Correct answer: B — transistor

A transistor is the fundamental semiconductor amplifying device. It uses a small input signal (at the base or gate) to control a much larger current flow through the device, producing voltage or current gain. Both bipolar junction transistors (BJTs) and field-effect transistors (FETs) operate on this principle.

• A. diode — A diode contains only a single PN junction and allows current to flow in one direction only. It can rectify or detect signals but cannot amplify them.
• C. PN junction — A PN junction is the basic building block within a semiconductor device. On its own it forms a diode, not an amplifier.
• D. silicon gate — A silicon gate is a structural element found in MOSFETs (the gate electrode), not a device in its own right.

Therefore, the transistor is the basic semiconductor amplifying device, capable of taking a weak input signal and producing a stronger output signal.

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Correct answer: voltage regulators

Zener diodes are designed to operate in the reverse breakdown region, where they maintain a nearly constant voltage across them over a range of current.

This property makes them suitable for use as voltage reference or regulation devices in power supplies.

• RF detectors typically use ordinary diodes for rectification.
• AF detectors are also based on rectification.
• Current regulation is not the primary application of a Zener diode.

Therefore, Zener diodes are normally used as voltage regulators.

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Correct answer: A — 0.3 V

Semiconductor diodes have a forward voltage drop determined by the material they are made from. Germanium (Ge) diodes have a lower forward voltage drop than silicon diodes because germanium has a narrower energy band gap. In practice, a conducting germanium signal diode drops approximately 0.3 V across its junction.

• B. 0.6 V — This is the typical forward voltage drop of a silicon signal diode, not germanium.
• C. 0.7 V — Also associated with silicon diodes; 0.7 V is often quoted as the "textbook" silicon junction voltage.
• D. 1.3 V — This is approximately the forward voltage of certain LED types (e.g., red LEDs), not a germanium signal diode.

Therefore, the approximately 0.3 V forward voltage drop is the characteristic value for a conducting germanium signal diode, compared to roughly 0.6–0.7 V for silicon devices.

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Correct answer: C — emitter, base and collector

A bipolar junction transistor (BJT) has three semiconductor regions, each connected to an external terminal: the emitter, the base, and the collector. The emitter supplies charge carriers, the base (a very thin middle layer) controls the flow, and the collector gathers the carriers. This three-layer structure is what distinguishes a BJT and gives it its current-amplifying properties.

• A (base, emitter and drain) is incorrect — "drain" is a terminal belonging to a field-effect transistor (FET), not a BJT.
• B (collector, base and source) is incorrect — "source" is likewise an FET terminal, not found on a BJT.
• D (drain, source and gate) is incorrect — all three of these terminals belong to a FET (MOSFET or JFET), not a bipolar transistor.

Therefore, the three terminals of a bipolar junction transistor are correctly named the emitter, base, and collector.

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Correct answer: collector, emitter, base

A bipolar junction transistor (BJT), including a PNP transistor, has three terminals:

• collector
• emitter
• base

These are used to control current flow through the device.

• Source and drain are used in FETs.
• Gate is also a FET terminal.

Therefore, the three leads are collector, emitter, base.

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Correct answer: A — amplification

Amplification is the process by which an active device (such as a transistor) takes a low-level input signal and produces a larger version of it at the output. The transistor uses a small input current or voltage to control a larger current from the supply, resulting in a signal with greater power or amplitude at the output.

• Detection is the process of extracting the original audio (or data) signal from a modulated carrier wave — not increasing signal level.
• Modulation is the process of impressing information onto a carrier wave by varying its amplitude, frequency, or phase.
• Rectification is the conversion of alternating current (AC) to direct current (DC) by allowing current to flow in only one direction — unrelated to signal gain.

Therefore, when a circuit produces a higher-level output signal from a lower-level input signal, the effect is called amplification.

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Correct answer: silicon

Modern power supply rectifiers almost exclusively use silicon diodes. Silicon devices offer:

• low cost and wide availability
• high reverse voltage ratings
• good current handling capability
• excellent reliability and long life
• stable operation over a wide temperature range

These characteristics make silicon ideal for mains rectification and general power supply use.

• lithium is not used as a rectifier material in practical power electronics.
• germanium diodes were used in early electronics but have lower voltage ratings, higher leakage current, and poorer temperature stability than silicon.
• copper-oxide rectifiers are obsolete and inefficient compared with modern semiconductor diodes.

Therefore, the rectifier diode almost exclusively used in power supplies is silicon.

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Correct answer: D — demodulation

Demodulation (also called detection) is the process of extracting the original audio or data signal from a modulated carrier wave. A simple diode detector works by rectifying the received RF signal and then filtering out the carrier frequency, leaving only the recovered audio or information signal. This is one of the most fundamental uses of a diode in radio receivers.

• A. biasing — Biasing sets the operating point of a transistor or diode in an amplifier circuit; it does not recover information from a signal.
• B. rejuvenation — This term refers to restoring worn vacuum tubes and has no relevance to signal recovery.
• C. ionisation — Ionisation is the process of freeing electrons from atoms (relevant to propagation and gas-filled tubes), not to extracting audio from a carrier.

Therefore, the correct term for using a diode to recover information from a transmitted signal is demodulation.

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Correct answer: B — flow from n to p

In a forward biased pn junction, the external voltage reduces the depletion region barrier, allowing majority carriers to cross the junction. Electrons (majority carriers in the n-type material) gain enough energy to cross into the p-type region, flowing from n to p. Simultaneously, holes flow in the opposite direction (from p to n). Conventional current flows from p to n (anode to cathode), while electron flow — being opposite to conventional current — goes from n to p.

• A. flow from p to n — This describes hole flow (conventional current direction), not electron flow. Electrons and holes move in opposite directions across the junction.
• C. remain in the n region — If carriers remained in their respective regions, no current would flow at all; this describes the unbiased or reverse-biased condition where the depletion barrier prevents carrier movement.
• D. remain in the p region — Similarly, holes do not remain stationary in the p region under forward bias; they migrate toward the junction and into the n region.

Therefore, under forward bias, electrons flow from the n-type region across the junction into the p-type region, constituting part of the diode's forward current.

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Correct answer: capacitor

A varactor diode is designed to operate in reverse bias, where the width of its depletion region changes with applied voltage. This change in depletion width causes the diode’s junction capacitance to vary as the reverse voltage changes.

As the reverse voltage increases, the depletion region widens and the capacitance decreases. As the reverse voltage decreases, the capacitance increases. This makes the device behave like a voltage-controlled variable capacitor.

• resistor is incorrect because the diode is reverse biased and ideally conducts very little current.
• voltage regulator is incorrect because the device does not regulate voltage, it varies capacitance.
• inductor is incorrect because no magnetic field or inductive behaviour is involved.

Therefore, a varactor diode acts like a variable capacitor.

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Correct answer: impurities

Doping is the process of adding very small amounts of specific impurity atoms to a pure semiconductor material such as silicon.

These impurity atoms:

• increase the number of free charge carriers
• improve the electrical conductivity

Depending on the impurity added, the semiconductor becomes:

• n-type (extra electrons)

• p-type (extra holes)

• Electrons and protons are not added directly.

• Ions are formed after doping, not added as such.

Therefore, a semiconductor is doped by adding impurities.

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Correct answer: anode and cathode

A semiconductor diode has two terminals:

• Anode
• Cathode

Current flows from the anode to the cathode when the diode is forward biased.

• Cathode and drain are associated with FETs.
• Gate and source are FET terminals.
• Collector and base are bipolar transistor terminals.

Therefore, the connections to a semiconductor diode are anode and cathode.

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Correct answer: B — 3 connecting leads

A bipolar junction transistor (BJT) has three semiconductor regions — the emitter, base, and collector — each requiring its own connecting lead. This gives the device three terminals, which is the fundamental physical structure of all standard bipolar transistors, whether NPN or PNP type. The base is the control terminal; a small current into the base controls a much larger current flowing between collector and emitter.

• A. 4 connecting leads — Incorrect. Four-terminal devices include components such as FETs in some packages or integrated circuits, but a standard BJT has only three regions and three leads.
• C. 2 connecting leads — Incorrect. Two-terminal devices include diodes, resistors, and capacitors. A BJT requires a third terminal to provide the control (base) signal.
• D. 1 connecting lead — Incorrect. No active semiconductor switching device operates with a single lead; a single terminal cannot form a useful circuit path.

Therefore, bipolar transistors always have three connecting leads, one for each of the emitter, base, and collector regions.

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Correct answer: C — bipolar transistor

A device described as "NPN" is a bipolar junction transistor (BJT). The letters NPN describe the sandwich of semiconductor layers: N-type / P-type / N-type, forming the emitter, base, and collector regions. The "general purpose audio" label simply tells you its intended application. BJTs are current-controlled devices widely used in audio amplification and switching circuits.

• A. Triode — A triode is a vacuum tube with three electrodes (cathode, grid, anode). It is not a semiconductor device and has no NPN designation.
• B. Silicon diode — A diode is a two-terminal semiconductor device (anode and cathode) used for rectification; it has no NPN structure and cannot amplify.
• D. Field effect transistor — A FET is a three-terminal semiconductor, but it is described by channel type and gate construction (e.g., N-channel JFET or MOSFET), not as NPN or PNP. FETs are voltage-controlled, not current-controlled.

Therefore, "general purpose audio NPN device" unambiguously identifies a bipolar junction transistor.

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Correct answer: NPN and PNP types

Bipolar junction transistors (BJTs) are classified based on the arrangement of their semiconductor layers:

• NPN: a layer of P-type material between two N-type layers
• PNP: a layer of N-type material between two P-type layers

These determine the direction of current flow and biasing requirements.

• P-channel and n-channel refer to FETs.
• Diode and triode are different types of devices.
• Varicap and zener are types of diodes.

Therefore, the two basic types are NPN and PNP.

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Correct answer: excessive heat

Transistors have maximum temperature limits.

Excessive heat can:

• damage semiconductor junctions

• cause breakdown of materials

• lead to permanent failure

• Light is generally not destructive (except in specialised devices).

• Saturation and cut-off are normal operating states.

Therefore, a transistor can be destroyed by excessive heat.

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Correct answer: B — at the emitter potential

Cut-off is the transistor state in which collector current is essentially zero — the device is fully "off." For a bipolar junction transistor (BJT) to reach cut-off, the base–emitter junction must have no forward bias. This is achieved when the base is held at (or below, for an NPN) the same potential as the emitter, so the junction voltage V_BE ≈ 0 V and no base current flows. With no base current, there is no collector current and the transistor is cut-off.

• A — at the collector potential: The collector sits at a much higher potential (for NPN). Driving the base to the collector voltage would heavily forward-bias the base–collector junction, pushing the transistor toward saturation, not cut-off.
• C — mid-way between collector and emitter potentials: This would place a significant positive voltage on the base relative to the emitter, turning the transistor on into its active region, not off.
• D — mid-way between the collector and supply potentials: Similarly, this would apply a forward bias to the base–emitter junction, producing base and collector current — the transistor would be conducting, not cut-off.

Therefore, biasing the base at the emitter potential removes all forward bias from the base–emitter junction, driving the transistor into cut-off and stopping collector current flow.

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Correct answer: A — n-channel and p-channel

A Field Effect Transistor (FET) controls current flow through a semiconducting channel using an electric field applied at the gate terminal. The two basic types are defined by the polarity of that channel: an n-channel FET carries current via electrons (negative charge carriers), while a p-channel FET carries current via holes (positive charge carriers). This classification applies to both the JFET and MOSFET families of FETs.

• B. NPN and PNP — These are the two types of bipolar junction transistors (BJTs), not FETs. BJTs are current-controlled devices, whereas FETs are voltage-controlled.
• C. Germanium and silicon — These are semiconductor materials used to manufacture transistors (and diodes), not types of FET.
• D. Inductive and capacitive — These are types of reactance in AC circuits and have no relevance to transistor classification.

Therefore, the two basic types of FET are n-channel and p-channel, distinguished by the polarity of the conducting channel.

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Correct answer: field-effect transistor

A semiconductor device with terminals labelled gate, drain, and source is a field-effect transistor (FET).

In a FET:

• The gate controls the current flow
• The current flows between the drain and source

The gate voltage creates an electric field that controls the conductivity of the channel between drain and source.

• A bipolar transistor uses base, collector, and emitter terminals.
• A silicon diode has only two terminals.
• “Gated transistor” is not a standard device classification.

Therefore, the device described is a field-effect transistor.

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