Semiconductors
3.3.24 State that materials can be divided into three broad categories
according to their electrical properties – conductors, insulators and
semiconductors.
3.3.25 Give examples of conductors, insulators and semiconductors.
3.3.26 State that the addition of impurity atoms to a pure semiconductor (a
process called doping) decreases its resistance.
Conductors
These materials have many electrons which are free to move within the material
and so electrons are available to carry an electrical current. Examples – all metals
and carbon (graphite). The electrical resistance of conductors is low.
Insulators
In these material very few electrons are free to move so they cannot easily carry
and electrical current. Examples - most non-metals, plastics, wood, glass, rubber.
The electrical resistance of insulators is high.
Semiconductors
The materials are like insulators when in a pure form, having very few free
electrons to carry an electrical current. Some semiconductors called intrinsic
semiconductors can conduct a little when heated. Others can be made to conduct
by adding impurity atoms, which is called DOPING. Examples – silicon, germanium,
gallium arsenide. Their electrical resistance is high when pure and decreased when
impurities are added.
3.3.27 Explain how doping can form an n-type semiconductor in which the
majority of the charge carriers are negative, or a p-type semiconductor
in which the majority of the charge carriers are positive.
One example of a semiconductor is silicon. Silicon has four electrons in its outer
shell which are either available to make bonds with other atoms or carry an
electrical current.
Silicon atom with four electrons in its
outside shell.
Pure silicon forms a crystal lattice, where all the
electrons pair with electrons from other silicon
atoms to form bonds which stop the electrons
moving. There are almost no free electrons in
pure silicon to carry an electrical current, and
therefore it has a high resistance.
N-Type Semiconductors
N-type semiconductors are formed by doping silicon with a small amount of an
element having five electrons in its outside shell (e.g. phosphorus or arsenic).
Free electron
Phosphorous atom with five electrons in its
outside shell.
Four of the phosphorus electrons form bonds with
the surrounding silicon atoms. This leaves one
electron free to carry an electrical current. As
electrons are negatively charged the material is
called an n-type semiconductor.
P-Type Semiconductors
P-type semiconductors are formed by doping silicon with a small amount of an
element with three electrons in its outer shell (e.g. boron or indium).
Boron atom with three electrons in its
outside shell.
Hole
All three electrons in the outside shell of the boron
atom form bonds with the surrounding silicon atoms.
This leaves gap in the crystal lattice called a hole.
When a voltage is applied, an electron from another
silicon atom can jump into this hole leaving a hole
elsewhere in the crystal. This moving hole behaves
like a positive charge moving through the material.
This type of semiconductor is called a p-type
semiconductor.
Majority and Minority Charge Carriers
Pure silicon does not have an infinitely high resistance. Some of the electron pair
bonds are broken by the thermal motion of the atoms, leaving a free electron and a
hole. These free charge carriers can then carry an electrical current. The
resistance of pure silicon reduces as the temperature increases because more
electron pair bonds are broken with increasing temperature.
This process also occurs in n-type and p-type semiconductors. In n-type
semiconductors, the majority (of) charge carriers are electrons. There are
however some holes produced by thermal activity and these are called minority
charge carriers.
In p-type semiconductors the situation is reversed. The majority charge carriers
are holes and the minority charge carriers are electrons.
3.3.28 Describe the movement of the charge carriers in a forward/reverse
biased p-n junction diode.
P-N Junction Diode
A p-n junction diode is an electronic component which allows current to flow in one
direction only.
A p-n junction diode is produced by doping a single piece of semiconductor so that
it is p-type at one end and n-type at the other.
Free electron
Hole
P-type
semiconductor
N-type
semiconductor
Overall both the p-type and n-type pieces semiconductors are electrically neutral.
Some of the free electrons and holes near the junction will cross to the other type
of semiconductor and combine with holes and electrons on the other side. This
leaves an area near the junction with no free charge carriers called the depletion
layer. This movement also leaves the n-type semiconductor positively charged and
the p-type negatively charged. This small voltage then prevents any further
movement of charges across the junction. The voltage across the junction is
usually about 0.7V for silicon diodes and 0.3V for germanium diodes. This voltage is
often called the “forward voltage drop”.
Positively
charged
end
Negatively
charged
end
P-type
semiconductor
N-type
semiconductor
Depletion layer
No charge carriers
Biasing a P-N Junction Diode
Biasing means placing a voltage across the diode. This can be done in two ways;
Forward Biasing
Voltage due to
depletion layer
0.7V
P
N
P
P-type semiconductor
N
N-type semiconductor
When the diode is forward biased no current will flow if the applied voltage is less
than the voltage across the depletion layer. Above this voltage electrons in the ntype semiconductor are driven across the depletion layer to the positive end of the
battery, and holes are attracted across toward the negative end of the battery.
So current will flow through a forward biased diode provided the voltage applied is
above the forward voltage drop.
Reverse Biasing
Voltage due to
depletion layer
0.7V
P
N
When the diode is reverse biased the positive voltage of the battery attracts
electrons in the n-type semiconductor and the negative voltage attracts the holes
in the p-type. This makes the depletion layer wider and prevents movement of
charge carriers through the diode. So no current will flow through the diode.
Due to the minority carriers and thermally induced electron/hole pairs there is
always a very small current when a diode is reversed biased. This is called the
reverse leakage current. In practice this is usually of no importance but is used in
the photoconductive mode of a photodiode (see later).
The electrical symbol for a p-n junction diode is
Electrons will flow
in this direction.
3.3.29 State that in the junction region of a forward-biased p-n junction
diode, positive and negative charge carriers may recombine to give
quanta of radiation
Light Emitting Diodes (LED)
An LED is a p-n junction diode with the junction near the surface of the diode so
that light can be emitted. When it is forward biased, some of the holes and
electrons crossing the p-n junction can recombine. Each recombination releases
energy which can occur as a photon of light. The quantity of energy released
depends on the type of material used to make the diode, so the frequency of light
(remember E=hf) emitted if fixed.
The electrical symbol for a LED is
3.3.30 State that a photodiode is a solid-state device in which positive and
negative charges are produced by the action of light on a p-n junction.
Photodiodes
Photodiodes like LDRs have a p-n junction close to the surface. When light is
incident upon the p-n junction, the energy of each photon can produce an
electron/hole pair.
The electrical symbol for a photodiode is
D3.3.31 State that in the photovoltaic mode, a photodiode may be used to
supply power to a load
Photodiodes can be used in two different ways;
Photovoltaic mode
There is a voltage across the depletion layer at p-n junction. When a photon of
light produces an electron/hole pair, the electron is attracted towards the n-type
material and the hole towards the p-type. The voltage caused by this separation of
electrons and holes can be used to drive a load (eg an electric motor). A solar cell
is a photodiode in photovoltaic mode.
3.3.32
State that in the photoconductive mode, a photodiode may be used
as a
light sensor.
3.3.33 State that the leakage current of a reverse biased photodiode is
directly proportional to the light irradiance and fairly independent of
the reverse-biasing voltage, below the breakdown voltage.
3.3.34 State that the switching action of a reverse biased photodiode is
extremely fast.
Photoconductive mode
In this mode the photodiode is reversed biased and is used as a light sensor.
A
A sensitive ammeter can be used to detect the reverse leakage current. When a
photon of light hits the p-n junction of the diode, an electron/hole pair is formed.
The electron is attracted towards the positive end of the battery and the hole
towards the negative end, producing a small
reverse leakage current  A
current.
This current is directly proportional to the
irradiance as each photon produces one
electron/hole pair. The reverse biased
current does not vary much with reversed
biased voltage. The switching action is
extremely fast i.e. the current changes
immediately when the irradiance changes.

Irradiance Wm2

3.3.35 Describe the structure of an n-channel enhancement MOSFET using the
terms: gate, source, drain, substrate, channel, implant and oxide layer.
3.3.36 Explain the electrical ON and OFF states of an n-channel enhancement
MOSFET
3.3.37 State that an n-channel enhancement MOSFET can be used as an
amplifier.
MOSFET
An n-channel MOSFET consists of;





a substrate made of a p-type semiconductor.
two n-type implants, called the source and drain with metal contacts.
A metal contact lying between the source and drain called the gate.
An insulating oxide layer which separates the gate from the substrate.
A connection between the source and substrate.
source
gate
drain
metal contact
oxide layer
n
n
n-type implant
p-type substrate
substrate contact
MOSFET Off
When the MOSFET is used in a circuit the drain is always more positive than the
source. If there is no voltage applied to the gate, no current can flow between the
source and drain as;
 the p-type substrate and the n-type implant at the drain form a reversed
biased p-n junction.
 the p-type substrate and the n-type implant at the source form a forward
biased p-n junction. However, the connection between the source and
substrate ensures that the forward voltage across this junction is zero. (see
section on p-n junction diodes).
gate
source
Forward biased p-n
junction (zero volts
across the junction)
n
drain
p
p
n
Reversed biased
p-n junction
MOSFET On
When a positive voltage is applied to the gate, electrons in the p-type
substrate are attracted towards the gate (or think of it as holes being repelled).
This forms a channel of n-type semiconductor between the source and drain.
As there are now no p-n junctions to prevent the flow of electrons, current can
flow through the MOSFET.
Like p-n junctions there is a minimum voltage to make the MOSFET conduct.
It is usually around 2V. If the voltage of the gate is increased the size of the ntype channel increases allowing more current to flow through the device.
source
n
gate
drain
n
n-type channel
VS
Electrons
will flow
in this
direction.
MOSFETs in Circuits
drain (+)
source (-)
gate
Circuit symbol of an
n-channel
enhancement
MOSFET
MOSFET as a Switch
When the gate voltage VG is below 2V the MOSFET is off. When VG rises above
2V the MOSFET will switch on. The current through the device from the source
to the drain is called the drain current (ID).
+Vs
Load. eg lamp,
electric motor etc
ID
VG
0V
S
MOSFET as an Amplifier
Once Vin is greater than 2V the drain current is proportional to the input voltage so
Vout will also vary with Vin.
+Vs
ID
VOut
Vin
0V