Semiconductors and the Electronics Mission: Notes

The correct answer is C, about 1.1 eV. The energy gap between the valence band and the conduction band decides whether a solid conducts: in silicon the gap is roughly 1.1 electron volts, small enough for heat or light to lift some electrons across it, which is what makes silicon a semiconductor. Option A is wrong because 0.3 volt is the forward barrier voltage of a germanium diode, a different quantity altogether. Option B is wrong because about 0.7 eV is the band gap of germanium, the other elemental semiconductor, and it is also the forward voltage drop of a silicon diode, which is why candidates confuse the two numbers. Option D is wrong because about 5.5 eV is the gap in diamond, an insulator. Learn the trio together: germanium 0.7 eV, silicon 1.1 eV, gallium arsenide about 1.43 eV.

The correct answer is C, the number of holes equals the number of electrons. In an intrinsic, that is a chemically pure, semiconductor every free electron is produced by breaking a covalent bond, and breaking a bond leaves behind exactly one hole, so electrons and holes are created strictly in pairs and their concentrations must be equal. Option A is wrong because a product of two concentrations cannot equal a bare number one; the product is fixed by temperature and the band gap, not by unity. Option B is wrong because holes can outnumber electrons only in a p-type crystal, which needs a trivalent dopant. Option D is wrong because electrons outnumber holes only in an n-type crystal, which needs a pentavalent dopant. Equal numbers are therefore the signature of the pure crystal, and any inequality means doping has taken place.

The correct answer is C, phosphorus. Silicon has four valence electrons, so adding a pentavalent atom such as phosphorus, arsenic, antimony or bismuth leaves one electron over after four bonds are formed; that spare electron is loosely held and becomes a free carrier, making electrons the majority carriers and the crystal n-type. Option A is wrong because boron is trivalent and creates a vacancy, giving a p-type crystal. Option B is wrong because aluminium is also trivalent and is another p-type dopant. Option D is wrong because indium is trivalent as well. Note that an n-type crystal is not negatively charged: the donor atom keeps its positive nucleus, so the crystal as a whole stays electrically neutral, and examiners test exactly this point in statement-based questions.

The correct answer is B, holes. A trivalent dopant such as boron, aluminium, gallium or indium forms only three covalent bonds with the surrounding silicon atoms, leaving one bond incomplete; that vacancy behaves as a mobile positive carrier called a hole, and holes therefore become the majority carriers while thermally generated electrons remain the minority carriers. Option A is wrong because electrons are the majority carriers in an n-type crystal and only the minority carriers here. Option C is wrong because protons are bound inside nuclei and never move through a crystal lattice. Option D is wrong because the dopant ions are fixed in the lattice and cannot carry current, even though they are charged. Remember also that a p-type crystal is electrically neutral overall, exactly like an n-type crystal.

The correct answer is A, Bardeen, Brattain and Shockley. John Bardeen, Walter Brattain and William Shockley built the first working transistor at Bell Laboratories in the United States in 1947, and the three shared the Nobel Prize in Physics in 1956 for the discovery of the transistor effect; the device replaced the bulky vacuum tube and made all later electronics possible. Option B is wrong because Jack Kilby and Robert Noyce independently developed the integrated circuit in 1958 and 1959, and Gordon Moore gave the industry its famous doubling rule. Option C is wrong because Faraday, Maxwell and Hertz belong to the history of electromagnetism. Option D is wrong because Townes, Basov and Prokhorov won the Nobel Prize of 1964 for work on the maser and the laser.

The correct answer is A, collector current to emitter current. In a bipolar junction transistor the emitter injects carriers, the thin lightly doped base lets most of them through, and the collector gathers them, so the emitter current divides into a large collector current and a small base current; alpha is defined as the collector current divided by the emitter current and is therefore always a little less than one, typically between 0.95 and 0.99. Option B is wrong because it inverts the ratio and would give a value greater than one. Option C is wrong because the base to collector ratio is a small fraction with no standard name. Option D is wrong because the collector current divided by the base current is beta, the current gain, which runs into the tens or hundreds. Keep alpha below one and beta well above one.

The correct answer is A, the collector current divided by the base current. Beta is the current gain of a bipolar junction transistor in the common emitter configuration: a very small change in the base current produces a much larger change in the collector current, and the ratio of the two is beta, which commonly lies between about twenty and several hundred. This is why a transistor can act as an amplifier. Option B is wrong because the base current divided by the emitter current is a small fraction with no standard symbol. Option C is wrong because it inverts the alpha ratio. Option D is wrong because the collector current divided by the emitter current is alpha, which is slightly less than one. The two quantities are linked, since beta equals alpha divided by one minus alpha.

The correct answer is B, the doubling of the number of transistors on a chip over a regular period. Gordon Moore, a co-founder of Intel, observed in 1965 that the number of components that could be packed economically into an integrated circuit was doubling at a steady rate, and the observation, later quoted as a doubling roughly every two years, became the planning rule of the whole industry. Option A is wrong because the behaviour of a pure semiconductor at absolute zero, where it acts as an insulator, follows from band theory and not from any law of Moore. Option C is wrong because the blocking voltage of a diode is its breakdown rating. Option D is wrong because electron speed in a vacuum tube belongs to an earlier technology that the transistor replaced. Moore's law is an empirical trend, not a law of physics.

The correct answer is C, the Ministry of Electronics and Information Technology. The India Semiconductor Mission was set up to carry out the Semicon India programme, which the Union Cabinet approved in December 2021 with an outlay of 76,000 crore rupees, and it works as a specialised division under this ministry, handling the schemes for semiconductor fabs, display fabs, compound semiconductor and packaging units, and design linked incentives. Option A is wrong because the Ministry of Heavy Industries handles capital goods and the incentive scheme for electric vehicles. Option B is wrong because the Ministry of Science and Technology runs research departments such as the Department of Science and Technology and the Department of Biotechnology. Option D is wrong because the Ministry of Commerce and Industry handles trade policy and the wider Make in India effort rather than this mission.

The correct answer is B, Assembly, Testing, Marking and Packaging. Chip making has three broad stages: design, which is done by fabless companies and design houses; fabrication, in which circuits are built on a silicon wafer in a fab; and the back end, in which the wafer is cut into individual chips that are then assembled, tested, marked and packaged, which is what ATMP means. Firms that do only this back end work for others are called OSAT units, that is outsourced semiconductor assembly and test. Option A is wrong because it is an invented expansion. Option C is wrong because thermal metal plating is a different industrial process. Option D is wrong because it too is invented. India's first approved projects under the Semicon India programme were largely of this back end kind, which makes the term examinable.

The correct answer is A, Mohali. The Semi-Conductor Laboratory at Mohali in Punjab works under the Ministry of Electronics and Information Technology and is India's own fabrication unit, producing chips at an older process node for strategic users such as the Indian Space Research Organisation and the Defence Research and Development Organisation rather than for the consumer market. Option B is wrong because Hyderabad is a large centre of chip design and of pharmaceutical manufacturing, not the home of this laboratory. Option C is wrong because Bengaluru holds the greatest concentration of semiconductor design centres in India but not this government fab. Option D is wrong because Pune is an engineering and software hub without such a facility. The distinction between a design centre and a fabrication plant is itself a favourite examination point.

The correct answer is B, decreases. Heating a semiconductor gives more electrons enough energy to cross the forbidden gap into the conduction band, and each one that crosses leaves a hole behind, so the number of charge carriers rises sharply and the resistance falls; a semiconductor therefore has a negative temperature coefficient of resistance. Option A is wrong because resistance rises with temperature in metals, where the carrier number is already fixed and heating only increases collisions, and candidates lose the mark by carrying the metal rule over to semiconductors. Option C is wrong because the change is large and easily measured. Option D is wrong because resistance never becomes zero; that is superconductivity, a separate phenomenon seen in certain materials below a critical temperature. At absolute zero a pure semiconductor behaves as an insulator.