301. In an \(LED\), the colour of emitted light is mainly determined by:
ⓐ. the length of the external connecting wire only
ⓑ. the reverse saturation current in darkness
ⓒ. the band gap of the semiconductor material
ⓓ. the collector current of a transistor
Correct Answer: the band gap of the semiconductor material
Explanation: In an \(LED\), photons are produced when electrons and holes recombine. The energy of the emitted photons is related to the energy gap of the semiconductor material. A larger band gap generally corresponds to higher photon energy and shorter wavelength. This is why different semiconductor materials are used for different \(LED\) colours. The colour is not decided mainly by the external wire length or by transistor current.
302. A semiconductor used for an \(LED\) has a larger band gap than another \(LED\) material. The emitted light is expected to have:
ⓐ. lower photon energy and longer wavelength
ⓑ. zero photon energy in every case
ⓒ. higher photon energy
ⓓ. no relation with the band gap
Correct Answer: higher photon energy
Explanation: The photon energy emitted by an \(LED\) is related to the band gap energy. If the band gap is larger, the recombination energy available to the photon is larger. Higher photon energy corresponds to shorter wavelength because \(E=\frac{hc}{\lambda}\). Therefore, the material with the larger band gap tends to emit shorter-wavelength light. This is a qualitative band-gap relation, not a statement that every recombination process is perfectly identical.
303. A solar cell is best described as a device that:
ⓐ. emits light when forward biased
ⓑ. detects light only by reverse-bias leakage current
ⓒ. regulates voltage only in reverse breakdown
ⓓ. converts light directly into electrical energy
Correct Answer: converts light directly into electrical energy
Explanation: A solar cell is a photovoltaic device. When light falls on its \(p\)-\(n\) junction, electron-hole pairs are generated. The internal junction field separates these carriers and produces an emf across the device. It can deliver electrical power to an external load without needing an external bias source for its basic action. This distinguishes it from a photodiode detector and from an \(LED\) emitter.
304. A solar cell usually operates without external bias because:
ⓐ. it is just a reverse-biased Zener diode
ⓑ. it needs forward bias to emit photons
ⓒ. junction field separates carriers
ⓓ. it has no depletion region when illuminated
Correct Answer: junction field separates carriers
Explanation: In a solar cell, incident light generates electron-hole pairs in the semiconductor. The built-in electric field near the junction separates these carriers. This separation creates a potential difference and can drive current through an external load. No external bias is required for the basic photovoltaic action. The device uses light as the energy input, unlike an \(LED\), which uses electrical energy to produce light.
305. Match the optoelectronic device with its usual operating idea.
| Column I | Column II |
|---|
| P. Photodiode | 1. Forward-biased light emitter |
| Q. \(LED\) | 2. Reverse-biased light detector |
| R. Solar cell | 3. Photovoltaic power source without external bias |
| S. Zener diode regulator | 4. Reverse-breakdown voltage stabiliser |
The suitable matching is:
ⓐ. P-1, Q-2, R-4, S-3
ⓑ. P-2, Q-1, R-3, S-4
ⓒ. P-2, Q-3, R-1, S-4
ⓓ. P-4, Q-1, R-2, S-3
Correct Answer: P-2, Q-1, R-3, S-4
Explanation: A photodiode is usually operated in reverse bias and used as a light detector. An \(LED\) is operated in forward bias and emits light due to electron-hole recombination. A solar cell uses the photovoltaic effect to convert light energy into electrical energy without an external bias for its basic operation. A Zener diode regulator uses reverse breakdown to maintain nearly constant voltage. The comparison is based on energy conversion and bias condition, not only on all being junction devices.
306. A device is needed for automatic switching of a street light when the surrounding light level changes. The most suitable semiconductor device from the given choices is:
ⓐ. \(LED\)
ⓑ. photodiode
ⓒ. Zener diode used only as a regulator
ⓓ. ordinary forward-biased rectifier diode
Correct Answer: photodiode
Explanation: A photodiode is designed to respond to incident light. Its reverse current changes with light intensity, so it can provide an electrical signal related to illumination level. Such a signal can be used in a control circuit for automatic switching. An \(LED\) emits light instead of detecting it, and a Zener diode mainly provides voltage regulation in reverse breakdown. The detector must produce an electrical response to light, so the photodiode is the natural choice.
307. A bipolar junction transistor is called “bipolar” because its operation involves:
ⓐ. only electrons as charge carriers
ⓑ. only holes as charge carriers
ⓒ. only fixed donor and acceptor ions as moving carriers
ⓓ. both electrons and holes as charge carriers
Correct Answer: both electrons and holes as charge carriers
Explanation: A bipolar junction transistor involves both kinds of mobile charge carriers: electrons and holes. In an \(npn\) transistor, electron flow is dominant, while in a \(pnp\) transistor, hole flow is dominant. The word bipolar does not mean that both carriers are always present in equal amounts. It means that the device action depends on both carrier types in the semiconductor structure. Fixed ions help set up junction behaviour, but they do not move through the device as ordinary current carriers.
308. A transistor contains three semiconductor regions arranged as \(n\)-\(p\)-\(n\). This device is:
ⓐ. a \(pnp\) transistor
ⓑ. an intrinsic semiconductor slab
ⓒ. an \(npn\) transistor
ⓓ. a single \(p\)-\(n\) junction diode
Correct Answer: an \(npn\) transistor
Explanation: The name of a bipolar junction transistor follows the order of its three semiconductor regions. An \(n\)-\(p\)-\(n\) arrangement is called an \(npn\) transistor. A \(p\)-\(n\)-\(p\) arrangement would be called a \(pnp\) transistor. A single diode has only one \(p\)-\(n\) junction, while a transistor has two junctions. The middle region is the base, so the order of regions also helps identify the transistor type.
309. In a junction transistor, the three terminals are:
ⓐ. emitter, base, and collector
ⓑ. anode, cathode, and gate
ⓒ. source, drain, and filament
ⓓ. positive plate, negative plate, and dielectric
Correct Answer: emitter, base, and collector
Explanation: A bipolar junction transistor has three terminals called emitter, base, and collector. These terminals are connected to three semiconductor regions of the device. Anode and cathode are diode terminals, so they do not describe all three transistor terminals. Source and drain are more closely associated with field-effect transistor terminology, not the basic junction transistor introduced here. The three-terminal structure is what allows a small base-related input to control a larger collector-related current.
310. Use the arrangement described below.
A semiconductor device has three regions in the order \(p\)-\(n\)-\(p\). The middle region is very thin and is connected to a terminal used for control.
The middle region is called the:
ⓐ. base
ⓑ. emitter
ⓒ. collector
ⓓ. cathode
Correct Answer: base
Explanation: In both \(npn\) and \(pnp\) transistors, the middle region is called the base. The base is made very thin so that most carriers injected from the emitter can pass across it toward the collector. It is not the cathode, because cathode is a diode terminal name. The emitter and collector are the two outer transistor regions. The controlling role of the middle region is why the base is central to transistor action.
311. The emitter of a bipolar junction transistor is usually heavily doped because its main role is to:
ⓐ. collect heat from the external circuit only
ⓑ. remain completely inactive during transistor operation
ⓒ. inject majority carriers into the base
ⓓ. block all carriers from entering the base
Correct Answer: inject majority carriers into the base
Explanation: The emitter is designed to supply or inject majority carriers into the base region. Heavy doping gives the emitter a large concentration of carriers available for injection. In an \(npn\) transistor, it injects electrons; in a \(pnp\) transistor, it injects holes. The base is made thin and lightly doped so that only a small fraction of these carriers recombine there. The emitter is therefore not a passive end terminal; it is the strong carrier-supplying region.
312. The base of a transistor is made thin and lightly doped mainly so that:
ⓐ. all injected carriers recombine inside the base
ⓑ. most injected carriers reach collector
ⓒ. the device becomes a single diode
ⓓ. the collector can be removed from the circuit
Correct Answer: most injected carriers reach collector
Explanation: The base lies between the emitter and collector. If the base were thick or heavily doped, many carriers injected from the emitter would recombine inside it. A thin and lightly doped base reduces recombination and allows most carriers to pass through to the collector. This is essential for transistor action, because a small base current can then control a much larger collector current. The base is a control region, not a region meant to absorb all injected carriers.
313. In a transistor, the collector is usually made physically larger than the emitter-base region because it:
ⓐ. must inject all carriers into the base
ⓑ. must be the thinnest and lightest-doped region
ⓒ. behaves as the anode of a single diode only
ⓓ. must collect carriers and dissipate more heat
Correct Answer: must collect carriers and dissipate more heat
Explanation: The collector receives most of the carriers injected by the emitter and passing through the base. Because it handles the output side of the transistor, it often has to dissipate more heat than the other regions. It is therefore made larger in area in practical transistor structure. The emitter is the main injecting region, while the base is the thin control region. The collector is not simply an anode of a diode; it is part of a three-region transistor device.
314. Match the transistor region with its usual structural feature or role.
| Column I | Column II |
|---|
| P. Emitter | 1. Thin and lightly doped control region |
| Q. Base | 2. Heavily doped carrier-injecting region |
| R. Collector | 3. Larger region that collects carriers |
| S. Transistor as a whole | 4. Three-region, two-junction device |
The suitable matching is:
ⓐ. P-2, Q-1, R-3, S-4
ⓑ. P-1, Q-2, R-4, S-3
ⓒ. P-3, Q-1, R-2, S-4
ⓓ. P-2, Q-3, R-1, S-4
Correct Answer: P-2, Q-1, R-3, S-4
Explanation: The emitter is heavily doped so that it can inject majority carriers effectively. The base is thin and lightly doped, which reduces recombination and makes control possible. The collector is generally larger and collects carriers arriving through the base. A bipolar junction transistor has three regions and two junctions. The matching keeps structural design connected with the function of each region.
315. A transistor has two \(p\)-\(n\) junctions because:
ⓐ. it has only two terminals like an ordinary diode
ⓑ. forms emitter-base and collector-base junctions
ⓒ. its emitter and collector are directly shorted together
ⓓ. it contains no \(p\)-type or \(n\)-type material
Correct Answer: forms emitter-base and collector-base junctions
Explanation: A junction transistor has three regions: emitter, base, and collector. The boundary between emitter and base forms one \(p\)-\(n\) junction. The boundary between collector and base forms the second \(p\)-\(n\) junction. These two junctions are part of one connected transistor structure, not two independent diodes simply placed side by side. Transistor action depends on the interaction of the two junctions through the thin base region.
316. A claim says: “A transistor is just two separate diodes connected back-to-back, so it should work the same way as two ordinary diodes.” The claim is:
ⓐ. suitable because the base region has no special role
ⓑ. suitable only when the collector is made larger
ⓒ. unsuitable because a transistor has only one semiconductor junction
ⓓ. unsuitable because transistor action needs a thin shared base
Correct Answer: unsuitable because transistor action needs a thin shared base
Explanation: A transistor does contain two \(p\)-\(n\) junctions, but they are not two independent diodes operating separately. The two junctions share a thin base region. Carriers injected from the emitter can cross the base and be collected by the collector. This carrier transfer is what allows a small base current to control a larger collector current. The two-diode picture misses the physical coupling through the base, so it cannot fully explain transistor action.
317. Study the table for transistor structure.
| Row | Feature | Suitable description |
|---|
| P | Number of regions | Three |
| Q | Number of junctions | Two |
| R | Base | Very thick and heavily doped to maximise recombination |
| S | Emitter | Heavily doped for carrier injection |
The row that is not suitable is:
ⓐ. Row P
ⓑ. Row R
ⓒ. Row Q
ⓓ. Row S
Correct Answer: Row R
Explanation: Row R is not suitable because the base is made thin and lightly doped, not very thick and heavily doped. A thick or heavily doped base would increase recombination of carriers injected from the emitter. Rows P and Q correctly describe a transistor as a three-region, two-junction device. Row S is also suitable because the emitter is heavily doped to supply carriers efficiently. The special base design is essential for the transistor to work as a controlled device.
318. Consider the following statements about transistor structure.
I. An \(npn\) transistor has two \(n\)-type regions separated by a thin \(p\)-type base.
II. A \(pnp\) transistor has two \(p\)-type regions separated by a thin \(n\)-type base.
III. The base is usually made thick to force all carriers to recombine before reaching the collector.
The valid statements are:
ⓐ. II and III only
ⓑ. I and III only
ⓒ. I, II, and III
ⓓ. I and II only
Correct Answer: I and II only
Explanation: Statement I correctly describes the region order of an \(npn\) transistor. Statement II correctly describes the region order of a \(pnp\) transistor. Statement III is not valid because the base is made thin and lightly doped to reduce recombination. If all carriers recombined in the base, the collector current would be very small and transistor action would be poor. The base must control carrier flow without stopping most carriers from reaching the collector.
319. The emitter arrow in a transistor symbol is placed on the:
ⓐ. emitter terminal
ⓑ. base terminal
ⓒ. collector terminal
ⓓ. load resistor
Correct Answer: emitter terminal
Explanation: In a transistor symbol, the arrow is drawn on the emitter terminal. The arrow helps identify both the emitter and the transistor type. For an \(npn\) transistor, the emitter arrow points outward. For a \(pnp\) transistor, the emitter arrow points inward. This symbol convention is about conventional current direction at the emitter, not about electron flow direction alone.
320. The emitter arrow points outward in the symbol of:
ⓐ. an \(npn\) transistor
ⓑ. a \(pnp\) transistor
ⓒ. an ordinary resistor
ⓓ. a reverse-biased Zener diode only
Correct Answer: an \(npn\) transistor
Explanation: The standard symbol convention is that an \(npn\) transistor has an emitter arrow pointing outward. A \(pnp\) transistor has the emitter arrow pointing inward. The arrow is a useful visual clue for identifying the transistor type in a circuit diagram. It is not drawn on a resistor, and it is not the symbol rule for a Zener diode. The phrase “arrow out” can be linked with \(npn\) to remember the symbol direction.
