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S. S. Calbio http://www.izifundo.weebly.com 2015

BISHOP ANSTEY HIGH SCHOOL & TRINITY COLLEGE EAST

SIXTH FORM

CXC CAPE PHYSICS, UNIT 2

Ms. S. S. CALBIO – LESSON #24 notes 11/11/2015

Semiconductors and Diodes

Semiconductors, Electron and Hole Charge Carriers 1st video up to 1:45s

Semiconductors are a class of materials which have a resistivity about ten million times higher

than that of a good conductor such as copper. Silicon and germanium are examples of semiconductor

elements widely used in the electronics industry.

Silicon and germanium atoms are tetravalent. They have four electrons in their outermost shell,

called valence electrons. One valence electron is shared with each of four surrounding atoms in a

tetrahedral arrangement, forming ‘covalent bonds’ which maintain the crystalline solid structure. Figure

1(i) is a two-dimensional diagram of the structure, showing four silicon atoms, each having four valence

electrons round them.

Figure 1

At 0K, all the valence electrons are firmly bound to the nucleus of their particular atoms. At room

temperature, however, the thermal energy of a valence electron may become greater than the energy

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binding to its nucleus. The covalent bond is then broken. The electron leaves the atom, B say, and

becomes a free electron. This leaves B with a vacancy or hole, Figure 1(ii). Since B now has a net positive

charge, an electron in a neighbouring atom C may then be attracted. So the hole appears to move to C. So

C now has a positive charge and therefore attracts an electron from D. This leaves D with a hole.

In this way we can see that, due to the movement of valence electrons from atom to atom, holes

spread throughout the semiconductor. Since an electron carries a negative charge –e, a hole, moving in

the opposite direction to the electron, is equivalent to a positive charge +e.

So moving holes are equivalent to moving positive charges +e.

Summarising: In semiconductors, then, there are two kinds of charge carriers: a free

electron (-e) and a hole (+e). In contrast, a metal such as pure copper, a good conductor, has only

one kind of charge carrier, the free electron. In semiconductors, the escape of a valence (bound)

electron from an atom produces electron-hole pairs of charge carriers.

Effect of Temperature Rise

1st video 1:10 – 1:43

As we have seen, the charge carriers in a metal such as copper are only free electrons. As the

temperature of the metal rises, the amplitude of vibration of the atoms increases and more ‘collisions’

with atoms are then made by the drifting electrons. So the resistance of a pure metal increases with

temperature rise.

In the case of a semiconductor, however, the increase in thermal energy of the valence electrons

due to temperature rise enables more of them to break the covalent bonds and become free electrons.

Thus more electron-hole pairs are produced which can act as carriers of current. Hence, in contrast to a

pure metal, the electrical resistance of a pure semiconductor decreases with temperature rise. This is one

way of distinguishing between a pure metal and a pure semiconductor. Note that the pure or intrinsic

semiconductor always has equal numbers of electrons and hole, whatever its temperature.

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N-Semiconductors

1st video 1:45- 2:40

A pure or intrinsic semiconductor has charge carriers which are thermally generated. These are

relatively few in number. By ‘doping’ a semiconductor with a tiny amount of impurity such as one part in

a million, forming a so-called extrinsic or impure semiconductor, an enormous increase can be made in

the number of charge carriers. The impure semiconductor is widely used in the electronics industry.

Arsenic atoms for example, have five electrons in their outermost or valence band. When an atom

of arsenic is added to a silicon crystal, the atom settles in a lattice site with four of its electrons shared

with neighbouring silicon atoms. The fifth electron may thus become free to wander through the crystal.

Since an impurity atom may provide one free electron. For example, 1 milligram of arsenic has about

8 x 1018 atoms and so provides this large number of free electrons in addition to the relatively small

number of thermally-generated electrons.

Since there are a great number of negative (electron) charge carriers, the impure semiconductor is

called an ‘n-type semiconductor’ or n-semiconductor, where ‘n’ represents the negative charge on an

electron. Thus the majority carriers in an n-semiconductor are electrons. Positive charges or holes are

also present in an n-semiconductor, Figure 2. These are thermally generated, as previously explained, and

since they are relatively few they are called the minority carriers. The impurity (arsenic) atoms are called

donors because they donate electrons as carriers.

Figure 2: N- semiconductor

The conductivity σ of an n-semiconductor increases with temperature from very low

temperatures to about 250 K as more electron-hole pairs are formed. But from 250 K to about 650 K, σ

decreases with temperature rise. In this temperature range the increase in electron-hole pairs is more

than counteracted by the increase in the metal lattice vibration, which oppose the motion of the carriers.

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P-Semiconductors

1st video 2:40- 3:26

P-semiconductors are made by adding foreign atoms which are trivalent to pure germanium or silicon.

Examples are boron or iridium. In this case the reverse happens to that previously described. Each

trivalent atom in a lattice site attracts an electron from a neighbouring atom, thereby completing the four

valence bonds and forming a hole in the neighbouring atom. In this way an enormous increase occurs in

the number of holes. So in a p-semiconductor, the majority carriers are holes or positive charges. The

minority carriers are electrons, negative charges, which are thermally generated, Figure 3. The impurity

atoms are called acceptors in this case because each ‘accepts’ an electron when the atom is introduced

into the crystal.

Figure 3: P- semiconductor

Summarising:

In an n-semiconductor, conduction is due mainly to negative charges or electrons, with

positive charges (holes) as minority carriers.

In a p-semiconductor, conduction is due mainly to positive charges or holes, with negative

charges (electrons) as minority carriers.

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P-N Junction

1st video 3:30- 4:57, 2nd video 0-1:23, 3rd video 0:1:29

By a special manufacturing process, p- and n- semiconductors can be melted so that a boundary or

junction is formed between them. This junction is extremely thin and of the order 10-3 mm. It is called a

p-n junction, Figure 4.

Figure 4: p-n junction

When a scent bottle is opened, the high concentration of scent molecules in the bottle cause the

molecules to diffuse into the air. In the same way, the high concentration of holes (positive charges) on

one sides of a p-n junction, and the high concentration of electrons on the other side causes the two

carriers to diffuse respectively to the other side of the junction, as shown diagrammatically in Figure 4.

1st video 6:09- 7:29, 2nd video 1:23-2:39, 3rd video 1:29-3:10

The electrons which move to the p-semiconductor side recombine with holes there. These holes

therefore disappear and an excess negative charge A appears on this side, as shown in Figure 5.

Figure 5: barrier p.d.

In a similar way, an excess of positive charge builds up in the n-semiconductor when holes diffuse across

the junction. Together with the negative charge A on the p-side, an e.m.f. or p.d. is produced which

opposes more diffusion of charges across the junction. This is called a barrier p.d. and when the flow

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ceases it has a magnitude of a few tenths of a volt. The narrow region or layer at the p-n junction which

contains the negative and positive charges is called the depletion layer. The width of the depletion layer is

of the order 10-3 mm.

Junction Diode as Rectifier

1st video 7:30- 8:31, 2nd video 2:41-4:07

When a battery B, with an e.m.f. greater than the barrier p.d., is joined with its positive pole to the p-

semiconductor, P, and its negative pole to the n-semiconductor, N, p-charges (holes) are urged across the

p-n junction from P to N and n-charges (electrons) from N to P, Figure 6.

Figure 6: Junction Diode characteristics (i)

We can understand the movement if we consider the +ve pole of the battery to repel +ve charges (holes)

in the p-semiconductor and the –ve pole to repel the –ve charges (electrons) in the n-semiconductor. So

an appreciable current OA is obtained. The p-n junction is now said to be forward-biased, and when the

applied p.d. is increased, the current increases along the curve OA.

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Figure 7: Junction Diode characteristics (ii)

When the poles of the battery are reversed, only a very small current flows, Figure 7. In this case the

p-n junction is said to be reversed-biased. This time only the minority carriers, negative charges in the

p-semiconductor and positive charges in the n-semiconductors, are urged across the p-n junction by the

battery. Since the minority carriers are thermally-generated, the magnitude of the reverse current OB

depends on the temperature of the semiconductors. It may also be noted that the reverse-bias p.d.

increases with the width of the depletion layer, since it urges more electrons in the p-semiconductor and

holes in the n-semiconductor further away from the junction.

Figure 8: Junction Diode characteristics (iii)

The characteristic (I-V) curve AOB in Figure 8 shows that the p-n junction acts as a rectifier. It has

a low resistance in one direction of p.d., + V, and a high resistance in the opposite p.d. direction, -V.

It is called a junction diode. In the diode symbol in Figure 6, the low resistance is from left to right

(towards the triangle point) and the high resistance is in the opposite direction. The junction diode has

several advantages, for example, it needs only a low voltage battery B to work; it does not need time to

warm up; it is not bulky; and it is cheap to manufacture in large numbers.

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