BASIC ELECTRONICS
BASIC ELECTRONICS
CHAPTER 1 SEMICONDUCTOR MATERIALS AND PN JUNCTION
As the name suggests semiconductor materials (SMCMs) have properties between those of insulators and
conductors. In other words they behave as insulators or conductors under certain conditions.
Semiconductor properties are displayed by the elements of Group IV of the Periodic Table and some
compounds. The most commonly used SMCMs are Silicon (Si – atomic number of 14), Germanium (Ge –
atomic number 32) and Gallium Arsenate (GaAs). GaAs combines an element from Group III and element
of Group V of the Periodic Table.
Atoms of the same element could arrange themselves in different ways. The SMCMs that are used in
electronic semiconductor devices must have regular, repetitive structure called lattice or crystal structure.
In such structures the SMCMs atoms are bonded by covalent bonds. Each atom needs four additional
electrons to complete the outmost shell. In covalent bonding this as achieved by each atom sharing its
electrons with four neighbour atoms.
1.1 Intrinsic Semiconductor
An intrinsic semiconductor is a pure semiconductor, formed by only one type of atoms. The simplified
structure for a Si (or Ge) crystal is shown in Figure 1.1.
Si (or Ge) core atom
Covalent bond
Valence electron
Figure 1.1
This is a typical structure of a SMCM when no external (like thermal) energy is applied to the material. All
valence electrons are in covalent bonds, therefore there are no free charged particles, and the material is an
insulator. It needs to be noted that the negative charge is equal to the positive, therefore the material is
electrically neutral. Regardless of the fact that there is no external energy, the electrons in the atoms posses
certain amount of energy which increases with their distance from the nucleus. The energy is measured in
Joules, but as it is very small a more convenient unit of electron-volt (eV) is introduced, where
1 eV = 1.602 x 10-19 J.
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When external energy (usually in form of heat) is applied to the material the energy level of the electrons
increases and some valence electrons leave covalent bonds and become ‘free’ moving at random within the
crystal (Figure 1.2). As a result, a covalent bond is broken and a charge carrier (or current carrier) is
produced.
Si (or Ge) core atom
Covalent bond
Free electron
Hole
Figure 1.2
The place where an electron vacated a covalent bond has the property of attracting an electron in order to
restore the bond. Small amount of energy (much less that is needed to produce a free electron) is needed in
addition to the electrical force of attraction, for an electron to brake from a neighbour covalent bond and fill
the vacancy. However, another vacancy is created with the same property.
It can be observed that a vacancy has the following properties:
- It attracts an electron; therefore it has the equivalent to an electron positive charge.
- It ‘moves’ in random within the crystal.
The above properties satisfy the definition for a current carrier. Such current carriers in SMCMs are called
holes.
Thus in intrinsic semiconductors, unlike in conductors, there are two types of current carriers – electrons
which carry negative charge and holes which carry equal positive charge. The charge on an electron is
equal to 1.602 x 10-19 C. At any given temperature, above 0o K, the number of free electrons is equal to the
number of holes because when a free electron is produced a hole is produced, or
ni  p i ,
Where ni and pi are the electron and hole concentrations respectively.
It needs to be noted that the movement of holes is actually a movement of valence electrons in opposite
direction.
The minimum amount of energy needed to create a free electron is different for different materials. For Si it
is of minimum of 1.1 eV above the valence electron energy, for Ge it is of 0.67 eV. The values are
approximate and decrease with increase in the temperature.
The process of creating a free electron and a hole is called generation of an electron - hole pair.
In their motion within the crystal electrons collide with the atoms of the crystal and lose energy. Such
electron is ‘captured’ by a hole and both disappear as current carriers. The process is called recombination
of current carriers.
At any temperature above 0o K the two processes take place simultaneously.
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It is found that the increase in the current carrier concentration with the temperature is exponential. Small
increases in temperature lead to substantial increase in the concentration, which makes the conductivity of
intrinsic semiconductors difficult to control, as the conductivity is directly proportional to the number of
current carriers in a unit volume.
If an external voltage source is applied to the material, electrons and holes move in opposite directions, thus
producing what is known as a drift current.
Another type of current associated with SMCMs is the diffusion current. It is produced when
concentration gradient exists in the material for some reason. Concentration gradient means that there is a
higher concentration of current carriers in some part of the material compared to the rest. Electrons and
holes move in the same direction from the place of higher concentration to the place of lower
concentration. Diffusion current ceases when the distribution of current carriers within the material
becomes even.
1.2 Extrinsic Semiconductor
Intrinsic SMCMs have limited practical applications in fabricating devices for reasons stated above. In
practice they serve as a base for obtaining extrinsic semiconductors that are employed in electronic
semiconductor devices. Extrinsic means impure, of SMCMs with impurities added to the intrinsic material.
There are two types extrinsic semiconductors namely N-type and P-type.
1.2.1 N-type Semiconductor
An N-type semiconductor is obtained by introducing atoms of element of valence V to the intrinsic
semiconductor so that an impurity atom replaces a Si (or Ge) atom. Such impurities are called donors. The
most common donor atoms for Si are the phosphorus (P) atoms. (Figure 1.3).
The phosphorus atom forms covalent bonds with the four neighbour Si atoms. The fifth electron however is
not bonded and therefore lightly attached to its parent atom. A small amount of energy (approximately 0.04
eV) is needed for this electron and become free within the crystal.
The properties and characteristics for N-type semiconductors can be summarized as follows:
At 0o K there are no current carriers in the material. It is an insulator.
- When a free electron from a donor atom is produced, no hole is produced.
- A donor atom, which has lost an electron, becomes a positive ion. Such ion is not a current carrier.
- The material is electrically neutral at any temperature.
- The electrons in an N-type semiconductor are called majority current carriers. Their concentration is
denoted as nn.
- The concentration of free electrons is almost constant over a wide temperature range (up to
approximately 150o C for Si and 80o C for Ge) and equal to the number of donors in the material.
- Further increase in temperature leads to generation of electron-hole pairs. Holes in an N-type
semiconductor are called minority current carriers, pn.
- At very high temperatures the concentration of minority current carriers exceeds the concentration of
minority current carriers many times and the material becomes an intrinsic.
- The practical use of N-type SMCMs is limited to the certain temperature region where the conductivity
is relatively constant and can be controlled.
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Si (or Ge) core atom
Free electron
P
Hole
P
Donor (phosphorus) atom
Figure 1.3
1.2.2 P-Type Semiconductor
The description for an N-type semiconductor is to a large extend is valid for a P-type semiconductor.
To obtain a P-type semiconductor an element of valence III (as boron, B or aluminum, Al) is introduced to
the material, so that an impurity atom replaces a Si (or Ge) atom in the crystal. Such impurities are called
acceptors. The acceptor atom forms covalent bonds with three neighbour Si atoms. The fourth covalent
bond is incomplete. A very small amount of energy (approximately 0.045 eV) is needed for an electron
from a neighbour Si atom to move and complete the covalent bond. Thus a hole is created in the material
and the acceptor atom becomes a negative ion. No free electron is produced.
In a P-type semiconductor the holes are majority current carriers (pp), the electrons are the minority current
carriers (pn). The practical use is also restricted to a certain temperature region.
1.3 PN Junction
A PN junction is formed when in a crystal of semiconductor material both N-type and P-type areas are
formed next to each other. The border between the two areas is called a PN Junction. Another way of
illustrating a PN junction is to assume that a P-type and N-type semiconductors are brought into contact as
shown in Figure 1.4.
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P-side
+
+
N-side
+
+
Positive ion
+
+
+
+
+
Negative ion
+
+
+
+
+
+
+
+
+
+
+
+
+
Free electron
+
+
+
+
+
+
+
+
+
+
+
Hole
+
PN junction
Figure1.4
It can be seen that in the P-side of the junction there are more holes than in the N-side of the junction.
Similarly, there are more electrons in the N-side of the junction than in the P-side. As soon a contact is
formed between the two parts two diffusion currents start flowing; one from electrons with direction from
N- to P-side, and one from holes in opposite direction.
The first to cross the border are the electrons from the N-side closest to the junction. Each electron moving
from the N-side to the P-side leaves behind a positive impurity ion. A layer of such ions is formed on the
N-side of the junction. At the P-side, the electrons recombine with holes closest to the junction. A layer of
negative impurity ions is formed next to the junction.
Vb
P-side
+
Eint
+
N-side
+
+
+
+
+
positive ion
+
negative ion
+
+
+
+
+
+
+
+
+
+
free electron
+
+
+
+
+
+
+
+
+
+
+
hole
+
Depletion layer
Figure 1.5
Neutral areas
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The layer of positive and negative ions give rise to an electric field, called internal electric field (Eint,
Fig.1.5) and potential difference, called a barrier potential (Vb). The barrier potential prevents the further
motion of majority current carriers across the junction and the diffusion currents cease. The barrier
potential however, is not a barrier for the minority current carriers. As their concentration in both sides of
the junction is the same, the net current from minority current carriers across the junction is zero.
The layer formed by the positive and negative ions is called a depletion layer, as there are no current
carriers in it. It could symmetrical or non-symmetrical depending on the doping levels of the N- and P-side.
The resistance of the depletion layer is very high, of order of MΩs.
The areas outside the depletion layer are called neutral areas. Their resistance is low, of order of Ωs.
1.3.1 Biasing PN Junction
Applying external voltage to a PN junction is known as biasing. There are two ways to apply voltage to a
PN junction:
- The positive terminal is connected to the P-side of the junction, the negative terminal is connected to
the N-side of the junction. It is known as reverse biased PN junction.
- The positive terminal is connected to the N-side of the junction, the negative terminal is connected to
the P-side of the junction. It is known as forward biased PN junction.
1.3.1.1 Forward Biased PN Junction
Forward-biased PN junction is illustrated in Figure 1.6.
Vb
P
_
_
_
_
_
_
R
+
+
+
+
+
+
N
E
Figure 1.6
As the resistance of the neutral areas is much smaller than the resistance of the depletion layer, the entire of
the external voltage (potential difference) E drops on it. Thus there are two voltages across the junction
acting in opposite direction. With increase in the external voltage from 0 V the resultant voltage drop
across the junction decreases. More majority current carriers have enough energy to overcome the reduced
potential barrier. As a result a drift current flows through the junction whose value increases exponentially.
When the external voltage reaches the barrier potential value (0.7 V), the potential barrier disappears and
large current flows through the junction limited only by the resistance of the neutral areas. Even very small
further increases in the source voltage lead to large increase in the current.
It practice a current-limiting resistor (R) should be connected between the source and the junction to
prevent it from destruction.
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The change in the barrier potential does not effect the movement of minority current carriers through the
junction.
1.3.1.2 Reverse Biased PN Junction
When a PN junction is reverse biased, the external and the barrier voltage across the junction act in the
same direction. The resultant potential barrier for the majority current carriers increases and the current is
practically zero.
The current from minority current carriers is however not affected as their concentration is constant at any
given temperature (> 0o K). The current through a reverse biased PN junction is called leakage or
saturation current (Is). It has much smaller value compared to the current in forward direction and is
constant for a large range of voltages at certain temperature.
1.4 Review Questions
1.
2.
3.
4.
5.
6.
7.
8.
Explain the process of generation of electrons and holes in an intrincis semiconductor material
using the simplified structures.
Explain the term ‘recombination of current carriers’.
Explain how majority current carriers are generated in:
(a) An N-type semiconductor.
(b) A P-type semiconductor.
Explain the properties of an N-type semiconductor at temperature:
(a) 0 oK.
(b) 300 oK
(c) 300 oC
Discuss the conductivity of an intrinsic and extrinsic semiconductor as a function of the
temperature.
Define the following currents associated with semiconductor materials:
(a) Drift current
(b) Diffusion current
Give a definition for a PN junction and explain the formation of the depletion layer.
Explain the behaviour of a PN junction when:
(a) Forward biased.
(b) Reverse biased.
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