DIODES & TRANSISTORS
AIMS
The aims of this unit are:
.
To introduce the physical operation of semiconductors.
.
To explain the operation of diodes and transistors.
INTRODUCTION
The last 40 years or so have seen very significant advances in electronics. In that time it has
become possible to manufacture electronic circuits containing millions of electronic devices such as diodes, transistors, resistors, etc. - on a single, small piece of silicon, only a few
millimetres square. The net result has been that electronic equipment has become smaller, more
reliable, and cheaper to buy and operate.
In this unit we will be studying the physical operation of the basic semiconductor devices, diodes
and transistors, which are the fundamental building blocks of electronic circuitry.
BASIC SEMICONDUCTOR DEVICES
OBJECTIVES
After studying this sub-unit, you should be able to:
1. Define or explain the meaning of the following terms:
.


.
.
.
.


.

.
.
.
.

.
.
acceptor atom
atomic number
atomic weight
covalent bond
donor atom
dopants
doping
energy bands
energy band diagram
extrinsic semiconductor
free electron
hole
intrinsic semiconductor
majority carrier
minority carrier
negative ion
n-type semiconductor
pentavalent atom
1


.
.
.
.
.

periodic table
positive ion
p-type semiconductor
semiconductor
tetravalent atom
trivalent atom
valence electron
valency
2. Define the properties of a semiconductor in relation to conductors and insulators.
CONDUCTORS, INSULATORS AND SEMICONDUCTORS
A complete understanding of the operation of semiconductor devices such as diodes, transistors
and integrated circuits requires some knowledge of atomic theory. We already know from our
previous study that the atom consists of a central nucleus, containing protons and neutrons,
around which electrons orbit, as shown in Figure 1.
-
+
-
+
+
-
+
-
Electron
+
Proton
Neutron
Nucleus
-
Figure 1
Atomic Structure
In a given element, each atom has a specific number of protons and electrons. For example, the
simplest atom is that of hydrogen has one proton and one electron, whereas the semiconductor
material silicon has 14 protons in its nucleus and 14 orbiting electrons. Each electron possesses a
negative charge of 1.6 × 10-19C, while each proton possesses an equal positive charge. A single
atom is therefore electrically neutral. The forces of attraction between the positive and negative
charges hold the atom together. The forces of attraction on electrons become progressively
weaker as their distance from the nucleus increases.
All elements can be arranged in a so-called periodic table according to the number of orbiting
electrons in an electrically neutral atom of the element. This is referred to as the atomic number.
The elements can also be arranged by their atomic weight, which is approximately the number of
protons and neutrons in the nucleus. For example, hydrogen has an atomic number of 1 and an
atomic weight of 1, whereas silicon has an atomic weight of 28 (14 protons + 14 neutrons).
2
The electrons revolve around the nucleus in a number of fixed ‘orbits’ or ‘shells’, as shown in
Figure 2. A given atom has a fixed number of shells. The shells are designated K, L, M, N and so
on with K being closest to the nucleus. Each shell has a fixed maximum number of electrons at
permissible energy levels (orbits). The differences in energy levels within a shell are substantially
smaller than the energy difference between shells.
Energy
level
2nd
shell
W6
W5
W4
W3
Electrons
r3
1st
shell
W2
W1
r4
r1
r6
r2
r5
W = energy
Nucleus
K
shell
L
shell
R = distance from nucleus
Figure 2
Energy Shells for Orbiting Electrons In An Atom
The electrons in any particular orbit or shell have a kinetic energy, which lies within a certain
range or band. Electrons in orbits closest to the nucleus have the least energy. Those electrons in
the outermost orbit have the greatest energy and are known as valence electrons, and the number
of electrons in the outermost shell of an atom determines its valency. Electrons farthest from the
nucleus are less tightly bound to the atom since the force of attraction between the positively
charged nucleus and the negatively charged electron decreases with increasing distance.
An electron may change orbits if it absorbs sufficient additional energy from an external source.
When an atom absorbs energy from a heat or light source, for example, the energy levels of the
electrons are increased. If the energy gain is sufficient they may move to a different orbiting
shell. Since the valence electrons possess more energy and are less tightly bound to the nucleus,
they can more readily jump to higher orbits. If a valence electron acquires a sufficient amount of
energy, it can be completely removed from the outer shell and the atom’s influence. This causes a
previously neutral atom to have an excess of positive charge. Such an atom is referred to as a
positive ion. The escaped electron is referred to as a free electron. If this free electron falls into
the outer shell of another neutral atom then we have a negative ion.
We have seen that the electrons of an atom can only orbit within prescribed energy bands. Each
shell around the nucleus corresponds to a certain energy band and is separated from adjacent
3
shells by energy gaps in which no electrons can orbit. This is illustrated in the energy band
diagram of Figure 3, in which the highest energy band is called the conduction band. These
energy levels contain electrons which have gained sufficient energy to escape the forces of
attraction of the nucleus and are free to migrate through the material, i.e. available for the
conduction of electricity through the material.
Energy
Conduction
band
Energy gaps
(no electrons)
Valence
band
2nd
band
1st
band
Figure 3
General Energy Band Diagram for a Material
In insulators, the valence electrons are very tightly bound to the nucleus. Even large amounts of
external energy will fail to free electrons in sufficient numbers for the conduction of electricity.
In conductors, the valence electrons are very loosely bound and are free to move under the
influence of an applied emf, thereby giving rise to an electric current.
You will recall that the resistance of a piece of material to the flow of electric current is indicated
by its resistivity value, i.e. the higher the resistivity, the greater the resistance. Materials with
resistivity values which lie between that of a good conductor and a good insulator are called
semiconductors. The best-known semiconductor material is silicon, which is used in the
manufacture of a large proportion of electronic components such as diodes, transistors and
integrated circuits (ICs). Other materials used as semiconductors include germanium, galliumarsenide, cadmium-sulphide and lead-sulphide. Typical resistivity values for conductors,
insulators and semiconductors are as shown in Table 3.1.
Material
copper (Conductor)
pure silicon (Semiconductor)
mica (Insulator)
Resistivity ( m)
1.7 × 10-8
103
1012
Table 3.1
Resistivity values for a conductor, insulator and semiconductor
Figure 4 gives typical energy band diagrams for conductors, insulators and semiconductors. The
energy gap for an insulator is so large that very few electrons can acquire sufficient energy to
reach the conduction band. The valence band and the conduction band in a conductor, e.g.
4
copper, overlap so that there is always an abundance of electrons available for conduction. Even
without the application of external energy a large number of valence electrons in a conductor
have sufficient energy to jump into the conduction band. In a semiconductor there is an energy
gap between the valence and conduction bands but it is much narrower than that of an insulator.
Energy
Energy
Energy
Conduction
band
Conduction band
Energy gaps
Conduction band
Energy gaps
Valence
band
Overlap
Valence band
Valence band
(a) Insulator
(b) Semiconductor
(c) Conductor
Figure 4
Energy Band Diagram for Conductors, Insulators & Semiconductors
Another difference between conductors, insulators and semiconductors is the way in which their
resistance changes with temperature. This is summarised by the graphs of Figure 5.
As the temperature of a conductor is increased, its resistance increases slightly, due to the
increased vibration (random motion) of its free electrons. See Figure 5(a).
An insulator, in contrast, shows a very slight reduction in resistance as temperature is increased,
as illustrated in Figure 5(b). This is due to the production of a small number of free electrons,
which acquire sufficient energy to overcome the forces of attraction of the nucleus.
As the temperature of a semiconductor is increased, its resistance decreases very significantly.
See Figure 5(c). A rise in temperature causes large numbers of valence electrons to break free and
become available for conduction.
resistance
R
resistance
R
resistance
R
temperature
(a) Conductor
temperature
(b) Insulator
temperature
(c) Semiconductor
Figure 5
Variations of resistance with temperature in Conductors, Insulators & Semiconductors
The properties of a semiconductor may be summarised as follows:
A pure semiconductor has a resistivity between that of conductor and an insulator. Its resistivity
falls steeply with an increase in temperature.
5
INTRINSIC (PURE) SEMICONDUCTORS
Consider the structure of a silicon atom, as shown in Figure 6. Its nucleus contains 14 protons
and 14 neutrons.
Nucleus
14 P
14 N
Figure 6
Structure of a silicon atom
2
8
4
Valence Electrons
There are 14 electrons, 4 of which are valence electrons. Any element which has 4 valence
electrons is referred to as a tetravalent element, where tetravalent means having 4 valence
electrons.
Atoms which combine to form a solid arrange themselves into an orderly pattern called a lattice.
Such a lattice is illustrated in Figure 7(a), in which only the valence electrons are shown. Each
atom shares a valence electron with its neighbour, so that there are forces of attraction between
atoms, holding them together. These forces of attraction are called covalent bonds. A more
simplified schematic of covalent bonding is shown in Figure 7(b).
- -
- -
(a) Covalent bonding
(b) Simplified Schematic of
Covalent bonding
Figure 7
Lattice structure of silicon
In semiconductors the covalent bonds are fairly weak. At room temperature, a small proportion of
electrons in these bonds acquire sufficient thermal energy to escape and become available for
conduction. Consider Figure 8. When an electron acquires sufficient thermal energy and leaves a
bond, such as at A, a neighbouring valence electron, for example B, can transfer into the vacant
bond or hole, without acquiring extra energy.
The new hole at B can similarly transfer to C and thus wander throughout the bulk of the
6
material. Therefore, the actual series of electron displacements in one direction gives rise to the
apparent motion of a hole in the opposite direction. Holes can be treated as particles of positive
charge and a certain effective mass.
When a hole-electron pair is created by an electron with sufficient thermal energy to break a
covalent bond, the hole and free electron are said to be thermally-generated. If there is no
potential difference across the semiconductor, the movement of the electrons and holes is
random.
However, when an emf is applied across the material, the electrons will flow to the positive
terminal and the holes will appear to flow to the negative one. Therefore, the flow of current in a
pure semiconductor material is due to both electron and hole conduction. This is known as
intrinsic conduction.
A
-
-
- B
-
-
- C
-
-
-
-
-
-
-
-
-
-
Figure 8
Electron/hole conduction in a semiconductor
SAQ 1
(a) State whether the following statement is true or false: ‘Holes in a semiconductor material
appear to move’.
(b) The temperature increase in an intrinsic semiconductor will cause an intrinsic current to:
(i)
remain unchanged
(ii) decrease
(iii) increase.
EXTRINSIC (DOPED) SEMICONDUCTORS
The only current carriers in a pure semiconductor are the thermally-generated electron-hole pairs.
For most applications, an insufficient number of these exist to produce a usable current. Higher
current levels can be achieved only by adding small quantities of other elements into the lattice
structure, in order to increase the number of charge carriers. The added atoms form covalent
bonds with their silicon neighbours. The number of covalent bonds formed, and hence the type of
charge carriers produced, depends on the number of valence electrons or valency of the added
atoms. The process of adding extra atoms is known as doping and the impurities are called
dopants. A doped semiconductor is called an extrinsic semiconductor.
7
For use in diodes, transistors and integrated circuits, silicon is first purified to impurity
concentrations of less than 1 part in 1010. It is then doped with either pentavalent (valency 5) or
trivalent (valency 3) atoms, to concentrations of about 1 part in 105. This increases the number
of either free electrons or holes respectively. Common materials used for doping are listed in
Table 3.2.
Pentavalent Elements
Trivalent Elements
Arsenic
Antimony
Phosphorous
Boron
Aluminium
Indium
Gallium
Table 3.2
Some common dopants
N-TYPE SEMICONDUCTOR
Consider Figure 9. An n-type semiconductor is formed by adding pentavalent impurity atoms
(valency 5). Pentavalent dopants have 5 outer electrons, and hence there is one spare, when the
covalent bonds are full. This electron is not tightly bound and is available for conduction. At the
impurity concentrations normally used, these electrons far out-number the thermally-generated
electron-hole pairs. Therefore, in an n-type semiconductor the electrons are referred to as the
majority carriers, while holes are called the minority carriers. Pentavalent impurities are often
called donor impurities, since they ‘donate’ electrons for conduction.
-
-
-
-
-
-
Impurity Atom
(5 Valence electrons)
-
-
-
One free electron
from impurity atom
Figure 9
Doping of silicon with pentavalent impurity atoms
8
P-TYPE SEMICONDUCTOR
A p-type semiconductor is formed when it is doped with trivalent atoms (valency 3), as shown in
Figure 10.
-
-
-
-
-
-
Impurity Atom
(3 Valence electrons)
-
Hole
Figure 10
Doping of silicon with trivalent impurity atoms
Some atoms are now a bonding electron short, causing the presence of a hole. In this case, the
holes far out-number the free electrons. Thus in a p-type semiconductor the holes are the majority
carriers, while the electrons are the minority carriers. Trivalent atoms are also known as
acceptor atoms, because each hole they contribute may ‘accept’ an electron, to complete the
bond.
SAQ 2
Fill in the missing words in the following statements:
(a) Valency 5 atoms are called ............. atoms.
(b) ........... atoms are called acceptor atoms.
(c) In an n-type semiconductor electrons are ........ carriers and holes are .......... carriers.
(d) In a p-type semiconductor ........ are majority carriers and ......... are minority carriers.
SUMMARY
1. A semiconductor is a material which is neither a good conductor nor a good insulator.
2. With an increase in temperature, the resistance of a conductor increases slightly, while the
resistance of an insulator decreases slightly.
The resistance of a pure semiconductor decreases dramatically with an increase in
temperature.
3. The sharing of valence electrons by atoms in the lattice of a solid material creates forces of
9
attraction between atoms, which are called covalent bonds.
4. In an intrinsic (pure) semiconductor, there are equal numbers of free electrons and holes,
and both take part equally in conduction. At room temperature, these thermally-generated
electron-hole pairs are insufficient in number to form a usable electric current.
5. In an extrinsic (doped) semiconductor, small quantities of impurities are added, producing a
surplus of either free electrons or holes. These far exceed the number of thermally-generated
electron-hole pairs, and, if an emf is applied, will form an electric current.
6. An n-type semiconductor is formed by adding pentavalent (5-valence electrons) impurity
atoms, which results in an excess of free electrons. The electrons are majority carriers, while
holes are minority carriers.
7. A p-type semiconductor is formed by adding trivalent (3-valence electrons) impurity atoms,
which results in the material containing an excess of holes. The holes are majority carriers,
while the electrons are minority carriers.
ANSWERS TO SAQS
SAQ 1
(a) true
(b) increase
SAQ 2
(a) donor (pentavalent)
(b) valency 3 (trivalent)
(c) majority, minority
(d) holes, electrons
10
THE P-N JUNCTION
OBJECTIVES
After studying this sub-unit, you should be able to:
1. Define or explain the meaning of the following terms:
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
anode
avalanche effect
base
bias
breakdown voltage
cathode
depletion layer
forward-biased
forward characteristic
junction potential
leakage current
peak inverse voltage (PIV)
p-n junction
reverse-biased
Zener effect
Zener voltage
2. Draw a p-n junction diode connected in forward-biased and reverse-biased modes, indicating
the direction of current flow in the diode and the external circuit.
3. Explain the principle of operation of the following diodes:
.
.
.
.
.
junction diode
Zener diode
light emitting diode (LED)
photodiode
varicap (varactor) diode
UNBIASED JUNCTION
When a semiconductor material changes from p-type to n-type somewhere along its length, the
boundary where the p-type and n-type regions meet is called the p-n junction. Consider Figure
11.
The p-side has many holes and the n-side many conduction electrons. To avoid confusion, no
minority carriers are shown. But it is important to realise that there are a few conduction electrons
on the p-side and a few conduction holes on the n-side. The p-n junction shown in Figure 11 is
unbiased; that is, there is no external voltage applied to it.
11
Virtual Battery
(barrier potential)
Holes
Junction
Electrons
N
P
+ + + + + + +
- - - - - - -
+ + + + + + + - + - - - - - - -
+ + + + + + +
- - - - - - -
+ + + + + + + - + - - - - - - -
+ + + + + + +
- - - - - - -
+ + + + + + + - + - - - - - - -
+ + + + + + +
- - - - - - -
+ + + + + + + - + - - - - - - -
P
N
Negatively-charged
Ions
Protons > Electrons
Positively-charged
Ions
Protons < Electrons
Depletion Layer
Ii
Io
(a) Before diffusion
(b) After diffusion
Figure 11
p-n junction
The conduction holes in the p-region and conduction electrons in the n-region are mobile, and
can migrate (diffuse) across the junction to recombine with majority carriers on the opposite side.
This process cannot continue indefinitely, as the migration causes the build-up of positive charge
in the n-region and negative charge in the p-region, in the vicinity of the junction. In other words,
every electron which moves away from the n-region leaves a positively charged ion behind, and
every hole which move away from the p-region leaves a negatively charged ion behind. This
disturbs the electrical balance of the material, which normally has equal numbers of negative
(electrons) and positive (holes) charges (but with only one type free to move).
There is now a voltage, called the junction potential, between the n-type and p-type material at
the junction. This potential opposes the movement of more majority carriers and an equilibrium
position is reached. It might be thought that this potential should cause the p-n junction to act as a
battery, but once the circuit is completed externally, the total emf around the circuit is zero.
Minority carriers (holes on the n-side, electrons on the p-side) are helped across by the junction
potential, giving rise to a current I0, but this current is exactly balanced by the current Ii, due to
the small proportion of majority carriers that have sufficient energy to cross in spite of the
junction potential.
The region near the junction, which is emptied of movable charges, is called the depletion layer.
FORWARD-BIASED JUNCTION
Bias refers to the application of a voltage between the p and n layers of the junction. If the
applied voltage is connected as shown in Figure 12(a), with the positive terminal connected to the
p-type material and the negative terminal connected to the n-type material, the junction is said to
be forward-biased. The junction potential (potential barrier) is lowered, allowing more majority
carriers to cross, giving a net current flowing from p-type to n-type.
12
Original depletion
layer
Reduced
depletion
layer
Current flow
across junction
due to majority
carriers
P
N
I
I
Current flow in
external circuit
Current I (mA)
40
30
20
10
V
0.5
(a) Bias Arrangement
Voltage
1.0 (volts)
(b) Forward Characteristic
Figure 12
Forward-biased p-n junction
Consider Figure 12(b). As the applied voltage is increased, the size of the depletion region
becomes smaller, until the amount of forward bias equals the junction potential - about 0.6V for
silicon, when there is no longer any depletion and the junction can conduct. At slightly higher
forward bias, carriers will move across the junction, so that current flows in the circuit.
A p-n junction is the basis of an important electronic device known as a diode. Note that a diode
does not obey Ohm’s law: the ratio of voltage to current is not constant. The sketch of Figure
12(b) is referred to as the forward characteristic of the diode.
REVERSE-BIASED JUNCTION
If a voltage is applied with the positive terminal connected to the n-type material and the negative
terminal to the p-type material, as shown in Figure 13(a), the junction is said to be reversebiased. As the junction potential is increased, the depletion layer widens, reducing the number of
majority carriers crossing. If the applied voltage is sufficient, the number of majority carriers
crossing effectively becomes zero, leaving a very small net current (leakage current) from n to p
of I, due to minority carriers. See Figure 13(b).
When the reverse voltage is increased to a critical value, known as the breakdown voltage, the
junction suddenly begins to conduct and the current level increases very rapidly. This breakdown
of the junction may be caused by one of two mechanisms:
(a) Zener effect, in which some of the covalent bonds are broken, due to the high value of the
applied electric field.
(b) Avalanche effect, in which the charge carriers are accelerated by the electric field to such an
extent that they break covalent bonds through collision, thereby releasing more charge
carriers, which are in turn accelerated.
13
The maximum value of reverse voltage that can be applied across a p-n junction without causing
breakdown is called the peak inverse voltage (PIV).
Small current flow
across junction
due to majority
carriers
Increased depletion
layer
Breakdown voltage
Original
depletion
layer
Voltage
V
(volts)
-30
-20
-10
10
20
I
Small Current flow in
external circuit
30
I
Current I
(μA)
V
(a) Bias Arrangement
(b) Reverse Characteristic
Figure 13
Reverse-biased p-n junction
JUNCTION DIODE
A p-n junction, in which the two semiconductor regions have leads for connection to an external
electric circuit, is called a junction diode. The standard circuit symbol for a diode is shown in
Figure 14. (The word diode is a contraction of di electrode, where di means two.)
Anode
Cathode
Figure 14
Junction Diode Symbol
The p-region is called the anode and the n-region the cathode. The arrowhead indicates the
direction of conventional current flow through the diode, from the positive to the negative of the
supply.
14
SAQ 3
State whether the following statements are true or false:
(a) In a forward-biased p-n junction diode, the cathode is positive with respect to the anode.
(b) In a reverse-biased p-n junction diode, the anode is negative with respect to the cathode.
(c) The current in a forward-biased p-n junction diode is independent of the voltage across it.
(d) The current through a reverse-biased p-n junction diode is zero.
SAQ 4
The current in a p-n junction diode consists of the movement of:
(a)
(b)
(c)
(d)
electrons only
holes and electrons
holes only
atoms
ZENER DIODE
We have already seen that if the reverse-bias voltage across a diode is increased above a certain
critical value, the current flowing through it increases rapidly. This causes breakdown of the
junction and results in damage to the diode. A Zener diode is specially manufactured so that it
remains undamaged when operating in the breakdown region - provided that the current is kept
within a certain limit.
The construction and symbol for a Zener diode, together with a typical current/voltage (reverse)
characteristic, is shown in Figure 15. Its forward characteristic is similar to that of an ordinary,
junction diode.
The principal feature of the reverse characteristic is that it has an almost constant voltage drop
(Zener voltage) across it for a wide range of different current levels through it. It is this property
which makes it very useful for keeping steady (stabilising) the voltage output of a power supply.
By controlling the doping, Zener diodes with specific Zener voltages can be manufactured, for
example, 3.0V, 3.9V, 5.1V ..... and so on.
To avoid damaging a Zener diode, the current through it must be kept below a certain limit. This
limit is normally indicated by the specified power rating of the diode. Thus, the maximum
reverse current IR (max) can be determined from the formula:
IR (max) = power rating/Zener voltage
SAQ 5
A 5.6V Zener diode has a maximum power dissipation of 400mW. Determine the maximum
reverse current that can be safely passed through the diode.
15
Breakdown (Zener) voltage
Reverse
voltage
V
Reverse
current
I
(a) Symbol
(b) Characteristic
Figure 15
Zener diode
LIGHT-EMITTING DIODE (LED)
Recombination of electrons and holes occurs in a forward-biased p-n junction when electrons
travel from the n-type region to the p-type region and vice versa. This recombination (free
electrons entering into covalent bonds) results in the release of radiant energy which, in the case
of silicon, is low and causes a small temperature rise in the material.
However, if the p-n junction is manufactured of a compound semiconductor material, such as
gallium-arsenide-phosphide, and the junction is formed close to the surface of the material, the
amount of radiant energy released on recombination in the forward-biased junction is increased to
a visible level.
This is the principle used in the manufacture of diodes designed to give out light. These are called
light-emitting diodes (LEDs). The colours of light (red, yellow or green) emitted depends on the
relative composition of gallium, arsenide and phosphide used.
The structure, symbol and typical characteristic of an LED is shown in Figure 16.
Coloured
translucent
plastic case
Cathode
lead
Current
I (mA)
Lens
20
15
Anode
lead
(a) Physical Structure
10
5
(b) Symbol
Voltage
(volts)
(c) Characteristics
Figure 16
Light-emitting diode (LED)
16
In a practical situation, a LED must be connected in series with a resistor, as shown in Figure 17,
in order to limit the amount of current flowing through it.
resistor R
I
V
Figure 17
Circuit to operate an LED
For a typical LED sufficient light output can be obtained with a forward current, IF, in the range 5
- 25mA. A typical forward current is IF = 10mA, which corresponds to a voltage drop across the
LED of 2V. We will return to perform simple calculations on the circuit of Figure 17 in the next
sub-unit.
PHOTODIODE
As its name implies, a photodiode (Figure 18(a)) is a diode which is sensitive to light. It consists
of a normal p-n junction with a transparent ‘window’, through which light can enter. The device
is normally operated in the reverse-bias mode, so that when no light falls on it, a very small
current (dark current) flows. See Figure 18(b).
Reverse
voltage
Dark current
Breakdown
voltage
Increasing
light intensity
on diode
(a) Symbol
Reverse
current
(b) Characteristics
Figure 18
Photodiode
As the intensity of the light falling on the device is increased, the number of electron-hole pairs
rises as a result of the light energy breaking covalent bonds in the crystal lattice. Thus, the
magnitude of the reverse current in the device increases in proportion to the intensity of the
incident light.
17
The symbol for a photodiode together with a typical set of characteristics is shown in Figure 18.
The forward characteristic is that of a normal diode.
VARICAP (VARACTOR) DIODE
In our discussion of the p-n junction earlier, we noted the existence of a region depleted of charge
carriers (depletion region) in the vicinity of the junction. This means that there are effectively two
conductors separated by an insulator, which gives rise to a capacitive effect across the junction.
As the bias voltage varies, the width of the depletion region, and hence the capacitance, varies.
Diodes which are manufactured specifically to exhibit a marked variation in their junction
capacitance with bias voltage are called variable capacitance diodes or, more generally, varicap
or varactor diodes.
For a reverse-biased junction, the junction capacitance, C, is inversely proportional to the square
root of the bias voltage, V. Mathematically:
C
1
V
or C  k
V
Where k is a constant
The symbol and typical characteristic for a varicap diode are shown in Figure 19.
Capacitance C
C max
Reverse
voltage (V)
(a) Symbol
-20 -15 -10 -5
0
(b) Typical Characteristic
Figure 19
Varicap diode
SAQ 6
A varicap diode has a capacitance of 6pF when its bias voltage is 9V. Calculate the value of
capacitance when the reverse bias voltage is decreased to 4V.
18
SUMMARY
1. A p-n junction is the boundary between the p-type and the n-type region in the same
semiconductor material.
2. When a p-n junction diode is forward-biased, a high level of current flows, due mainly to
majority carriers.
3. When a p-n junction diode is reverse-biased, current flow due to majority carriers is
inhibited. There is, however, a very small leakage current due to minority carriers.
4. Breakdown of a p-n junction diode occurs if the reverse voltage is increased beyond a critical
level called the breakdown voltage.
5. The maximum value of reverse voltage that can be applied across a p-n junction diode
without causing breakdown is called the peak inverse voltage (PIV).
6. A Zener diode is designed to operate in the breakdown region without being damaged. Its
principal feature is that it has an almost constant voltage drop (Zener voltage) across it for a
wide range of different current levels through it.
7. A light emitting diode (LED) produces light energy when forward-biased.
8. A photodiode has the following property: its reverse leakage current increases in proportion
to the light intensity falling on the diode.
9. A varicap or varactor diode has the following property: the capacitance across its junction
varies with the reverse-bias voltage.
ANSWERS TO SAQS
SAQ 3
(a)
(b)
(c)
(d)
False
True
False
False
SAQ 4
(b) Holes and electrons
SAQ 5
Power dissipation = voltage across device  current through device
P = V×I
19
I =
=
=
=
P/V
(400 ×10-3)/5.6
71.4 × 10-3 A
71.4mA
Therefore, the maximum current through Zener = 71.4mA.
SAQ 6
k
V
C
6  10
 12

k
9
k
3
k  18  10 12

When V = 4V:
C
 12
18  10
4
 12
 18  10
F
 9 pF
20
THE BIPOLAR TRANSISTOR
OBJECTIVES
After studying this sub-unit, you should be able to:
1. Define or explain the meaning of the following terms:
.
.
.
.
.
bipolar transistor
collector
emitter
base
n-p-n transistor
p-n-p transistor
2. Sketch n-p-n and p-n-p junction transistors, showing normal bias conditions, and indicating
the direction of current flow in the transistor and the external circuit.
3. State the equations governing the current relationships in a bipolar transistor, given the
appropriate data.
4. Perform simple calculations on the current relationships in a bipolar transistor, given the
appropriate data.
5. Understand and describe the operation of a bipolar transistor amplifier.
INTRODUCTION
Transistors are the most important semiconductor devices. They are manufactured both as
discrete (separate) components, and as part of complex integrated circuits (ICs) such as
amplifiers, memories, microprocessors, etc. These ICs may contain many thousands of transistors
on a small piece of silicon called a ‘chip’.
There are two basic types of transistor:
(a) the bipolar junction transistor (BJT)
(b) the unipolar or field-effect transistor (FET)
In this sub-unit we will be studying the basic construction and characteristics of the bipolar
transistor. We will be studying the field-effect transistor in the next sub-unit.
CONSTRUCTION AND OPERATION
Bipolar junction transistors or BJTs consist of a thin region of either n-type or p-type silicon,
sandwiched between two thicker regions of the other type, as shown in Figure 20.
When a p-type region is sandwiched between two n-type regions, as shown in Figure 20(a), it is
called an n-p-n transistor. When an n-type region is sandwiched between two p-type regions, as
21
in Figure 20(b), it is called a p-n-p transistor.
Emitter
N
Base
P
Collector
N
Emitter
P
(a) N-P-N transistor
Base
N
Collector
P
(b) P-N-P transistor
Figure 20
Types of bipolar transistor
The thin region is lightly doped and is called the base (B). The two thicker regions are much
more heavily doped and are known as the emitter (E) and collector (C). The difference between
the collector region and the emitter region is mainly in terms of size and shape. The collector
region is made physically larger in most transistors, since it normally has to dissipate a greater
power. The symbols used to represent n-p-n and p-n-p transistors are shown in Figure 21. Note
that the two types of transistor are distinguished by the direction of the arrowhead on the emitter
lead.
Collector
C
Base
B
Collector
C
Base
B
Emitter
E
(a) N-P-N
Emitter
E
(a) P-N-P
Figure 21
Bipolar transistor symbols
In the n-p-n type, it points from B to E; in the p-n-p, it points from E to B. As we will shortly see,
the arrowhead indicates the direction of conventional current flow in the emitter.
Let us now consider the operation of an n-p-n transistor. The transistor consists of two p-n
junctions. Normal transistor operation occurs when the base-emitter junction is forward-biased
and the collector-base junction is reverse-biased. This is shown in Figure 22 where the directions
of the various currents which flow in the transistor are also indicated.
22
E
Electrons
Electrons
N
P
N
C
Ic
Vcb
C
B
A few holes
B
Ie
Ic
Holes
Ib
Vbe
Ib
Vbe
E
Ie
Vcb
(a) Physical circuit
(b) Symbolic circuit
Figure 22
Operation of an n-p-n transistor
Initially, consider the case where the base-emitter voltage Vbe is zero. There will be no net current
flow across the base-emitter junction. Since the collector-base junction is reverse biased, the only
current flowing across it is a very small leakage current, due to minority carriers, which we can
ignore. In other words, when Vbe = 0, all currents in the transistor are zero and the transistor is
said to be ‘off’.
When the base-emitter voltage Vbe is increased to about 0.7V, the base-emitter junction becomes
forward-biased. Electrons from the emitter cross the base-emitter junction into the base. The loss
of these electrons is replenished by electrons flowing from the external circuit through the emitter
lead, giving rise to the emitter current, Ie. This current also consists of holes from the p-type base.
However, since the base is lightly doped, this current is small compared with the electron flow in
the opposite direction. In short, electrons are the majority carriers in an n-p-n transistor.
The loss of a small number of holes in the p-type base region is replenished by a flow of holes
from the external supply giving rise to a small base current, Ib.
Because of the relatively small physical size of the base region, large numbers of electrons
flowing into the base from the emitter reach the vicinity of the collector-base junction where they
are attracted into the collector region by the positive bias on the collector, giving rise to the
collector current Ic.
The operation of a p-n-p transistor is very similar to that of the n-p-n type, except for the
following: in a p-n-p transistor holes are the majority carriers. The polarity of the bias supplies
and the directions of the currents are as shown in Figure 23.
SAQ 7
Fill in the blanks in the following statements:
(a) In a normally-operating transistor, the collector-base junction is ......... and the ...... junction is
forward-biased.
(b) In an n-p-n transistor, the majority carriers are ........ whereas in a p-n-p transistor, the
majority carriers are ......
23
E
Holes
Holes
P
N
P
C
A few
electrons
B
Ie
Electrons
Ic
Ib
Figure 23
Operation of a p-n-p transistor
Vbe
Vcb
SAQ 8
Using the transistor symbols, sketch an n-p-n and p-n-p transistor, showing the forward-biased
base-emitter junctions and reverse-biased base-collector junctions. Also indicate the direction of
conventional current flow in the circuit.
CURRENT RELATIONSHIPS
Studying again the transistor circuits in Figure 22 or Figure 23, we can say that the current
flowing into the transistor must be equal to the current flowing out of it. Hence the emitter
current Ie is equal to the sum of the collector and base currents, Ic and Ib respectively. That is:
Ie = Ic + Ib
From the previous description of transistor operation, we have seen that a large fraction (typically
99%) of the emitter current Ie constitutes the collector current, Ic. Mathematically:
Ic =  Ie
where  is typically 0.99. Therefore:
Ib = Ie - Ic
= Ie -  Ie
= (1 - )Ie
Typically,  = 0.99, (1 -  ) = 0.01 and Ib = 0.01Ie. Therefore Ib is very very small compared to Ie
or Ic.
If we regard the small base current Ib as the input current to the transistor, and the larger collector
current Ic as the output current, then the circuits of Figure 22 and Figure 23 are essentially current
amplifiers. As we shall see shortly, this is basically how signal amplification occurs in a
transistor amplifier.
24
The DC current gain  (sometimes called hFE) is an important quantity for a transistor and is
defined by the equation:
 = Ic /Ib
Since Ic =  Ie and Ib = (1 - )Ie
 = Ic /Ib
=  Ie /(1- )Ie
=  /(1 - )
For  = 0.99
 = 0.99/(1 - 0.99)
= 0.99/0.01
= 99
Although this is a typical value for , it is important to realise that for a wide range of transistors,
the value of  may be anywhere in the range 10 to 1000.
Since Ic =  Ib, it follows that when Ib = 0, Ic = 0. Thus, the small base current Ib ‘turns on’
and†controls a large collector current Ic. A junction transistor is therefore a current-operated
device.
SAQ 9
In a transistor, the DC emitter and collector currents are 1mA and 0.99mA respectively.
Determine:
(a) the DC base current
(b) the DC current gain
TRANSISTOR AMPLIFIER
The circuit diagram of a single-stage bipolar transistor amplifier is shown in Figure 24. The
resistors R1, R2, RC and RE are dc biasing components, i.e. their values are such that they cause
the base-emitter junction of the transistor to be forward-biased, the collector-base junction to be
reverse-biased, and establish suitable values for the dc emitter, collector, and base currents.
Capacitors C1 and C2 are coupling capacitors which block the dc voltages on the base and
collector of the transistor from the input ac signal source and the output load respectively.
Capacitor CE is a bypass capacitor and is a short circuit to ac signals and eliminates the
detrimental effect of resistor RE on the ac amplification ability of the amplifier.
The small input voltage signal to be amplified causes the base current to vary proportionately
about its nominal dc value, i.e. the ac variation is superimposed on the dc value. This variation in
ac base current causes a corresponding amplified ac variation in the emitter and collector currents
due to the transistor action described above. The amplified ac variation in collector current is
25
converted into a corresponding amplified voltage variation across the collector resistance R c
which in turn causes an amplified ac voltage to appear at the output of the amplifier.
+VCC = + 15 V
RC
R1
5 k C2
100k
C1
Input
signal
.1μF
Vi
amplified
output
signal
.22μF
47μF
50k
R2
Vo
CE
RE
4.7k
Figure 24
Bipolar Transistor Amplifier
SUMMARY
1. A bipolar (junction) transistor or BJT consists of three semiconductor regions, called the
base, emitter and collector.
2. Bipolar transistors may be of the n-p-n or p-n-p type.
3. In normal operation, a bipolar transistor has its collector-base junction reverse-biased and its
base-emitter junction forward-biased.
4. The following current relationships apply to bipolar transistors:
Ie = Ic + Ib
Ic =  Ie
Ic =  Ib
 =  /(1 -  )
where Ie = emitter current
Ic = collector current
Ib = base current
 = fraction of emitter current which flows in the collector (typically 0.99)
 = the collector-base current gain (typically 100)
26
ANSWERS TO SAQS
SAQ 7
(a) reverse-biased, base-emitter
(b) electrons, holes
SAQ 8
Your answer should be as shown in Figure 25.
Vcb
Vcb
Ic
Ic
Ib
Ib
Vbe
Vbe
Ie
(a) n-p-n transistor
Ie
(b) p-n-p transistor
Figure 25
SAQ 9
(a) Emitter current = collector current + base current
Ie = Ic + Ib
Ib = Ie - Ic
= 1 - 0.99
= 0.01mA
= 10µA
(b) DC current gain  = Ic /Ib
= 0.99/0.01
= 99.
27
THE FIELD EFFECT TRANSISTOR
OBJECTIVES
After studying this sub-unit, you should be able to:
1. Define or explain the meaning of the following terms:
.
.
.
.
.
.
.
.
channel
drain
field-effect transistor (FET)
metal-oxide semiconductor field-effect transistor (MOSFET)
n-channel MOSFET
p-channel MOSFET
threshold voltage
source
2. Sketch the structure of a n-channel metal-oxide semiconductor field effect transistors
(MOSFET), showing normal bias conditions and indicating the directions of current flow in
the MOSFET and its external circuit.
3. Understand and describe the operation of a MOSFET amplifier.
INTRODUCTION
The bipolar (junction) transistor or BJT is very widely used in electronics. However, there are
certain applications when another type of transistor, the unipolar or field effect transistor
(FET), is preferred. In a BJT the input (base) current controls the output (collector) current. In an
FET the input voltage controls the output current. The input current is virtually negligible and the
FET therefore has a very high input resistance (ri =1010). This is a desirable feature if the input
signal source, for example a crystal pick-up in a record player, cannot supply much current.
There are two basic types of field effect transistor - the junction field effect transistor (JFET)
and the metal-oxide semiconductor field-effect transistor (MOSFET). The MOSFET, which is
used mainly in digital integrated circuits, can be subdivided into two different types - the
depletion MOSFET and the enhancement MOSFET. The JFET and both types of MOSFET
differ slightly in construction and operation. Only the enhancement MOSFET will be considered
here.
OPERATION OF ENHANCEMENT MOSFET
The basic structure of an n-channel enhancement metal-oxide-semiconductor field effect
transistor is shown in Figure 26(a). The main bulk of the device consists of a substrate doped
with p-type charge carriers. The left-hand-side of the substrate consists of three distinct regions.
At the top and bottom left hand corners there are two very heavily doped n-type regions, at the
edges of which metal electrodes are attached. External wire connections to these electrodes are
called the drain (D) and the source (S) respectively. At the left-hand-side of the device, there is a
also a third, larger, metal electrode, to which another external wire connection called the gate (G)
28
is attached. Between the gate electrode and the edge of the p-type substrate there is a thin
insulating layer of silicon oxide. It is this structure of metal, oxide and semiconductor which
gives the device its name, that is metal-oxide-semiconductor field-effect transistor (MOSFET).
When a positive dc voltage VGS is applied between the gate and the source, then the properties of
the surface layer are changed. The holes in this layer are repelled and electrons are attracted into
it creating an n-type channel between the gate and the source as shown in Figure 27. The value
of the dc voltage at which this channel begins to be created is typically 1 to 2V and is called the
threshold voltage Vt. When a channel is in existence, a voltage applied between the two heavily
diffused regions causes current to flow between the drain and the source.
Since the created channel consists of n-type charge carriers (electrons), the device is referred to
as an n-channel MOSFET. It is also possible to construct a MOSFET in which the substrate is
made from n-type semiconductor material, the heavily diffused drain and source regions consist
of p-type charge carriers (holes) and the created channel is p-type. This device is referred to as a
p-channel MOSFET.
Source (S)
Gate (G)
Drain (D)
Oxide
(SiO2)
Metal
Channel
region
L
p-type substrate
(body)
n+
Figure 26
Structure of a MOSFET
n+
Body
B
Figure 27 shows the way in which an n-channel MOSFET is biased for normal operation.
VDS
Very small
gate
current
S
Is
Gate electrode
+
VGS
Oxide
(SiO2)
G
Ig
n+
Induced
n-type
channel
D
Id
n+
L
p-type substrate
Depletion region
Figure 27
Normal biasing of a N-channel JFET
B
29
A positive voltage VGS (typically 1 to 5V) applied between the gate and the source junctions
creates the n-type channel. Because of the insulating silicon oxide layer a negligibly small current
flows in the gate lead. The greater the gate voltage the greater the number of charge carriers the
channel contains. Hence the conductivity of the device between the drain and the source
increases.
Thus, the gate voltage controls the current between the drain and source. The MOSFET is
therefore a voltage-controlled device. The current from drain to source through the channel gives
rise to a drain current Id flowing in the external circuit, as shown.
The schematic symbols normally used to represent n-channel and p-channel enhancement
MOSFETs are shown in Figure 28.
D
Channel
S
Channel
G
G
S
D
(a) n-channel MOSFET
Figure 28
MOSFET Circuit Symbols
(a) n-channel MOSFET
The thin vertical line represents the channel, to which the drain (D) and source (S) leads are
connected. In each case, the arrowhead on the source points in the direction of conventional
current flow.
The circuit of a complete MOSFET amplifier is shown in Figure 29. You can see that the
structure is very similar to that of the bipolar amplifier in Figure 24. Indeed its operation is very
similar, except that, since the MOSFET is a voltage operated device, the ac input voltage causes a
corresponding variation in the gate-source voltage Vgs which in turn causes a corresponding
variation in the drain and source currents, with the variation in drain current being converted into
an amplified output voltage through the drain resistor RD. Compared to the single-stage bipolar
transistor amplifier though, the MOSFET amplifier has a lower voltage gain, but has a higher
input resistance because of the very small current which flows in the gate terminal.
RD
R1
2 M
5 k C2
C1
Input
signal
.1μF
Vc
amplified
output
signal
.22μF
R2
1 M
Vo
Rs
22μF
1 k
Figure 29
MOSFET Amplifier
30
SAQ 10
Show how the normal biasing of a p-channel MOSFET would be arranged.
SAQ 11
Fill in the blanks in the following statements:
(a) A BJT is a ...... -controlled device, whereas a FET is a ....... - controlled device.
(b) In an n-channel MOSFET, the more positive the gate-source voltage, the ........... the draincurrent.
SUMMARY
1. A field-effect transistor (FET) is a voltage-operated transistor with a very high input
resistance.
2. There are two basic types of field effect transistor - the junction field effect transistor
(JFET) and the metal-oxide semiconductor field effect transistor (MOSFET).
3
In a MOSFET the value of Vgs which just causes a channel to be produced is called the
threshold voltage Vt, which is a constant for a particular transistor.
ANSWERS TO SAQS
SAQ 10
The biasing arrangement for a p-channel MOSFET is as shown in Figure 30.
Vgs
-
sis = id
+
0
+
Vds
id
Figure 30
Biasing of p-channel MOSFET
SAQ 11
(a) current, voltage
(b) greater
31