Transistors

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Transistors
Three-terminal, solid-state electronic device
used for amplification and switching. The
transistor is an arrangement of semiconductor
materials that share common physical boundaries.
Materials most commonly used are silicon,
gallium-arsenide, and germanium, into which
impurities have been introduced by a process
called doping. In n-type semiconductors the
impurities or dopants result in an excess of
electrons, or negative charges in p-type
semiconductors the dopants lead to a deficiency
of electrons and therefore an excess of positive
charge carriers or "holes. The PNP and NPN
Junction Transistor The Field-Effect Transistor
The Metal-Oxide Semiconductor Field-Effect
Transistor (MOSFET)
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The First Transistor
The bipolar junction transistor was the first
solid-state amplifier element and started the
solid-state electronics revolution. Bardeen,
Brattain and Shockley at the Bell Laboratories
invented it in 1948 as part of a post-war effort
to replace vacuum tubes with solid-state devices
(Nobel Prize in 1956).
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Bipolar Junction Transistors
A Bipolar Transistor essentially consists of a
pair of PN Junction Diodes that are joined
back-to-back. This forms a sort of a sandwich
where one kind of semiconductor is placed in
between two others. There are therefore two kinds
of Bipolar sandwich, the NPN and PNP varieties.
The three layers of the sandwich are
conventionally called the Collector, Base, and
Emitter.
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Bipolar Junction Transistors
The direction of the emitter arrow defines the
type transistor. Biasing and power supply
polarity are positive for NPN and negative for
PNP transistors. The transistor is primarily used
as an current amplifier. When a small current
signal is applied to the base terminal, it is
amplified in the collector circuit.
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Figure 1 shows the energy levels in an NPN
transistor when we aren't externally applying any
voltages. In each of the N-type layers conduction
can take place by the free movement of electrons
in the conduction band. In the P-type (filling)
layer conduction can take place by the movement
of the free holes in the valence band. However,
in the absence of any externally applied electric
field, we find that depletion zones form at both
PN-Junctions, so no charge wants to move from one
layer to another.
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When we apply a moderate voltage between the
Collector and Base and the polarity of the
applied voltage is chosen to increase the force
pulling the N-type electrons and P-type holes
apart. (i.e. we make the Collector positive with
respect to the Base.) This widens the depletion
zone between the Collector and base and so no
current will flow. In effect we have
reverse-biased the Base-Collector diode junction.
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When we apply a relatively small Emitter-Base
voltage whose polarity is designed to
forward-bias the Emitter-Base junction. This
'pushes' electrons from the Emitter into the Base
region and sets up a current flow across the
Emitter-Base boundary.
Once the electrons have managed to get into the
Base region they can respond to the attractive
force from the positively-biased Collector
region. As a result the electrons which get into
the Base move swiftly towards the Collector and
cross into the Collector region. Hence we see a
Emitter-Collector current whose magnitude is set
by the chosen Emitter-Base voltage we have
applied.
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Characteristic curves from a working Bipolar
Transistor
Each curve shows how the collector current, IC,
varies with the Collector-Emitter voltage, VCE,
for a specific fixed value of the Base current,
IB.
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FET (Field Effect Transistor)
The field-effect transistor (FET) controls the
current between two points but does so
differently than the bipolar transistor.  The FET
operates by the effects of an electric field on
the flow of electrons through a single type of
semiconductor material. There are two basic types
of FET.  The J-FET (Junction Field Effect
Transistor ) and the MOS-FET (Metal-Oxide-Semicond
uctor FET) are voltage controlled devices that
is a small change in input voltage causes a large
change in output current. FET operation
involves an electric field which controls the
flow of a charge (current ) through the device.
In contrast, a bipolar transistor employs a small
input current to control a large output current.
The source, drain, and gate terminal of the FET
are analogous to the emitter, collector,  and
base of a bipolar transistor . The terms
n-channel and p- channel refer to the material
which the drain and source are connected.
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J-FET (Junction Field Effect Transistor)
In the junction FET (JFET), the gate material is
made of the opposite polarity semiconductor to
the channel material (for a P-channel FET the
gate is made of N-type semiconductor material). 
Current is high if the junction is forward
biased and is extremely small when the junction
is reverse biased. 
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JFET
In an n-channel device, the channel is made of
n-type semiconductor, so the charges free to move
along the channel are negatively charged - they
are electrons. In a p-channel device the free
charges which move from end-to-end are positively
(hence p) charged - they are holes. In each case
the source puts fresh charges into the channel
while the drain removes them at the other end.
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JFET
Placing an insulating layer between the gate and
the channel allows for a wider range of control
(gate) voltages and further decreases the gate
current (and thus increases the device input
resistance) MOSFET
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Metal Oxide Semiconductor Field Effect Transistor
(MOSFET)
The insulator is typically made of an oxide (such
as silicon dioxide, SiO2), This type of device is
called a metal-oxide-semiconductor FET (MOSFET).
The general structure is a lightly doped p-type
substrate, into which two regions, the source and
the drain, both of heavily doped n-type
semiconductor have been embedded. The symbol n
is used to denote this heavy doping.
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MOSFET
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The p-type doped substrate is only very lightly
doped, and so it has a very high electrical
resistance, and current cannot pass between the
source and drain if there is zero voltage on the
gate. Application of a positive potential to
the gate electrode creates a strong electric
field across the p-type material even for
relatively small voltages, as the device
thickness is very small and the field strength is
given by the potential difference divided by the
separation of the gate and body electrodes.
Since the gate electrode is positively charged,
it will therefore repel the holes in the p-type
region. For high enough electrical fields, the
resulting deformation of the energy bands will
cause the bands of the p-type region to curve up
so much that electrons will begin to populate the
conduction band. The population of the p-type
substrate conduction bands in the region near to
the oxide layer creates a conducting channel
between the source and drain electrodes,
permitting a current to pass through the device.
The population of the conduction band begins
above a critical voltage, VT, below which there
is no conducting channel and no current flows. In
this way the MOSFET may be used as a switch.
Above the critical voltage, the gate voltage
modulates the flow of current between source and
drain, and may be used for signal amplification.
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The bias voltage on the gate terminal either
attracts or repels the majority carriers of the
substrate across the PN junction with the
channel.  This narrows (depletes) or widens
(enhances) the channel, respectively, as VGS
changes polarity.  For N-channel MOSFETs,
positive gate voltages with respect to the
substrate and the source (VGS gt 0) repel holes
from the channel into the substrate, thereby
widening the channel and decreasing channel
resistance.  Conversely, VGS lt 0 causes holes to
be attracted from the substrate, narrowing the
channel and increasing the channel resistance.
If we apply a positive voltage to the gate we'll
set up an electrostatic field between it and the
rest of the transistor. The positive gate voltage
will push away the holes inside the p-type
substrate and attracts the moveable electrons in
the n-type regions under the source drain
electrodes. This produces a layer just under the
gate's insulator through which electrons can get
into and move along from source to drain. The
positive gate voltage therefore creates a
channel in the top layer of material. Increasing
the value of the positive gate voltage pushes the
p-type holes further away and enlarges the
thickness of the created channel. As a result we
find that the size of the channel we've made
increases with the size of the gate voltage and
enhances or increases the amount of current which
can go from source to drain