Semi Conductors
Diodes
P-N Junction Diodes & Other Diodes
Transistor
Gates
Semiconductors are a unique class of materials in which the number of charge carriers,
the particles that move electric charge, is
intermediate between those of
insulators and those of
conductors. A
Conductor is a material that contains a large number of
free electrons that are able to
move freely when a voltage is applied. An
Insulator is a material that has very
few free electrons
and hence is a
poor conductor of electricity.
Two of the most common
pure semiconductor materials are
Germanium and
Silicon.
Each of these materials has
atoms with
four valence electrons that are shared with the valence electrons
of its four nearest neighbors in the solid state.
Types of Semi Conductor
There are
two main types of semiconductors:
Intrinsic & Extrinsic Semiconductors
For pure semiconductors as the
temperature is
increased the
resistance decreases and so
raising the temperature
increases the
conducting power of these materials. These types of
semiconductor materials are called
Intrinsic semiconductors.
The
conducting power of intrinsic semiconductors is greatly
influenced by
temperature and sometimes
light making them particularly interesting.
The other type of semiconductor is one in which the charge carriers are created by an impurity.
These types of semiconductors are known as
Extrinsic semiconductors. The addition of impurities,
such as arsenic or indium, has a significant influence on the conductive properties of semiconductors.
The
addition of such
impurities is known as
Doping.
Doped Semiconductor
A semiconductor crystal is called
n-type if the addition of an impurity element results in a
large number of
free electrons (negative charge carriers) available for conduction. Each
impurity atom is called a
donor atom since it donates an electron. The electron is free to
move and can contribute to an electric current. The positive ion left behind is fixed and cannot
take part in conduction.
A semiconductor crystal can be made
p-type by doping it with a different element so that there are a
large number of positive charge carriers available for conduction. The positive charge carriers
actually correspond to vacancies or deficiencies of electrons in the bonds holding the atoms in the
crystal lattice. The positive charges are called
holes.
Current flow in Semiconductors
An electric current can flow through a semiconductor as a result of the
movement of holes
and/or free electrons. There are
two important
processes that account for current flow in
semiconductors. These processes are called
Drift and
Diffusion.
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Diodes
Diodes that are composed of a
combination of semiconductor materials, with
p-regions
and
n-regions, are good conductors when a voltage is applied so that a current is driven
in one direction but a poor conductor if the voltage is reversed.
V-I Characteristics of Diodes
Forward Characteristics of Diodes
This results in the
depletion layer becoming
very thin and narrow and which now
represents a
low impedance path thereby producing a very
small potential barrier and
allowing
high currents to flow. The point at which this takes place is represented on the
static V-I characteristics curve above as the
"knee" point.
Reverse Characteristics of Diodes
This results in a
wide depletion layer.
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Imagine that a
p-type block of silicon can be placed in perfect contact with an
n-type block.
Free electrons from the
n-type region will diffuse across the
junction to the
p-type side
where they will recombine with some of the many holes in the
p-type material. Similarly,
holes will
diffuse across the junction in the opposite direction and recombine.
The
recombination of free electrons and holes in the vicinity of the junction
leaves a
narrow region on either side of the junction that contains no mobile charge.
This narrow region which has been depleted of mobile charge is called the
Depletion layer.
It
extends into both the
p-type and n-type regions.
Biasing in P-N Junction Diode
There are
3 possible
"biasing" conditions for the standard Junction Diode and these are:
Zero Biasing
When a diode is connected in a Zero Bias condition,
no external potential energy is applied to the PN-junction.
Forward Biasing
If a battery is connected to such a device so that the
positive end is
connected to the
p side of the device and the
negative end is
connected to the
n side of the device,
the situation is referred to as a
Forward-biased junction. The charges across the p-n
junction with ease, since there is an accelerating field setup, with electrons driven
towards the positive end of the battery and positive holes driven towards the negative
end of the battery, that allows them to overcome the opposing electric field.
This condition represents the
low resistance direction in a PN-junction allowing very
large currents to flow through the diode with only a small increase in bias voltage.
The actual
potential difference across the junction or diode is kept constant by the action
of the depletion layer at about
0.3v for
Germanium and about
0.7v for
Silicon diodes. Since
the diode can conduct "infinite" current above this knee point as it effectively becomes
a short circuit, resistors are used in series with the device to limit its current flow.
Reverse Biasing
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. The positive
voltage applied to the N-type material attracts electrons towards the positive electrode and
away from the junction, while the holes in the P-type end are also attracted away from the
junction towards the negative electrode. 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. The result is that a
high potential barrier is created thus
preventing current
from flowing through the semiconductor material.
This condition represents the
high resistance direction of a PN-junction and practically
zero current flows through the diode with an
increase in
bias voltage. However, a very
small leakage current does flow through the junction which can be measured in microamperes, (μA)
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Zener Diode
A Zener Diode is a special kind of diode which permits
current to flow in the
forward direction
as normal, but will also allow it to flow in the
reverse direction when the
voltage is above a
certain value -
the breakdown voltage known as the
Zener voltage.
Zener Effect
With the application of sufficient reverse voltage, a p-n junction will experience a
rapid
avalanche breakdown and conduct current in the reverse direction.The illustration shows this
phenomenon in a Current vs. Voltage graph. With a zener diode connected in the forward direction,
it behaves exactly the same as a standard diode - i.e. a small voltage drop of 0.3 to 0.7V with
current flowing through pretty much unrestricted. In the reverse direction however there is a very
small leakage current between 0V and the Zener voltage - i.e. just a tiny amount of current is able
to flow. Then, when the voltage reaches the breakdown voltage (Vz), suddenly current can flow freely
through it.
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Light Emitting Diodes(LEDs)
LEDs
emit light when an electric
current passes through them.
An LED must have a
resistor connected in
series to limit the current through the LED, otherwise it will burn out almost instantly.
LEDs must be connected the correct way round, the diagram may be labelled
a or + for
anode and
k or
- for
cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and
there may be a slight flat on the body of round LEDs.
Note: Never connect an LED directly to a battery or power supply!
Colours of LEDs
LEDs are available in
red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.
The colour of an LED is
determined by the
semiconductor material, not by the colouring of the 'package
' (the plastic body).
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The transistor is a special semiconductor device discovered by
John Bardeen, Walter Brattain (1902-1987),
and William Shockley(1910-1989) in 1948. Their discovery revolutionized the world of electronics and
the three were awarded the Nobel Prize in physics in 1956.
There are two
types of transistors:
p-n-p transistors and
n-p-n transistors.
The
p-n-p transistor consists of a semiconducting material with a very narrow n region sandwiched
between two p regions. The
n-p-n transistor consists of a p region sandwiched between two n regions.
The letters refer to the layers of semiconductor material used to make the transistor.
Most transistors used today are NPN because this is the easiest type to make from silicon.
The leads are labelled
Base (B), Collector (C) and Emitter (E).
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Logic gates process signals which represent
true or
false. Normally the positive supply voltage +Vs
represents true and 0V represents false. Other terms which are used for the true and false states
are shown in the table on the right. It is best to be familiar with them all.
Gates are identified by their function:
NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR. Capital letters
are normally used to make it clear that the term refers to a logic gate.
NOT Gate(Inverter)
The output Q is true when the input A is NOT true, the output is the inverse of the input:
Q = NOT A .
A NOT gate can only have
one input.
A NOT gate is also called an inverter.
AND Gate
The output Q is true if input A AND input B are both true:
Q = A AND B.
An AND gate can have
two or more inputs,
its output is true if all inputs are true.
OR Gate
The output Q is true if input A OR input B is true (or both of them are true):
Q = A OR B.
An OR gate can have
two or more inputs,
its output is true if at least one input is true.
NAND Gate (NAND = Not AND)
This is an AND gate with the output inverted, as shown by the 'o' on the output.
The output is true if input A AND input B are NOT both true:
Q = NOT (A AND B).
A NAND gate can have
two or more inputs,
its output is true if NOT all inputs are true.
NOR Gate (NOR = Not OR)
This is an OR gate with the output inverted, as shown by the 'o' on the output.
The output Q is true if NOT inputs A OR B are true:
Q = NOT (A OR B)
A NOR gate can have
two or more inputs,
its output is true if no inputs are true.
EX-OR (EXclusive-OR) Gate
The output Q is true if either input A is true OR input B is true, but not when both of them are true:
Q = (A AND NOT B) OR (B AND NOT A).
This is like an OR gate but excluding both inputs being true.
The output is
true if inputs A and B are
DIFFERENT.
EX-OR gates can only have
2 inputs.
EX-NOR (EXclusive-NOR) Gate
This is an EX-OR gate with the output inverted, as shown by the 'o' on the output.
The output Q is true if inputs A and B are the SAME (both true or both false):
Q = (A AND B) OR (NOT A AND NOT B)
EX-NOR gates can only have
2 inputs.
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