurities e.g., boron, aluminum, indium, and gallium have 3 valence
electrons.
• When boron is added to Si, every boron atom’s three valence electrons are locked up
in covalent bond with valence electrons of three neighboring Si atoms. However, a
vacant spot “hole” is created within the covalent bond between one boron atom and
a neighboring Si atom. The holes are considered to be positive charge carriers.
When a voltage is applied across the silicon-boron mixture, a hole moves toward the
negative voltage end while a neighboring electron fills in its place.
• When boron is added to Si to yield the above effect, we say that Si is doped with
boron. The resulting mixture is called P-type silicon (P: positive charge carrier
silicon).
• The trivalent impurities are referred to as acceptor impurities.
P-Type Silicon —II
• The hole of boron atom points towards the negative terminal.
• The electron of neighboring silicon atom points toward
positive terminal.
• The electron from neighboring silicon atom falls into the
boron atom filling the hole in boron atom and creating a “new”
hole in the silicon atom.
• It appears as though a hole moves toward the negative terminal.
Diode:
•A diode is a 2 lead semiconductor that acts as a one way gate to electron flow.
– Diode allows current to pass in only one direction.
•A pn-junction diode is formed by joining together n-type and p-type silicon.
•In practice, as the n-type Si crystal is being grown, the process is abruptly altered to
grow p-type Si crystal. Finally, a glass or plastic coating is placed around the joined
crystal.
•The p-side is called anode and the n-side is called cathode.
•When the anode and cathode of a pn-junction diode are connected to external voltage
such that the potential at anode is higher than the potential at cathode, the diode is said
to be forward biased.
–In a forward-biased diode current is allowed to flow through the device.
•When potential at anode is smaller than the potential at cathode, the diode is said to
be reverse biased. In a reverse-biased diode current is blocked.
Water Analogy of Diodes
• When water pressure on left overcomes the restoring force of spring, the gate is
opened and water is allowed to flow.
• When water pressure is from right to left, the gate is pressed against the solid stop
and no water is allowed to flow.
• Spring restoring force is analogous to 0.6V needed to forward bias a Si diode.
Diode: How it Works —I
• When a diode is
connected to a battery as
shown, electrons from
the n-side and holes from
the p-side are forced
toward the center by the
electrical field supplied
by the battery. The
electrons and holes
combine causing the
current to pass through
the diode. When a diode
is arranged in this way, it
is said to be forward-
biased.
Diode: How it Works—II
• A diode’s one-way gate feature does not work all the time.
• Typically for silicon diodes, an applied voltage of 0.6V or greater is needed,
otherwise, the diode will not conduct.
• This feature is useful in forming a voltage-sensitive switch.
• I-V characteristics for silicon and germanium diodes is shown below.
Diode: How it doesn’t work
• When a diode is
connected to a battery as
shown, holes in the n-
side are forced to the
left while electrons in
the p-side are forced to
the right. This results in
an empty zone around
the pn- junction that is
free of charge carries
creating a depletion
region. This depletion
region acts as an
insulator preventing
current from flowing
through the diode.
When a diode is
arranged in this way, it
is said to be reverse-
biased.
Diode Applications —Half Wave Rectifier
•Diode converts ac input voltage to a pulsed dc output voltage.
•Whenever the ac input becomes negative at diode’s anode, the diode blocks current
flow.
– o/p voltage become zero.
•Diode introduces a 0.6V drop so o/p peak is 0.6V smaller than the i/p peak.
•The o/p frequency is same as the i/p frequency.
Diode Applications —Full Wave Rectifier
• A full-wave rectifier does not block negative swings in the i/p voltage, rather it
transforms them into positive swings at the o/p.
• To gain an understanding of device operation, follow current flow through pairs of
diodes in the bridge circuit.
• It is easily seen that one pair (D3-Rout-D2) allows current flow during the +ve half
cycle of Vin while the other pair (D4-Rout-D1) allows current flow during the -ve half
cycle of Vin.
– o/p voltage peak is 1.2V below the i/p voltage peak.
– The o/p frequency is twice the i/p frequency.
Diode Applications —AC2DC Power Supply
•An AC2DC power supply
is built using a transformer
and a full-wave rectifier.
•Transformer is used to
step down the voltage i/p.
•Rectifier converts AC to
pulsed DC.
•A filter capacitor is used
to smooth out the pulses.
•Capacitor must be large
enough to store sufficient
charge so as to provide a
steady current supply to the
load:
f is rectified signal’s
frequency (120Hz).
Transistor
• A three lead semiconductor device that acts as:
– an electrically controlled switch, or
– a current amplifier.
• Transistor is analogous to a faucet.
– Turning faucet’s control knob alters the flow rate of water coming out from the faucet.
– A small voltage/current applied at transistor’s control lead controls a larger current flow
through its other two leads.
Transistor Types: BJT, JFET, and MOSFET
• Bipolar Junction Transistor (BJT)
– NPN and PNP
• Junction Field Effect Transistor (JFET)
– N-channel and P-channel
• Metal Oxide Semiconductor FET (MOSFET)
– Depletion type (n- and p-channel) and enhancement type (n- and p-channel).
BJT Types
• NPN and PNP.
– NPN: a small input current and a positive voltage applied @ its base (with VB>VE
)
allows a large current to flow from collector to emitter.
– PNP: a small output current and a negative voltage @ its base (with VB<VE
) allows a
much larger current to flow from emitter to collector.
NPN BJT: How it works — I
• When no voltage is applied at
transistor’s base, electrons in the
emitter are prevented from passing
to the collector side because of the
pn junction.
• If a negative voltage is applied to
the base, things get even worse as
the pn junction between the base
and emitter becomes reverse-
biased resulting in the formation of
a depletion region that prevents
current flow.
NPN BJT: How it works — II
• If a positive voltage (>0.6V) is
applied to the base of an npn
transistor, the pn junction between
the base and emitter becomes
forward-biased. During forward
bias, escaping electrons are drawn
to the positive base.
• Some electrons exit through the
base, but because the p-type base
is so thin, the onslaught of
electrons that leave the emitter get
close enough to the collector side
that they begin jumping into the
collector. Increasing the base
voltage increases the emitter-to-
collector electron flow.
• Recall, positive current flow is in
the direction opposite to the
electron flow current flows from
collector to emitter.
JFET
• Junction field effect transistors like BJTs are three lead
semiconductor devices.
• JFETs are used as:
– electrically controlled switches,
– current amplifiers, and
– voltage-controlled resistors.
• Unlike BJTs, JFETs do not require a bias current and are controlled
by using only a voltage.
• JFETs are normally on when VG
JFET Types
• Two types of JFETs:
– n-channel and p-channel.
• In n-channel JFET, a –ve voltage applied @ its gate (with VG < VS
) reduces current
flow from drain to source. It operates with VD > VS
.
• In p-channel JFET, a +ve voltage applied @ its gate (with VG > VS
) reduces current
flow from source to drain. It operates with VS > VD
.
• JFETs have very high input impedance and draw little or no input current
– if there is any circuit/component connected to the gate of a JFET, no current is drawn
away from or sunk into this circuit.
MOSFET
• Metal oxide semiconductor FET.
• Similar to JFET.
• A metal oxide insulator is placed @ the gate to obtain a high input impedance @ the
gate
– gate input impedance approx. 1014Ω.
• Use of insulator as described above yields a low gate-to-channel capacitance.
– If too much static electricity builds up on the gate, then the MOSFET may be damaged.
MOSFET Types
• Enhancement type:
– Normally off, thus no current flows through drain-source channel when VG = VS
.
– When a voltage applied @ the gate causes VG
VS the drain-source channel reduces
resistance to current flow.
• Depletion type:
– Normally on, thus maximum current flows through drain-source channel when VG = VS
.
– When a voltage applied @ the gate causes VG
VS the drain-source channel increases
resistance to current flow.
Optoelectronics
Light emitting diodes Infrared detector
• In optoelectronics we deal with 2 types of electronic devices.
• Light emitting electronic devices: ones that generate electromagnetic energy under
the action of electrical field. Example: light emitting diodes (visible and infrared
light).
• Light detecting devices: ones that transform electromagnetic energy input into
electrical current/voltage. Examples: photoresistors, photodiodes, phototransistors,
etc.
LED 101— I
• 2 lead semiconductor device.
• Light emitting PN-junction diode.
– Visible or infrared light.
• Has polarity.
• Recall diodes act as a one way gate to current flow.
– A forward-biased PN-junction diode allows current flow from anode to cathode.
• An LED conducts and emits light when its anode is made more positive (approx.
1.4V) than its cathode.
– With reverse polarity, LED stops conducting and emitting light.
LED 101— II
• Similar to diodes, LEDs are current-dependent devices.
– LED brightness is controlled by controlling current through LED.
• Too little current through LED LED remains OFF.
• Small current through LED dimly lit LED.
• Large current through LED brightly lit LED.
• Too much current through LED LED is destroyed.
• A resistor placed in series with LED accomplishes current control.
LED 101—III
• Let Vs
be the supply voltage.
• Let Vf
be the required forward bias voltage for the
LED.
• Let I be the desired current flow through LED.
• Then, the current limiting resistance R is sized as
follow:
• If R is chosen smaller than the above value, a
larger current will flow through the LED.
– LEDs can handle only limited current (varies from
20mA to 100mA).
– If current through LED is larger than the maximum
allowed value, than the LED will be damaged.
Visible-Light LED
• Inexpensive and durable.
• Typical usage: as indicator lights.
• Common colors: green (~565nm), yellow (~585nm), orange (~615nm), and red
(~650nm).
• Maximum forward voltage: 1.8V.
• Typical operating currents: 1 to 3mA.
• Typical brightness levels: 1.0 to 3.0mcd/1mA to 3.0mcd /2mA.
• High-brightness LEDs exist.
– Used in high-brightness flashers (e.g., bicycle flashers).
Blinking LED
• Contain a miniature integrated circuit that causes LED to flash from 1 to 6 times/second.
• Typical usage: indicator flashers. May also be used as simple oscillators.
Tricolor LED
• Two LEDs placed in parallel facing opposite directions.
• One LED is red or orange, the other is green.
• Current flow in one direction turns one LED ON while the other remains OFF due
to reverse bias.
• Current flow in the other direction turns the first LED OFF and the second LED ON.
• Rapid switching of current flow direction will alternatively turn the two LEDs ON
giving yellow light.
• Used as a polarity indicator.
• Maximum voltage rating: 3V
• Operating range: 10 to 20mA
7-Segment LED Display
• Used for displaying numbers and other characters.
• 7 individual LEDs are used to make up the display.
• When a voltage is applied across one of the LEDs, a portion of the 8 lights up.
• Unlike liquid crystal displays (LCD), 7-segment LED displays tend to be more
rugged, but they also consume more power.
How LED Works
• The light-emitting section of an LED is made by joining n-type and p-type semiconductors
together to form a pn junction.
• When the pn junction is forward-biased, electrons in the n side are excited across the pn
junction and into the p side, where they combine with holes.
• As the electrons combine with the holes, photons are emitted.
• The pn-junction section of an LED is encased in an epoxy shell that is doped with light
scattering particles to diffuse light and make the LED appear brighter.
• Often a reflector placed beneath the semiconductor is used to direct the light upward.
Photoresistors —I
• Light sensitive variable resistors.
• Its resistance depends on the intensity of light incident upon it.
– Under dark condition, resistance is quite high (M: called dark resistance).
– Under bright condition, resistance is lowered (few hundred ).
• Response time:
– When a photoresistor is exposed to light, it takes a few milliseconds, before it
lowers its resistance.
– When a photoresistor experiences removal of light, it may take a few seconds
to return to its dark resistance.
Photoresistors —II
• Some photoresistors respond better to light that contains
photons within a particular wavelength of spectrum.
– Example: Cadmium-sulfide photoresistos respond to
light within 400-800nm range.
– Example: Lead-sulfide photoresistos respond to infrared light.
How Photoresistor Works
• Special semiconductor crystal, such as cadmium sulfide or lead sulfide is used to make
photoresistors.
• When this semiconductor is placed in dark, electrons within its structure resist flow through
the resistor because they are too strongly bound to the crystal’s atoms.
• When this semiconductor is illuminated, incoming photons of light collide with the bound
electrons, stripping them from the binding atom, thus creating holes in the process.
• Liberated electrons contribute to the current flowing through the device.
Photoresistor Application —Light Activated Relay
• Light-sensitive voltage divider is being
used to trip a relay whenever the light
intensity change.
• Light-activated circuit:
– When the photoresistor is exposed to
light, its resistance decreases.
– Transistor’s base current and
voltage increase and if the base current
and voltage are large enough, the
collector-emitter pair of the transistor
conducts triggering the relay.
• The value of R1
in the light-activated
circuit should be around 1 KΩ but
may have to be adjusted.
• Dark-activated relay works in a similar
but opposite manner.
• R1
in the dark-activated circuit
(100KΩ) may also have to be adjusted.
• A 6 to 9-V relay with a 500Ω coil can
be used in either circuit.
Photodiode
• Photodiode is a 2 lead semiconductor device that transforms light energy to electric
current.
• Suppose anode and cathode of a photodiode are wired to a current meter.
– When photodiode is placed in dark, the current meter displays zero current flow.
– When the photodiode is expose to light, it acts a a current source, causing current flow
from cathode to anode of photodiode through the current meter.
• Photodiodes have very linear light v/s current characteristics.
– Commonly used as light meters in cameras.
• Photodiodes often have built-in lenses and optical filters.
• Response time of a photodiode slows with increasing surface area.
• Photodiodes are more sensitive than photoresistor.
How Photodiode Works
• Photodiode: A thin n-type
semiconductor sandwiched with a
thicker p-type semiconductor.
• N-side is cathode, p-side is anode.
• Upon illumination, a # of photons pass
from the n-side and into the p-side of
photodiode.
– Some photons making it into p-side
collide with bound electrons within p-
semiconductor, ejecting them and
creating holes.
– If these collisions are close to the pn-
interface, the ejected electrons cross the
junction, yielding extra electrons on the
n-side and extra holes on the p-side.
– Segregation of +ve and -ve charges
leads to a potential difference across the
pn-junction.
– When a wire is connected between the
cathode and anode, a conventionally
positive current flow from the anode to
cathode.
Photodiode Applications—Photovoltaic Current Source
• Photodiode converts light energy directly into electric current that can be
measured with meter.
• The input intensity of light and the output current are nearly linear.
Solar Cell—I
Solar cells are photodiodes with very large surface areas.
• Compared to usual photodiodes, the large surface area in photodiode of a
solar cell yields
– a device that is more sensitive to incoming light.
– a device that yields more power (larger current/volts).
• Solar cells yield more power.
• A single solar cell may provide up to 0.5V that can supply 0.1A when
exposed to bright light.
Solar Cell Basic Operation—Power Sources
• Each solar cell produces an
open-circuit voltage from
around 0.45 to 0.5 V and may
generate as much as 0.1 A in
bright light.
• Similar to batteries, solar cells
can be combined in series or
parallel.
• Adding cells in series, yields
output voltage that is the sum
of the individual cell voltages.
• Adding solar cells in parallel,
yields an increased output
current vis-à-vis a single solar
cell.
Solar Cell Basic Operation—Battery Charger
• Nine solar cells placed in series can
be used to recharge two 1.5 V NiCd
cells.
• The diode is added to the circuit to
prevent the NiCd cells from
discharging through the solar cell
during times of darkness.
• It is important not to exceed the safe
charging rate of NiCd cells. To slow
the charge rate, a resistor can be
placed in series with the batteries.
Phototransistor
• Phototransistor is a light sensitive transistor.
• In one common type of phototransistor, the base lead of a BJT is replaced by a light
sensitive surface.
• When the light sensitive surface @ the base is kept in darkness, the collector-emitter
pair of the BJT does not conduct.
• When the light sensitive surface @ the base is exposed to light, a small amount of
current flows from the base to the emitter. The small base-emitter current controls
the larger collector-emitter current.
• Alternatively, one can also use a field-effect phototransistor (Photo FET).
• In a photo FET, the light exposure generates a gate voltage which controls a
drain-source current.
How Phototransistor Works
• The bipolar phototransistor resembles a
bipolar transistor that has extra large p-type
semiconductor region that is open for light
exposure.
• When photons from a light source collide
with electrons within the p-type
semiconductor, they gain enough energy to
jump across the pn-junction energy barrier-
provided the photons are of the right
frequency/energy.
• As electrons jump from the p-region into the
lower n-region, holes are created in the p-
type semiconductor.
• The extra electrons injected into the lower n-
type slab are drawn toward the positive
terminal of the battery, while electrons from
the negative terminal of the battery are draw
into the upper n-type semiconductor and
across the np junction, where they combine
with the holes, the net result is an electrons
current that flows from the emitter to the
collector.
Phototransistor Applications—Light Activated Relay
• A phototransistor is used to control the base current supplied to a power-switching
transistor that is used to supply current to a relay.
• When light comes in contact with the phototransistor, the phototransistor turns on,
allowing current to pass from the supply into the base of the power-switching
transistor.
• This allows the power-switching transistor to turns on, and current flows through the
relay, triggering it to switch states.
• The 100K pot is used to adjust the sensitivity of device by controlling current flow
through the phototransistor.
Phototransistor Applications—Light Activated Relay
• A phototransistor is used to control the base current supplied to a power-switching
transistor that is used to supply current to a relay.
• When light comes in contact with the phototransistor, the phototransistor turns on,
allowing current to pass from the supply into the base of the power-switching
transistor.
• This allows the power-switching transistor to turns on, and current flows through the
relay, triggering it to switch states.
• The 100K pot is used to adjust the sensitivity of device by controlling current flow
through the phototransistor.
Phototransistor Applications—Dark Activated Relay
• A phototransistor is used to control the base current supplied to a power-switching
transistor that is used to supply current to a relay.
• When light is removed from the phototransistor, the phototransistor turns off,
allowing more current to enter into the base of the power-switching transistor.
• This allows the power-switching transistor to turns on, and current flows through
the relay, triggering it to switch states.
• The 100K pot is used to adjust the sensitivity of device by controlling current flow
through the phototransistor.
Phototransistor Applications—Tachometer
• A phototransistor is being used as a frequency counter or tachometer.
• A rotating disk is connected to a rotating shaft. The rotating disk has one hole in it.
• For the given setup, the disk will allow light to pass through the hole once every
revolution.
• The light passing through the disk triggers the phototransistor into conduction.
• A frequency counter is used to count the number of electrical pulses generated.
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