The forward voltage drop of a diode is the voltage required for current to cross the PN junction and conduct. That voltage is set by the semiconductor material and the diode construction, so different diode types have different forward drops. For example, germanium diodes typically have a forward drop around 0.25–0.3 volts, silicon diodes around 0.7 volts, and many LEDs around 1.4 volts (though LED drops vary with color and material).

Because the forward drop is an inherent property of the junction material and design, its value can differ significantly between diode types and must be considered when designing circuits (it reduces the voltage available to the load when the diode is conducting).

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A diode is a semiconductor component that conducts current easily in one direction but blocks it in the opposite direction. The most familiar diode for many people is the LED (Light Emitting Diode): it lets current flow one way and emits light when it does.

A resistor does not block one direction of current; it simply limits the amount of current in either direction. A fuse is a protective device that normally allows current to flow both directions but is designed to melt (open) if excessive current flows. A driven element is part of an antenna that is driven by the transmitter to radiate RF energy and is not a one-way electrical device.

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A transistor has three terminals; on the most common type of transistor (a bipolar transistor) these are Base, Collector, and Emitter. Current applied to the Base controls current flow between the Collector and Emitter, which allows it to act as an electronic switch. The amplifier capability is related: a small change in Base current produces a proportionally larger change in Collector–Emitter current.

A varistor (voltage-dependent resistor) changes resistance with applied voltage and is used for surge protection, not as a switch. A potentiometer is a variable resistor used to set voltages or adjust levels. A thermistor changes resistance with temperature. None of these three functions as an electronic switch in the same controlled way a transistor does.

Memory aids:

• (none provided)

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When a component is described as having three regions of semiconductor material, that refers to a transistor. Bipolar junction transistors (BJTs) are made from three semiconductor layers arranged as either N‑P‑N or P‑N‑P. Each letter stands for the type of semiconductor doping in one of the three regions (negative/positive/negative or positive/negative/positive).

In general usage, the word “semiconductor” often points to transistors specifically or to devices made from many transistors (for example, integrated circuits). The detailed operation of a transistor is beyond this brief explanation, but remembering the three-layer structure is enough to identify it as the component with three semiconductor regions.

Memory aids:

• NPN and PNP denote the three layers: Negative-Positive-Negative or Positive-Negative-Positive
• Think “three letters = three regions”

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A field-effect transistor (FET) is a transistor that uses an electric field to control current flow through a semiconductor channel. FETs have three terminals named source, gate, and drain. Applying a voltage to the gate changes the conductivity between the drain and source, thereby controlling the current.

Memory aids:

• Fields (field-effect transistors) have gates: gate, drain, and source
• "A field's fence has a gate" — Field-effect → Gate

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A semiconductor diode is a polarized component with an anode and a cathode. Because many diode packages are very small, manufacturers commonly mark the cathode lead with a visible stripe on the body so the polarity can be identified easily. The stripe on the package corresponds to the bar (the cathode) in the diode schematic symbol, so it shows which end is the cathode.

Memory aids:

• Think of a "striped cat" to remember the cathode is marked with a stripe.
• The stripe corresponds to the bar in the diode schematic symbol, which is the cathode.

Click for a list of schematic symbols: http://www.rapidtables.com/electric/electrical_symbols.htm

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Diodes conduct current when they are forward biased. A light-emitting diode (LED) is a semiconductor diode designed so that when forward current flows, electrons recombine with holes in the junction and release energy as photons (light). Reverse current does not produce light and can damage the device; capacitively or inductively coupled RF signals are not the mechanism that causes an LED to emit light.

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A Field Effect Transistor (FET) controls voltage and current like a regular transistor, but can operate on much smaller signals, which makes it ideal for radio receivers. It uses an electric field to control the shape and conductivity of the channel in the semiconductor material, allowing the device to amplify or switch signals with very high input impedance.

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The two electrodes of a diode are called the anode and the cathode. By convention (conventional current flow), current flows into the anode and out of the cathode.

Terms like “plus” and “minus” describe polarity of a power source and are not the names of diode electrodes. The names source, drain and gate are used for field-effect transistors (FETs). The names base, collector and emitter are used for bipolar junction transistors (BJTs).

Memory aids:

• "ode" is in the word diode — that can help remind you that the electrode names end in "-ode" (anode, cathode).
• Remember: current flows into the Anode and out of the Cathode.

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Transistor is the only component in this list that can provide power gain.

A transistor is an active device that uses energy from an external power supply (bias) so a small input signal can control a larger current or voltage from that supply. Because it draws power from the supply, the device can deliver more output power than the input signal provides, i.e. it can provide power gain.

A transformer can change (transform) voltage and current levels between windings, but an ideal transformer conserves power (neglecting losses) and therefore does not create power. A reactor (inductor) or other reactive component stores and returns energy but does not supply net power, and a resistor simply dissipates power as heat rather than providing gain.

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For amplifiers, gain just means “how much bigger is the output than the input?”

You can talk about that in three different ways:

• Voltage gain – output voltage compared to input voltage
• Current gain – output current compared to input current
• Power gain – output power compared to input power

All of those are valid ways to describe amplifier gain, depending on what you care about in that circuit. That’s why the correct answer is the one that says all of these are correct.

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A bipolar junction transistor (BJT) is a semiconductor device with three electrodes called the emitter, base, and collector. The base-emitter region and the base-collector region form two p–n junctions (for example NPN or PNP). A small change in voltage or current applied to the base controls a much larger current flowing between the collector and emitter, which is why these three electrode names are used for the device.

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