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Module Details
Module Title:
MICROELECTRONICS
Module Coordinator:
Anthony O
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It is therefore appropriate that we begin the discussion by examining the general
nature of semiconducting materials
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This bonding is called covalent
bonding where each atoms share each other’s electrons to one another
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Charge Carriers – as a review on the previous electronics course, semiconductors have two charge carries,
the electrons and the holes
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Electrons are negative and holes (absence of electron) is positive
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If a force/electromotive force is present, the free electrons will flow from high to low
potential, thus creating an electric current
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But if the energy level will drop, these free electrons will return to their parent atoms
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This
process is called recombination
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Doping – it is the process of adding impurities to a pure (intrinsic) semiconductor material to change its
resistance characteristics
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The
doped semiconductor material is now called an extrinsic material
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Two types of materials can
be created in the doping process, the p-type (positive) and n-type (negative)
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N-type can be created by adding
pentavalent elements such as Arsenic and Antimony
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What are the two types of a semiconductor charge carriers?
2
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What do you call the process of adding impurity atoms to a pure semiconductor material to
change its resistance characteristics?
4
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5
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It is composed of two extrinsic
semiconductor materials (p-type and n-type) joined together to form a single junction (as where the name
pn junction came from)
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P-type having holes as its
majority carrier and n-type as electrons
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These region will grow in width but since it is an

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insulator, a maximum width of these barrier will established
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Carrier Transport Under Applied Bias
We considered the changes in the electrostatic potential as we went around a circuit through a shortcircuited abrupt p-n diode and found that although there were steps up and down, the net change in potential
was zero
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It is not difficult to accept that there is little additional charge associated
with such a small current
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Thus, diode
is a device used to allow a current to flow in one direction only, which covers a very important applications
in the field of electronics
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Zero Bias – No external voltage potential is applied to the PN junction diode
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Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and
positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN
junction diode’s width
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Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and
negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN
junction diodes’s width
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However if the diodes terminals are shorted together, a few holes (majority carriers) in the Ptype material with enough energy to overcome the potential barrier will move across the junction against
this barrier potential
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This is known as the “Reverse Current” and is
referenced as IR
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The potential barrier that now exists discourages the diffusion of any more majority carriers across
the junction
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Then an “Equilibrium” or balance will be established when the majority carriers are equal and both
moving in opposite directions, so that the net result is zero current flowing in the circuit
...

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The minority carriers are constantly generated due to thermal energy so this state of equilibrium
can be broken by raising the temperature of the PN junction causing an increase in the generation of
minority carriers, thereby resulting in an increase in leakage current but an electric current cannot flow
since no circuit has been connected to the PN junction
Reverse Biased PN Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type
material and a negative voltage is applied to the P-type material
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The net result is that the depletion layer grows wider due to a lack of electrons and holes and
presents a high impedance path, almost an insulator
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Increase in the Depletion Layer due to Reverse Bias

This condition represents a high resistance value to the PN junction and practically zero current flows
through the junction diode with an increase in bias voltage
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One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough
value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect around the
junction
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Reverse Characteristics Curve for a Junction Diode

Sometimes this avalanche effect has practical applications in voltage stabilizing circuits where a
series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum
value thereby producing a fixed voltage output across the diode
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3

Forward Biased PN Junction Diode
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the Ntype material and a positive voltage is applied to the P-type material
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0
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3 volts for germanium, the potential
barriers opposition will be overcome and current will start to flow
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This results in a characteristics curve of zero current flowing up to this
voltage point, called the “knee” on the static curves and then a high current flow through the diode with
little increase in the external voltage as shown below
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The point at which this sudden increase in current takes place is represented
on the static I-V characteristics curve above as the “knee” point
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The actual potential difference
across the junction or diode is kept constant by the action of the depletion layer at approximately 0
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7v for silicon junction diodes
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Exceeding its
maximum forward current specification causes the device to dissipate more power in the form of heat than
it was designed for resulting in a very quick failure of the device
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Within a doped semiconductor material, there will be:
a
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b
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c
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d
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2
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the diode is in its forward conducting state
b
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there is no potential difference between the anode and cathode
3
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a
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Peak voltage
b
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Peak inverse voltage
4
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a negative voltage source connected to N-material
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a positive voltage source connected to N-material
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a negative voltage source connected to P-material
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all of these
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A PN-junction diode is primarily:
a
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c
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b
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d
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6
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maximum forward current rating
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maximum temperature rating
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maximum reverse voltage rating
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none of these
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The output frequency of a full-wave rectifier is ________ the input frequency
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one-half
b
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twice
d
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Give three reasons why Ge and Si have received the most attention in production of
semiconductor devices
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Determine the total discharge time for the capacitor in a clamper having C = 0
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10
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Each layer forming the transistor has a specific name, and each layer is
provided with a wire contact for connection to a circuit
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BJT transistor: (a) PNP schematic symbol, (b) physical layout (c) NPN symbol, (d) layout
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For any given state of operation, the current directions and voltage
polarities for each kind of transistor are exactly opposite each other
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In other words, transistors restrict the
amount of current passed according to a smaller, controlling current
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The small current that controls the main current goes from base to emitter,
or from emitter to base, once again depending on the kind of transistor it is (PNP or NPN, respectively)
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Small Base-Emitter current controls large Collector-Emitter current flowing against emitter arrow
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In other words, two types of charge carriers -- electrons and holes -- comprise this main current
through the transistor
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This is the
first and foremost rule in the use of transistors: all currents must be going in the proper directions for the
device to work as a current regulator
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Conversely, the large,
controlled current is referred to as the collector current because it is the only current that goes through the
collector wire
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No current through the base of the transistor, shuts it off like an open switch and prevents current
through the collector
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Collector current is primarily limited by the base current, regardless
of the amount of voltage available to push it
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Structure and principle of operation
A bipolar junction transistor consists of two back-to-back p-n junctions, who share a thin common
region with width, wB
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The structure of an npn bipolar
transistor is shown in figures below (a)
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(a) Structure and sign convention of an npn bipolar junction transistor
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Since the device consists of two back-to-back diodes, there are depletion regions between the quasi-neutral
regions
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2
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2
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2
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2
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2
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2
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2
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2
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2
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The base and collector current are positive
if a positive current goes into the base or collector contact
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This also implies that the emitter current, IE, equals the sum of the base
current, IB, and the collector current, IC:
(5
...
10)
The base-emitter voltage and the base-collector voltage are positive if a positive voltage is applied to the
base contact relative to the emitter and collector respectively
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We consider here only the forward active bias mode of operation,
obtained by forward biasing the base-emitter junction and reverse biasing the base-collector junction
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The corresponding energy band diagram is shown
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This carrier
diffusion is identical to that in a p-n junction
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Once the electrons arrive at the base-collector depletion
region, they are swept through the depletion layer due to the electric field
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In addition, there are two more currents, the base recombination current, indicated by the
vertical arrow, and the base-emitter depletion layer recombination current, Ir,d, (not shown)
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The total emitter current is the sum of the electron diffusion current, IE,n, the hole diffusion current, IE,p and
the base-emitter depletion layer recombination current, Ir,d
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2
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(5
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12)
The base current is the sum of the hole diffusion current, IE,p, the base recombination current, Ir,B and the
base-emitter depletion layer recombination current, Ir,d
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2
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2
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The current gain, , is defined as the ratio of the
collector and base current and equals:
(5
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15)
This explains how a bipolar junction transistor can provide current amplification
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The current gain, , can therefore
become much larger than one
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(5
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16)
The emitter efficiency, E, is defined as the ratio of the electron current in the emitter, IE,n, to the sum of
the electron and hole current diffusing across the base-emitter junction, IE,n + IE,p
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2
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(5
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18)
Recombination in the depletion-region of the base-emitter junction further reduces the current gain, as it
increases the emitter current without increasing the collector current
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When transistors are used in digital circuits they usually operate in the:
a
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breakdown region
c
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linear region
2
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gif of 250 and a base current, IB, of 20 mu
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The collector
current, IC, equals:
a
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5 mA
c
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5 A
3
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Beta
b
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alpha
d
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The C-B configuration is used to provide which type of gain?
a
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current
c
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power
5
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30 µA
b
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3 mA
d
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This ultra-thin insulated metal gate electrode can be thought of as one plate of a capacitor
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As the Gate terminal is isolated from the main current carrying channel “NO current flows into the gate” and
just like the JFET, the MOSFET also acts like a voltage controlled resistor were the current flowing through
the main channel between the Drain and Source is proportional to the input voltage
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Like the previous JFET tutorial, MOSFETs are three terminal devices with a Gate, Drain and Source and
both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available
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Depletion Type – the transistor requires the Gate-Source voltage, (VGS) to switch the device
“OFF”
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 2
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The enhancement mode MOSFET is equivalent to a “Normally Open” switch
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10

The four MOSFET symbols above show an additional terminal called the Substrate and is not normally
used as either an input or an output connection but instead it is used for grounding the substrate
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Usually in discrete type MOSFETs, this substrate lead is connected internally to the source terminal
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The line between the drain and source connections represents the semiconductive channel
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The direction of the arrow
indicates either a P-channel or an N-channel device
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Both
the Depletion and Enhancement type MOSFETs use an electrical field produced by a gate voltage to alter
the flow of charge carriers, electrons for N-channel or holes for P-channel, through the semiconductive
drain-source channel
...


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We saw in the previous tutorial, that the gate of a junction field effect transistor, JFET must be biased in
such a way as to reverse-bias the PN-junction
...

This makes the MOSFET device especially valuable as electronic switches or to make logic gates because
with no bias they are normally non-conducting and this high gate input resistance means that very little or
no control current is needed as MOSFETs are voltage controlled devices
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Depletion-mode MOSFET
The Depletion-mode MOSFET, which is less common than the enhancement types is normally switched
“ON” without the application of a gate bias voltage making it a “normally-closed” device
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Similar to the JFET types
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In other words, for an N-channel depletion mode MOSFET: +VGS means more electrons and more current
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The opposite is also true for the P-channel types
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Depletion-mode N-Channel MOSFET and circuit Symbols

The depletion-mode MOSFET is constructed in a similar way to their JFET transistor counterparts were
the drain-source channel is inherently conductive with the electrons and holes already present within the
N-type or P-type channel
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Enhancement-mode MOSFET
The more common Enhancement-mode MOSFET is the reverse of the depletion-mode type
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This results in the device
being normally “OFF” when the gate bias voltage is equal to zero
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This
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positive +ve gate voltage pushes away the holes within the channel attracting electrons towards the oxide
layer and thereby increasing the thickness of the channel allowing current to flow
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Increasing this positive gate voltage will cause the channel resistance to decrease further causing an increase
in the drain current, ID through the channel
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Then, the
enhancement-mode MOSFET is equivalent to a “normally-open” switch
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Enhancement-mode
MOSFETs are used in integrated circuits to produce CMOS type Logic Gates and power switching circuits
in the form of as PMOS (P-channel) and NMOS (N-channel) gates
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The MOSFET Amplifier
Just like the previous Junction Field Effect transistor, MOSFETs can be used to make single stage class
“A” amplifier circuits with the Enhancement mode N-channel MOSFET common source amplifier being
the most popular circuit
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This high input impedance is controlled by the gate biasing resistive network formed by R1 and R2
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When VG is high the transistor is
switched “ON” and VD (Vout) is low as shown
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The MOSFET circuit is biased in class A mode by the voltage divider network formed by
resistors R1 and R2
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Metal Oxide Semiconductor Field Effect Transistors are three terminal active devices made from different
semiconductor materials that can act as either an insulator or a conductor by the application of a small signal
voltage
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Then MOSFETs have the ability
to operate within three different regions:




1
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Linear (Ohmic) Region – with VGS > Vthreshold and VDS > VGS the transistor is in its constant
resistance region and acts like a variable resistor whose value is determined by the gate voltage, VGS
3
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The current IDS = maximum as the transistor acts as a closed circuit
Quiz
1
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As VGS decreases ID decreases
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As VGS increases ID remains constant
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As VGS increases ID increases
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As VGS decreases ID remains constant
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When an input signal reduces the channel size, the process is called:
a
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substrate connecting
b
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depletion
3
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zero biasing
c
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self biasing
d
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Calculate the value of RD
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Calculate the value of VDS
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Typical uses of the operational amplifier are to provide voltage
amplitude changes (amplitude and polarity), oscillators, timer circuits, filter circuits, comparator and many
types of instrumentation circuits
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A basic op-amp has two inputs and one output as would result in using differential amplifier input
stage
...

1
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The input is applied to the plus input (with minus input connected to the
ground), which results in an output with having the same polarity as the applied input signal
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Double-ended (Differential) Input – in addition to using only one input, it is possible to apply
signals at each input
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3
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An input signal applied to either input will result in outputs from both output terminal,
these outputs always being opposite in polarity
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The output is twice as large as
either of the two output voltage value because they are of opposite polarity and subtracting those
results in twice their amplitude
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Common Mode Operation – when the same input signal is applied to both inputs, common-mode
operation results, as shown in the figure below
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Practically, a small signal will result
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Common-Mode Rejection – a significant feature of a differential connection is that the signals
that are opposite at the inputs are highly amplified, whereas those that are common to the two inputs
are only slightly amplified - the overall operation being to amplify the difference signal while
rejecting the common signal at the two inputs
...

Differential Amplifier Circuit
The differential amplifier circuit is extremely popular connection used in IC units, this connection
can be describe by considering the basic differential amplifier shown below
...
Whereas most differential
amplifier circuits use two separate voltage supplies, the circuit can also operate using a single supply
...
”
If the same input is applied to both inputs, the operation is referred to as “common-mode
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An op-amp provides an output component that is due to amplification of
the difference of the signals applied to the plus and minus inputs and a component due to signals common
to both inputs
...

Differential Inputs
When separate inputs are applied to the op-amp, the resulting difference signal is the difference
between the two inputs
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𝑉𝑐 =

1
(𝑉 − 𝑉𝑖2 )
2 𝑖1

Output Voltage
Since any signals applied to an op-amp in general have both in-phase and out-of-phase components,
the resulting output can be expressed as
𝑉𝑜 = 𝐴 𝑑 𝑉 𝑑 + 𝐴 𝑐 𝑉𝑐
Where: Ad = differential gain
Vd = differential inputs
Ac = common-mode gain
Vc = common inputs
Common-Mode Rejection Ratio
Having obtained Ad and Ac, the CMRR can be calculated using the formula below
𝑪𝑴𝑹𝑹 =

𝑨𝒅
𝑨𝒄

The value of CMRR can also be expressed in logarithmic terms as:
𝐶𝑀𝑅𝑅 𝑑𝐵 = 20 log10

𝑨𝒅
𝑨𝒄

Quiz #5
1
...

2
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3
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4
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5
...
3 V
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The basic circuit is made using a difference
amplifier having two inputs
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For a basic op-amp circuit with one input resistor and
a feedback resistor, the configuration is a constant gain amplifier
...

𝑅𝑓
𝑉𝑜
= −
𝑉𝑖
𝑅1

Unity Gain
If Rf = R1, the gain is
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑔𝑎𝑖𝑛 = −

𝑅𝑓
= −1
𝑅1

so that the circuit provides a unity voltage gain with 180° phase shift
...

Practical Op-amp Circuits
The op-amp can be connected in a large number of circuits to provide various operating characteristics
...

1
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The output is obtain by multiplying the input by a fixed or constant gain,
set by the input resistor R1 and the feedback resistor Rf – this output also being inverted from the
input
...
Noninverting Amplifier – the figure below show the connection of a noninverting amplifier circuit
or constant-gain multiplier
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The voltage gain of the circuit can be determine by the
formula below
𝑽𝒐
𝑽𝒊

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=

𝑹𝟏
𝑹 𝟏+ 𝑹 𝒇

= 𝟏+

𝑹𝒇
𝑹𝟏

𝑽 𝒐 = (𝟏 +

𝑹𝒇
⁄𝑹 ) 𝑽𝒊
𝟏

Noninverting constant-gain amplifier
3
...
From the equivalent
circuit, it is clear that the output voltage is equal to the input and that the output is the same polarity or
phase as the input
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4
...
The circuit shows a three-input summing amplifier circuit, which provides a means of
algebraically summing (adding) three voltages, each multiplied by the constant-gain factor
...
If more inputs are used, they each add an additional component to the output
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Integrator – if the feedback component used is capacitor, the resulting connection is called an
integrator
...


𝑉𝑜 (𝑡) = −

1
∫ 𝑉1 (𝑡)𝑑𝑡
𝑅𝐶

The integration operation is one of summation, summing the area under the waveform or curve over a
period of time
...
Formula shows that the output voltage ramp is opposite in
polarity to the input voltage and is multiplied by the factor 1/RC
...


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6
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While not as useful as the other
circuit forms covered, the differentiator thus provide a useful operation, the resulting relationship for
the circuit being
𝑣 𝑜 (𝑡) = −𝑅𝐶

𝑑𝑣1 (𝑡)
𝑑𝑡

Where the scale factor is –RC

Differentiator circuit
Quiz # 6
1
...
5 V
...
What voltage value must be applied to the input of an op-amp non-inverting amplifier
circuit shown below to result in an output of 2
...
The op-amp is available
in number of packages, an 8-pin DIP and a 10-pin flatpack being among the more usual forms
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TABLE 1
Absolute Maximum Rating
Supply Voltage
± 22 V
Internal power dissipation
500 mW
Differential input voltage
± 30 V
Input voltage
±15 V
Electrical Characteristics
The manufacturer provides some combination of typical, minimum, or maximum values for
various parameters as deemed most useful
...
Input offset voltage (VIO): the input offset voltage is seen to be typically 1 mV, but can go as high
as 6 mV
...
If the worst condition
possible is of interest, the maximum value should be used
...

2
...

3
...

4
...

5
...
Depending on the circuit closed-loop gain, the input signal should
be limited to keep the output from varying by an amount larger than ±12 V
...
Large signal voltage amplification (AVD): this is the open-loop voltage gain of the op-amp
...
\
7
...
In a closed-loop circuit, this input
impedance can be much larger
...
Output resistance (ro): the op-amp output resistance is listed as typically 75 Ω
...
In closed-loop circuit, the output impedance can be
lower depending on the gain
...
Input capacitance (C1): for high frequency considerations, it is helpful to know that the input to
the op-amp has typically 1
...
An even small value compared to the stray wiring
...
Supply current (ICC): the op-amp draws a total of 2
...
7 mA
...

11
...
Since 90 dB is equivalent to 31,622
...

12
...

Reading List
[1] Jaeger & Blalock, Microelectronic Circuit Design (4th edition), McGraw Hill, 2010
...
ISBN 0201543931
[3] http://www
...
gatech
...
php?prmCourse=ECE3040
[4] http://en
...
org/wiki/Operational_Amplifier

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