Semiconductors and Basic Electronics: Exam Notes

The correct answer is B, decreases. A semiconductor has a small forbidden energy gap, so heating supplies thermal energy that pushes more electrons from the valence band into the conduction band, creating more electron-hole pairs. More charge carriers mean higher conductivity, and therefore lower resistance, which is why a semiconductor is said to have a negative temperature coefficient of resistance. Option A is wrong because resistance rises with temperature in metals, where the carriers are already free and increased lattice vibration merely obstructs them; students apply the metallic rule here by mistake. Option C is wrong because the resistance change in a semiconductor is in fact large and is used in thermistors. Option D is wrong because the resistance never becomes zero; it falls steadily but a semiconductor does not become a superconductor on heating.

The correct answer is C, arsenic. An n-type semiconductor is made by doping with a pentavalent element, that is one with five valence electrons, such as phosphorus, arsenic, antimony or bismuth. Four of the five electrons form covalent bonds with neighbouring silicon atoms and the fifth is left almost free, so the impurity acts as a donor and electrons become the majority carriers. Option A is wrong because boron is trivalent and gives p-type material. Option B is wrong because indium is also trivalent and is a classic acceptor impurity. Option D is wrong for the same reason, as gallium has three valence electrons. The memory aid is that pentavalent means donor and n-type, while trivalent means acceptor and p-type, and the doped crystal stays electrically neutral in both cases.

The correct answer is B, holes. A p-type semiconductor is made by adding a trivalent impurity such as boron, aluminium, gallium or indium. Because the impurity atom has only three valence electrons, one covalent bond is left incomplete and a vacancy called a hole is created; the impurity therefore acts as an acceptor and holes are the majority carriers, while the few thermally generated electrons are the minority carriers. Option A is wrong because electrons are the majority carriers in n-type material, not p-type. Option C is wrong because protons are bound in the nuclei and never act as charge carriers in a solid. Option D is wrong because the acceptor ions are fixed in the crystal lattice and cannot move, so they carry no current even though they are negatively charged.

The correct answer is B, about 1.1 eV. Silicon is the most widely used semiconductor and its forbidden energy gap, the separation between the top of the valence band and the bottom of the conduction band, is about 1.1 electron volts at room temperature. Option A is wrong because 0.7 electron volts is the band gap of germanium, the other elemental semiconductor, and the two values are the pair most often swapped in examinations. Option C is wrong because a gap of that size belongs to a wide band gap material rather than to silicon. Option D is wrong because 5.5 electron volts is the band gap of diamond, which is quoted as the standard example of an insulator. Gallium arsenide, for comparison, has a gap of about 1.43 electron volts.

The correct answer is A, rectifier. A p-n junction conducts freely in forward bias, where the potential barrier is reduced, and blocks current in reverse bias, where only a tiny leakage current flows. This one-way conduction is exactly what is needed to turn alternating current into direct current, so the diode's main use is as a rectifier, in half-wave, full-wave or bridge arrangements. Option B is wrong because amplification needs a three-terminal device such as a transistor; a two-terminal diode cannot supply power gain. Option C is wrong because an oscillator requires an amplifier with positive feedback, although special diodes can support oscillation. Option D is wrong because a transformer works by mutual induction between coils, handles only alternating current, and contains no semiconductor at all.

The correct answer is C, Zener diode. A Zener diode is heavily doped so that breakdown occurs at a low and sharply defined reverse voltage. Beyond that point the voltage across it stays almost constant even when the current through it changes a great deal, which is precisely the property a voltage regulator needs. Option A is wrong because a light emitting diode is worked in forward bias and converts electrical energy into light. Option B is wrong because a photodiode is indeed reverse biased, but it is used to detect light rather than to hold a voltage steady, and it is not operated in breakdown. Option D is wrong because a solar cell works with no external bias at all and generates its own electromotive force from incident light.

The correct answer is B, base. The base of a transistor is made very thin and only lightly doped so that most of the carriers injected by the emitter pass straight through it to the collector instead of recombining in it. This is what gives the transistor its large current transfer ratio. Option A is wrong because the emitter is the most heavily doped region, since its job is to supply carriers in plenty. Option C is wrong because the collector is moderately doped and is made physically the largest region so that it can dissipate the heat generated there. Option D is wrong because unequal doping is essential; it is also the reason a transistor cannot be built by simply joining two separate diodes back to back, since no thin common base would exist.

The correct answer is B, NAND and NOR. They are called universal gates because either one of them, used repeatedly, can build every other logic gate, including NOT, AND, OR, XOR and XNOR. This matters in practice because a manufacturer can lay down a whole integrated circuit using only one kind of gate. Option A is wrong because AND and OR, along with NOT, are the basic gates, and AND and OR by themselves cannot produce an inversion. Option C is wrong for the same reason; OR and NOT together can produce NOR and from it the rest, but the two gates are not individually universal in the sense asked. Option D is wrong because XOR and XNOR are derived gates built from the basic ones and are not universal.

The correct answer is C, 100 Hz. A full-wave rectifier conducts during both halves of the input cycle, since one pair of diodes handles the positive half and the other pair the negative half. The output therefore contains two pulses for every cycle of the input, so its fundamental ripple frequency is twice the input frequency, that is 100 hertz for a 50 hertz supply. Option A is wrong because the frequency is doubled, not halved. Option B is wrong because 50 hertz is the ripple frequency of a half-wave rectifier, which conducts in only one half of each cycle; this is the standard pair of values examiners test. Option D is wrong because the output of a rectifier is pulsating direct current and its ripple is zero only after a filter has smoothed it.

The correct answer is C, Bardeen, Brattain and Shockley. John Bardeen, Walter Brattain and William Shockley built the first working transistor at Bell Laboratories in 1947 and shared the Nobel Prize in Physics in 1956 for the discovery of the transistor effect. Option A is wrong because Jack Kilby and Robert Noyce are associated with the integrated circuit, developed from 1958, for which Kilby received the Nobel Prize in Physics in 2000. Option B is wrong because J. J. Thomson discovered the electron and Rutherford the nucleus, both long before electronics. Option D is wrong because Faraday and Maxwell belong to the nineteenth century work on electromagnetism. The word transistor itself is formed from transfer and resistor. The device they built was a point-contact transistor in germanium, and Shockley went on to describe the junction transistor that made mass production possible; the three names are usually asked together as a set.

The correct answer is C, it remains electrically neutral in both cases. Doping replaces a few host atoms with impurity atoms, and each impurity atom brings its own nucleus along with its electrons, so the total positive and negative charge in the crystal stays balanced. The names n-type and p-type describe which carrier is in the majority, not an overall charge. Option A is wrong because the extra electron contributed by a donor is matched by the extra proton in the donor nucleus. Option B is wrong for the same reason on the acceptor side. Option D is wrong because doping raises conductivity sharply, often by several orders of magnitude, which is the whole purpose of the process and the reason extrinsic semiconductors, not pure ones, are used in devices.