Download Semiconductor devices Electrons and Holes Intrinsic conduction

Electrons and Holes
Semiconductor devices
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In a metal wire electric current consists of a flow of
electrons from the more negative end to the more
positive end.
In semiconductors such as a silicon, electric current
appears to be carried not only by electrons but also by
HOLES.
Holes are positive charges created by missing
electrons in the covalent bond of the semiconductor
crystal. Holes behave like mobile positive charges.
Thus a the current consists of electrons moving from
negative to positive, and a different quantity of
positively charged holes moving from positive to
negative.
Intrinsic conduction
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Pure silicon has equal number of holes and electrons that
can contribute to conduction.
The relative proportion of the holes and electrons can be
changed by doping the silicon crystal with certain impurity
atoms.
When a battery is connected across a pure silicon, it
attracts free electrons towards the positive terminal and
the negative terminal supplies free electrons.
The free electron travel by hopping from one hole to
another. Since the freedom of the electrons is limited in
the covalent bonded crystal, the current in pure silicon is
very small.
Such stream of holes and electrons in opposite direction
constitutes an intrinsic conduction because the charges
carriers come from inside the crystal.
If the temperature of the semiconductor rises, more bonds
break and the intrinsic conductivity increases because
more free electrons and holes are produced. The
resistance of semiconductors thus decreases as
temperature rises, unlike conductors.
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n-type semiconductor
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This type is made by doping silicon with for example phosphorus
atoms.
Phosphorus has five valence electrons. When a phosphorus atom is
introduced to a silicon crystal, four of its valence electrons form covalent
bonds with four neighbouring silicon atoms. The fifth one is spared and can
take part in conduction. The impurity (phosphorus) is called donor as it gives
an electron for conduction. The impure silicon thus produced is called n-type
since the majority charge carriers are negative electrons.
The use of semiconductors in devises such as
diodes, transistors and integrated circuits
depends on increasing the conductivity of of
very pure (intrinsic) semiconductor material by
adding a small but controlled amount of
impurities. The semiconductor produced is
called extrinsic because the impurity supplies
the charge carriers. There are two kinds of
extrinsic semiconductors –n-type and p-type
semiconductors.
p-type semiconductor
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Here silicon or germanium is doped with for example boron which has three valence
electrons. When boron is introduced into a silicon crystal,its three valence electrons each
share an electron with three of the four silicon atoms surrounding it. One bond is
incomplete and the position of the missing electron (I.e. the HOLE) behaves like a positive
charge since it can attract another electron from a nearby silicon atom. Hence the impurity
(boron) atom is called acceptor and the impure silicon produced is called p-type where the
majority carries causing conduction are positive holes. In a p-type semiconductor,
electrons are minority carriers where as in an n-type semiconductor, holes are minority
carriers.
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Prepared by Dr Yonas M Gebremichael, 2005
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Extrinsic semiconductors
The structure of a pn junction
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The structure of a pn junction
A pn junction is a junction within the same crystal, between ptype silicon and n-type silicon –doping changes from primarily
acceptors (p-type) to primarily donors (n-type) at a plane within
the crystal.
The usual way to produce a pn junction is a diffusion process in
which a film of silicon dioxide is grown over the surface of silicon
by heating the silicon in an atmosphere of oxygen.
Then a window is etched in the oxide layer. The silicon is then
heated to over 1000C in an atmosphere of the dopant, the donor
or acceptor atoms diffuse into the silicon through this window in
the oxide. The density of the dopant atoms decreases with
distance from the surface, both depth and density can be
controlled by controlling the time and the temperature of the
process.
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Unbiased pn junction
Unbiased pn junction
A pn junction is shown below.
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As soon as the junction is made, free electrons near the junction in the
n-type material diffuse across the junction to the p-type material where
they fill the holes. As a result the n-type material near the junction
becomes positively charged. Similarly at the same time holes diffuse
from p-type material.
Reverse biased pn junction
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The diffusion soon stops because the negative charge on the p-type
material opposes further flow of electrons and the positive charges on
the n-type material stops further flow of holes. The region on either side
of the junction becomes fairly free of majority carriers and is called the
depletion (barrier) layer.
The situation is like a battery was placed across the junction of value
0.1V in germanium and 0.7V in silicon, called the junction voltage acting
from n to p.
If a battery is connected across a pn junction with its positive terminal joined
to the n-type side and the negative terminal to the p-type side, is adds to the
junction voltage.
Electrons and holes are repelled farther from the junction and the depletion
layer widens.
Only a few minority carriers (thermally generated) cross the junction and a
tiny current called leakage or reverse current flows. The resistance of the
junction is increased
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Forward biased pn junction
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If a battery is connected to oppose the junction voltage, with its positive
terminal to the p-type and its negative terminal to the n-type side, appreciable
current flows when the battery voltage overcomes the depletion voltage,
because majority carriers are able to cross the junction –electrons from the n
to the p-side and the holes in the opposite direction.
The junction is thus forward biased, a small current also flows because of
minority carriers that adds to the main majority carriers. The resistance of the
junction is very low.
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Prepared by Dr Yonas M Gebremichael, 2005
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If the starting material is n-type, and the diffused
material is acceptor such as boron, the surface is
converted to p-type since on the surface the acceptor
density exceeds the original donor density. But deeper
down, the donors still dominate, so a boundary is
formed between the p-region and the n-region which
the dopant changes from from donors to acceptors.
This region is called the transition region
Diode characteristics
The junction diode characteristics
A junction diode consists of a pn junction with one
connection to the p-side (anode) and another to the nside (the cathode)
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The characteristic curves show that the forward current is
very small until the forward voltage is 0.7V (silicon) and
0.1V(germanium), there after a small rise in the forward
voltage causes a large increase in the forward current.
The reverse currents are negligible and remain so as the
reverse voltage increases. However if the reverse voltage
in increased sufficiently, the insulation of the depletion
layer breaks down and the reverse current increased
suddenly and rapidly to cause permanent damage to the
diode. The reverse voltage lies from few volts up to 1000V.
Diode characteristics
Diodes as rectifiers
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Half-wave rectifier
Junction diodes are used in rectifier circuits such as those used in
power supplies to convert ac to dc.
Half-wave rectifier: - is the simplest form of rectifier circuit. As
shown in the circuit, for an ac input from the transformer:As the sinusoidal input voltage increases positively from zero
each cycle, the diode becomes forward biased. An appreciable
current will flow whenever the instantaneous value of the input
voltage is greater than ~0.7V.
However, when the input voltage goes negative, the diode is
reversed biased and only the minute reverse saturation current
flows. The diode behaves like an open circuit when the input
voltage drops below 0.7V.
The output voltage will therefore consist of positive half-cycles of
a sinusoid, as shown -provided that the input sine wave has an
amplitude much greater than the forward drop across the diode.
Capacitor Smoothing with Half-Wave Circuits
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Consider first a rectifier feeding into a capacitor, as shown in the Figure
below. Suppose that the capacitor is initially uncharged. During a negative
half-cycle of the supply voltage, the rectifier will be reverse biased and so
no current can flow in the circuit. During a positive half-cycle, however, the
rectifier becomes forward biased and current can now flow.
The rectifier acts effectively as a short circuit,
and so the voltage across the capacitor will
follow the supply voltage until it reaches its
peak. However, when the supply voltage falls
from its positive peak, the capacitor voltage is
unable to follow it. To do so, it would have to
discharge through the rectifier which is
prevented since (virtually) no current can flow
in the reverse direction. The capacitor will thus
remain charged up to the peak of the supply
voltage. Such a circuit is known as a peak
detector.
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Prepared by Dr Yonas M Gebremichael, 2005
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Capacitive smoothing
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Half-wave rectifier
As we have seen earlier, the waveforms produced by the rectifier
circuit alone consist of half sine waves and are unsuitable as a
form of dc supply.
Capacitive smoothing is used to convert the half sine wave
amplitude undulations into no more than a ripple.
A simplest dc power supply consists of a transformer, a half wave
rectifier and a capacitor as shown in the figure.
Current only flows through the diode when it is forward biased,
I.e when the transformer output is 0.7V more positive than the
voltage across the capacitor and the load.
During the positive half cycle, the current flows and charges up
the capacitor up to the peak value of the transformer output
minus the diode drop.
During the negative half cycle, the diode is reverse biased, the
capacitor discharges through the load. The value of the capacitor
must be sufficiently large so that it only lose a small fraction of
its charge through the load between each cycle.
Capacitive smoothing action
Neglecting any losses in the rectifier, on the positive
half-cycles of the input voltage, the capacitor will
charge up to the peak of the supply voltage as in the
previous circuit. However, the input voltage falls, the
diode becomes reversed biased and the capacitor
discharge into the load resistor, to keep the output
voltage relatively constant over the whole cycle. The
rectifier will only conduct when its anode is positive
with respect to its cathode, which as is shown, only
occurs for a small of part of the cycle. During the rest
of the cycle, the load current is being supplied by the
capacitor.
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Prepared by Dr Yonas M Gebremichael, 2005
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