Diodes and Semiconductor Devices

Semiconductor devices are where the subtest moves from passive components to active ones. The questions stay shallow: what does it do, and which way round.

The diode

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A diode conducts in one direction only.

It is made from a junction between p-type and n-type semiconductor material, which is why the conductors lesson comes first.

The two terminals are the anode and the cathode. Current flows in at the anode and out at the cathode, so forward bias means the anode is connected to the more positive side.

The symbol is a triangle pointing at a bar, and conventional current flows in the direction the triangle points, from anode to cathode. The bar is the cathode - the stop sign: current cannot come back through it. On a real diode the cathode end is marked with a painted band, and on an LED the longer lead is the anode.

A silicon diode needs about 0.7 volts across it before it conducts at all. Below that it blocks in both directions. That figure appears occasionally and is worth recognizing.

Reverse the polarity and the lamp goes out, which is the experiment the figure shows and the shape of most diode questions.

Rectification

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A rectifier converts AC to DC using diodes.

Half-wave rectification uses one diode. It passes the positive half of each cycle and blocks the negative half, so the output is a series of humps with gaps between them. It is DC in the sense that it never goes negative, and it is a long way from smooth.

Full-wave rectification uses four diodes in a bridge, and it flips the negative half up rather than discarding it, so the output is a continuous series of humps with no gaps. Full-wave is more efficient, because none of the input is thrown away.

What is left of the humps after smoothing is called ripple. Its frequency follows from the rectifier: from 60-hertz AC, half-wave gives 60 hertz of ripple and full-wave gives 120 hertz, because full-wave produces two humps per cycle.

A capacitor across the output smooths the humps into something close to steady DC. A larger filter capacitor holds its charge longer between pulses and leaves less ripple; a failing one, or a heavier load drawing it down faster, shows up as more ripple. That is why a power supply contains a transformer, a rectifier and a capacitor in that order, and a question naming all three is asking you to put them in sequence.

The order of stages is transformer, rectifier, filter, regulator, and it is asked in exactly that form.

A voltage regulator holds the output steady as the load or the supply varies. A linear regulator does it by burning off the excess voltage as heat: its heat is the voltage it drops times the current.

A 5-volt regulator runs from a 12-volt input and supplies 1 ampere. How much power does it dissipate?

It drops 7 volts at 1 ampere: 7 watts of heat. It also needs its input to sit some minimum amount above its output, called the dropout voltage. A switching regulator is more efficient because it switches fully on and off rather than burning off the difference. The common 7805 part is a 5-volt regulator - the "05" is the voltage.

Diodes must be rated above the peak reverse voltage they will see, called the peak inverse voltage; beyond it, reverse breakdown destroys an ordinary diode.

Other diodes

A zener is the one built to work in reverse breakdown. An ordinary diode is destroyed by it; a zener holds a fixed voltage there, which is why it is used as a simple regulator, with a series resistor to limit its current.

A diode across a relay coil - a flyback diode - absorbs the voltage spike the coil's collapsing magnetic field produces when the current is switched off, which would otherwise damage the transistor driving it.

LED stands for light-emitting diode, and it is still a diode, so it still conducts one way only. Fitting one backward gives no light rather than a dim one, which is a question.

Transistors

A transistor lets a small current or voltage control a much larger one.

It has three terminals, against a diode's two, and that third terminal is the control.

Two jobs, and the test asks about both:

As a switch. A small current at the control turns a large current on or off, with no moving parts. This is what every digital circuit is built from, by the billion.

As an amplifier. A small varying signal at the control produces a large varying current, which is a bigger copy of the input.

Both jobs are the same property used differently: control of a large current by a small one. Whether you call it switching or amplifying depends on whether you run it at the extremes or in between.

A transistor does not create energy. The large current comes from the power supply; the small signal only controls it. That is asked as a concept question, and it is the same conservation point that appears in transformers and in mechanical advantage.

Types: a bipolar transistor has a base, a collector and an emitter, and the base is the control. A field-effect transistor has a gate, a source and a drain, and the gate is the control. Recognizing which terminal is the control is the level the subtest asks at.

Bipolar transistors come in two types, NPN and PNP, named for the order of their layers. An NPN conducts when its base is made more positive than the emitter; a PNP when its base is made more negative. On the symbol, the arrow on the emitter points outward for NPN and inward for PNP; the usual memory aid is "NPN: Not Pointing iN".

A transistor switch is driven into saturation - fully on, with the smallest possible voltage across it - because a transistor half on is dropping voltage and getting hot. Fully off is cutoff.

A silicon controlled rectifier (SCR) is a semiconductor switch triggered by a pulse on its gate. Once triggered it stays on until the current through it falls below a holding value.

Integrated circuits

An integrated circuit is many components on one chip of semiconductor.

Transistors, diodes and resistors are built into a single piece of silicon rather than wired together. That is what makes modern electronics small, cheap and reliable, and it is the level of detail the subtest wants: what it is, not how it is made.

The MOSFET

A MOSFET - metal oxide semiconductor field effect transistor - is the field-effect transistor used in most switching today. A bipolar transistor is current-controlled (a base current turns it on); a MOSFET is voltage-controlled and its gate draws almost no current, which is why it is preferred for switching. Its gate sits on a very thin layer of insulation, so static electricity can puncture it - handle MOSFETs with static precautions.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• Semiconductor physics - doping levels, holes, depletion regions - comes up only a few times, and never beyond what the conductors lesson covers.
• Transistor gain calculations. The gain questions belong to the amplifier lesson and ask what gain means, not how to design for it. "Beta" never appears.
• Thyristor families. Triacs never appear; the SCR is the one switch of this kind that is asked.
• Reverse recovery time never appears. Know that Schottky diodes switch fast.

Where people lose points

Thinking a diode conducts both ways. One way only.

Reversing forward and reverse bias. Forward conducts.

Saying half-wave rectification is more efficient. Full-wave is; it uses both halves.

Saying a transistor amplifies power by creating it. It controls power from the supply.

Saying a rectifier changes DC to AC. That is an inverter.

Giving a transistor two terminals. Three.

Work one in under a minute

A power supply must take 120-volt AC from a wall outlet and deliver smooth 12-volt DC. Name the stages in order.

1. Transformer - steps 120 volts down toward 12.
2. Rectifier - diodes convert AC to pulsating DC.
3. Capacitor - smooths the pulses toward steady DC.
4. Regulator - holds the output steady as the load changes.

Each stage does exactly one thing, and the order cannot be rearranged: you cannot smooth AC, and rectifying at 120 volts then stepping down is not possible with a transformer, because a transformer will not work on DC.

Where this leads

Transistors are what amplifiers and logic gates are built from, which are the next two lessons.