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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 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. electrons . One valence electron is shared with each of four surrounding atoms in a
tetrahedral 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 S. S. Calbio
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binding to its nucleus. The covalent bond is then broken. The electron lea ves the atom, B say, and becomes a free a free electron. 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 attra cts 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 opposite direction to the electron, is equivalent to a positive charge +e. +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 ‘collision s’ 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 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. S. S. Calbio
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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) (electr on) 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 carriers in an n-semiconductor are electrons. Positive charges or holes are also present in an n-semiconductor, Figure 2. These are thermally generated, a s previously explained, and since they are relatively few they are called the minority carriers. carriers. The impurity (arsenic) atoms are called donors because 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 electrone lectron-hole pairs are formed. But from 250 K to about 650 K, σ decreases with temperature rise. In this temperature range the increase in e lectron-hole pairs is more than counteracted by the increase in the metal lattice vibration, which oppose the motion of the carriers. S. S. Calbio
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P-Semiconductors 1st video 2:40- 3:26 P-semiconductors are P-semiconductors are made by adding foreign atoms which are trivalent to 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 and forming a hole in 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, 2 nd video 0-1:23, 3 rd 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 ca uses 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, 7:29, 2 nd video 1:23-2:39, 3 rd 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 opposes more diffusion of charges across the junction. This is called a barrier p.d. and when the flow S. S. Calbio
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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 p -n junction which contains the negative and positive charges is called the t he depletion layer . The width of the depletion layer la yer is of the order 10-3 mm.
Junction Diode as Rectifier 1st video 7:30- 8:31, 2 nd video 2:41-4:07 When a battery B, with an e.m.f. e. m.f. greater than the barrier p.d., is joined with its positive pole to the psemiconductor, 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 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 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 reversed-biased. This time only the minority carriers, 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 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 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. S. S. Calbio
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