IN THIS CHAPTER

Digging deep into how semiconductors work

Looking at the simplest of semiconductors, the lowly diode

Understanding how diodes can change AC to DC

Seeing how some diodes can make light

So far in this book, all the components I tell you about were invented in the first 100 years or so since Alessandro Volta invented the first electric battery in 1800. The next 50 years of electronics, from roughly 1900 to 1950, were dominated by a technology that’s now all but obsolete: vacuum tubes, which were large, expensive, fragile (they were made of glass), and required a lot of current.

In the 1940s, researchers developed a new technology that lead to new components that were, literally, a quantum-leap improvement over vacuum tubes. These new components were called semiconductors.

This chapter introduces you to the most basic kind of semiconductor, called a diode. Diodes come in many shapes and sizes, but most of them resemble the ones shown in Figure 5-1. These diodes are about the size of resistors and are available from any electronics supply store, such as RadioShack.

Although diodes look a little like resistors, they behave very differently. Diodes have one ability that sets them apart: They let current flow freely in one direction, but block current if it tries to flow in the other direction. In other words, a diode is like a turnstile gate that you can walk through in one direction but not the other. This characteristic turns out to be incredibly useful in electronic circuits.

I want to warn you from the outset that the first part of this chapter is fairly difficult, as it delves just a little into the physics of how semiconductors work at the atomic level. You don’t really have to understand the physics of diodes to use them in your circuits, so you can skip over the entire first section if you want, and start with the section “Introducing Diodes,” later in this chapter. However, I recommend plowing through this first section no matter what. Don’t worry much if you have difficulty understanding how semiconductors work. Just take note of the key terms, such as p-type, n-type, and p-n junction, and move on.

What Is a Semiconductor?

As its name implies, a semiconductor is a material that conducts current, but only partly. The conductivity of a semiconductor is somewhere between that of an insulator, which has almost no conductivity, and a conductor, which has almost full conductivity. Most semiconductors are crystals made of certain materials, most commonly silicon.

To understand how semiconductors work, you must first understand a little about how electrons are organized in an atom. The electrons in an atom are organized in layers, kind of like the layers of an onion. These layers are called shells. The outermost shell is called the valence shell. The electrons in this shell are the ones that form bonds with neighboring atoms. Such bonds are called covalent bonds because they share valence electrons, which are also the electrons that sometimes go wandering off in search of other atoms.

Most conductors, including copper and silver, have just one electron in the valence shell. Atoms with just one valence electron have a hard time keeping that electron. That’s what makes copper and silver such good conductors: When valence electrons travel, they create moving electric fields that push other electrons out of their way. That’s what causes current to flow.

Semiconductors, on the other hand, typically have four electrons in their valence shell. The best known elements with four valence electrons are carbon, silicon, and germanium. Atoms with four valence electrons rarely lose one of them. However, they do like to share them with neighboring atoms.

If all the neighboring atoms are of the same type, it’s possible for all the valence electrons to bind with valence electrons from other atoms. When that happens, the atoms arrange themselves into neat and orderly structures called crystals. Semiconductors are made out of such crystals.

The most plentiful element with four valence electrons is carbon. However, carbon crystals are rarely used as semiconductors because they have other uses, such as in wedding rings. So instead, semiconductors are usually made from crystals of silicon and occasionally germanium.

Figure 5-2 shows the covalent bonds formed in a silicon crystal. Here, each circle represents a silicon atom, and the lines between the atoms represent the shared electrons. Each of the four valence electrons in each silicon atom is shared with one neighboring silicon atom. Thus, each silicon atom is bonded with four other silicon atoms.

Doping: It’s not just for athletes

By themselves, pure silicon crystals are pretty, but not all that useful electronically. With all four valence electrons interlocked with neighboring atoms, it isn’t easy to get current to flow through a pure silicon crystal. But things get interesting if you introduce small amounts of other elements into a crystal. Then, the crystal starts to conduct in an interesting way.

The process of deliberately introducing other elements into a crystal is called doping. The element introduced by doping is called a dopant. By carefully controlling the doping process and the dopants that are used, silicon crystals can transform into one of two distinct types of conductors:

• N-type semiconductor: Created when the dopant is an element that has five electrons in its valence layer. Phosphorus is commonly used for this purpose. The phosphorus atoms join right in the crystal structure of the silicon, each one bonding with four adjacent silicon atoms just like a silicon atom would. Because the phosphorus atom has five electrons in its valence shell, but only four of them are bonded to adjacent atoms, the fifth valence electron is left hanging out with nothing to bond to.

  The extra valence electrons in the phosphorous atoms start to behave like the single valence electrons in a regular conductor such as copper. They are free to move about, as shown in Figure 5-3. Because this type of semiconductor has extra electrons, it’s called an N-type semiconductor.

• P-type semiconductor: Happens when the dopant is an element (such as boron) that has only three electrons in the valence shell. When a small amount of boron is incorporated into the crystal, the boron atom is able to bond with four silicon atoms, but since it has only three electrons to offer, a gap is created where there would normally be an electron, as shown in Figure 5-4. This electron gap is called a hole. The hole behaves like a positive charge, so semiconductors doped in this way are called P-type semiconductors.

  Like a positive charge, holes attract electrons. But when an electron moves into a hole, the electron leaves a new hole at its previous location. Thus, in a P-type semiconductor, holes are constantly moving around within the crystal as electrons constantly try to fill them up. A P-type semiconductor is like crazed pieces of Swiss cheese in which you can’t quite get a fix on the holes because they’re always moving.

When voltage is applied to either an N-type or a P-type semiconductor, current flows, for the same reason that it flows in a regular conductor: The negative side of the voltage pushes electrons, and the positive side pulls them. The result is that the random electron and hole movement that’s always present in a semiconductor becomes organized in one direction, creating measurable electric current.

Introducing Diodes

A diode is a device made from a single p-n junction. As Figure 5-5 shows, leads are attached to the two ends of the p-n junction. These leads allow you to easily incorporate the diode into your circuits.

The lead attached to the n-type semiconductor is called the cathode. Thus, the cathode is the negative side of the diode. The positive side of the diode — that is, the lead attached to the p-type semiconductor — is called the anode.

When a voltage source is connected to a diode such that the positive side of the voltage source is on the anode and the negative side is on the cathode, the diode becomes a conductor and allows current to flow. Voltage connected to the diode in this direction is called forward bias.

But if you reverse the voltage direction, applying the positive side to the cathode and the negative side to the anode, current doesn’t flow. In effect, the diode becomes an insulator. Voltage connected to the diode in this direction is called reverse bias.

Forward bias allows current to flow through the diode. Reverse bias doesn’t allow current to flow. (Up to a point, anyway. As you discover in just a few moments, there are limits to how much reverse bias voltage a diode can hold at bay.)

The schematic symbol for a diode is shown in the margin. The anode is on the left, and the cathode is on the right. Here are two useful tricks for remembering which side of the symbol is the anode and which is the cathode:

• Think of the anode side of the symbol as an arrow that indicates the direction of conventional current flow — from positive to negative. Thus, the diode allows current to flow in the direction of the arrow.
• Think of the vertical line on the cathode side as a giant minus sign, indicating which side of the diode is negative for forward bias.

Figure 5-6 illustrates forward and reverse bias with two very simple circuits that connect a lamp to a battery with diodes. In the circuit on the left, the diode is forward biased, so current flows through the circuit and the lamp lights up. In the circuit on the right, the diode is reverse biased, so current doesn’t flow and the lamp remains dark.

Note that in a typical diode, a certain amount of forward voltage is required before any current will flow. This amount is usually very small. In most diodes, this voltage is around half a volt. Up to this voltage, current doesn’t flow. Once the forward voltage is reached, however, current flows easily through the diode.

This minimum threshold of voltage in the forward direction is called the diode’s forward voltage drop. That’s because the circuit loses this voltage at the diode. For example, if you were to place a voltmeter across the leads of the diode in the forward-biased circuit in Figure 5-6 (left), you would read the forward voltage drop of the diode. Then, if you were to place the voltmeter across the lamp terminals, the voltage would be the difference between the battery voltage (9 V) and the forward voltage drop of the diode.

For example, if the forward voltage drop of the diode was 0.7 V and the battery voltage was exactly 9 V, the voltage across the lamp would be 8.3 V.

Diodes also have a maximum reverse voltage they can withstand before they break down and allow current to flow backward through the diode. This reverse voltage (sometimes called PIV, for peak inverse voltage, or PRV for peak reverse voltage) is an important specification for diodes you use in your circuits, as you need to ensure that your diodes won’t be exposed to more than their PIV rating.

Besides the forward-voltage drop and peak inverse voltage, diodes are also rated for a maximum current rating. Exceed this current, and the diode will be damaged beyond repair.

The Many Types of Diodes

Diodes come in many different flavors. Some of them are exotic and rarely used by hobbyists, but many are commonly used. The following sections describe three types of diodes you’re likely to encounter in your electronic pursuits: rectifier, signal, and Zener diodes. Later in this chapter, you also learn about light-emitting diodes, or LEDs.

Using a Diode to Block Reverse Polarity

Project 15 presents a simple little construction project that demonstrates a diode’s ability to conduct current in only one direction. For this project, you wire up a rectifier diode in series with a 3 V flashlight lamp and a pair of AA batteries. I have you use a DPDT knife switch between the battery and the diode/lamp circuit so that when the switch is changed from one position to the other, the polarity of the voltage across the diode and lamp is reversed. Thus, the lamp lights in only one of the two switch positions. Figure 5-7 shows the completed project.

Project 15: Blocking Reverse Polarity

In this project, you build a simple circuit that uses a diode to allow current to flow through a lamp circuit in only one direction. The circuit uses a DPDT knife switch to reverse the polarity of the battery. When the polarity is reversed, the diode blocks the current so the lamp does not light up.

You need a small Phillips-head screwdriver, wire cutters, wire strippers, and a multimeter to complete this project.

Putting Rectifiers to Work

One of the most common uses for rectifier diodes is to convert household alternating current into direct current that can be used as an alternative to batteries. You learn all about creating complete power supplies for this purpose in Book 4, Chapter 2. For now, just concentrate on one part of a complete DC power supply: the rectifier circuit, which is typically made from a set of cleverly interlocked diodes. (A rectifier is a circuit that converts alternating current to direct current.)

In household current, the voltage swings from positive to negative in cycles that repeat 60 times per second. If you place a diode in series with an alternating current voltage, you eliminate the negative side of the voltage cycle, so you end up with just positive voltage, as shown in Figure 5-8.

If you look at the waveform of the voltage coming out of this rectifier diode, you’ll see that it consists of intervals that alternate between a short increase of voltage and periods of no voltage at all. This is a form of direct current because it consists entirely of positive voltage. However, it pulsates: First it’s on, then it’s off, then it’s on again, and so on.

Overall, voltage rectified by a single diode is off half of the time. So although the positive voltage reaches the same peak level as the input voltage, the average level of the rectified voltage is only half the level of the input voltage. This type of rectifier circuit is sometimes called a half-wave rectifier because it passes along only half of the incoming alternating current waveform.

A better type of rectifier circuit uses four rectifier diodes, in a special circuit called a bridge rectifier. Figure 5-9 shows a bridge rectifier circuit.

Look at how this rectifier works on both sides of the alternating current input signal:

• In the first half of the AC cycle, D2 and D4 conduct because they’re forward biased. Positive voltage is on the anode of D2 and negative voltage is on the cathode of D4. Thus, these two diodes work together to pass the first half of the signal through.
• In the second half of the AC cycle, D1 and D3 conduct because they’re forward biased: Positive voltage is on the anode of D1, and negative voltage is on the cathode of D3.

The net effect of the bridge rectifier is that both halves of the AC sine wave are allowed to pass through, but the negative half of the wave is inverted so that it becomes positive.

The resulting DC signal doesn’t drop to zero for half of the cycle. However, it still doesn’t provide a steady DC voltage level. Think about what you learn in Chapters 3 and 4 of this minibook, though. Both capacitors and inductors can be used to slow down changes in current and voltage. Thus, capacitors and inductors are often used in power supply circuits to improve the quality of DC voltage coming out of a rectifier circuit. You can learn more about how that’s done in Book 4, Chapter 2. But first, let’s look at rectifiers in action.

Building Rectifier Circuits

In Project 16, you build simple half-wave and bridge rectifier circuits to see how diodes can convert alternating current to direct current. You don’t light any lamps or anything with this circuit; instead, just use a voltmeter to verify that the diodes are doing their job. And to keep things safe, use a 9 V AC power adapter as the source for the alternating current. You should be able to find a 9 V AC power adapter for about $15 online, or you may find one for a dollar or two at a thrift store.

To make the voltage of the 9 V AC power adapter easier to work with, I suggest you cut off the power plug, separate the two wire leads, strip a bit of insulation from the ends, and attach alligator clips to the leads. The alligator clips make it much easier to connect the supply to your experimental circuits.

Before you build the project, use the multimeter to measure the AC voltage actually created by your power adapter. Although it may be marked as 9 V AC, the actual voltage you measure may be slightly more than 9 V AC.

You can find more information about rectifiers and how they’re used in power supply circuits in Book 4, Chapter 2. For the remainder of this chapter, turn your attention to another useful type of diode: the light-emitting kind.

Project 16: Rectifier Circuits

In this project, you build a simple half-wave rectifier and a bridge rectifier. You connect the rectifiers to a 9 V AC power source, then use your multimeter to measure the DC output voltage to verify that the AC voltage has been converted to DC.

Introducing Light Emitting Diodes

A light-emitting diode (also called an LED) is a special type of diode that emits visible light when current passes through it. The most common type of LED emits red light, but LEDs that emit blue, green, yellow, or white light are also available.

The word LED is pronounced by spelling the letters out (el-ee-dee), not like the word lead.

The schematic diagram symbol for an LED is shown in the margin, and Figure 5-10 shows typical LEDs. The two leads protruding from the bottom of an LED aren’t the same length: The shorter lead is the cathode, while the anode is the longer lead.

Whenever you use an LED in a circuit, you must provide some resistance in series with the LED, as shown in the schematic diagram in Figure 5-11. Otherwise, the LED will light brightly for an instant, and then burn itself out. In this example, the LED is connected to a 9 V DC supply through a resistor.

To determine the value of the resistor you should use, you need to know these three things:

• The supply voltage: For example, 9 V.
• The LED forward-voltage drop: For most red LEDs, the forward-voltage drop is 2 V. For other LED types, the voltage drop may be different. Check the specifications on the package if you use other types of LEDs.
• The desired current through the LED: Usually, the current flowing through the LED should be kept under 20 mA.

Once you know these three things, you can use Ohm’s law to calculate the desired resistance. The calculation requires just four steps, as follows:

1. Calculate the resistor voltage drop.

   You do that by subtracting the voltage drop of the LED (typically 2 V) from the total supply voltage. For example, if the total supply voltage is 9 V and the LED drops 2 V, the voltage drop for the resistor is 7 V.

2. Convert the desired current to amperes.

   In Ohm’s law, the current must be expressed in amperes. You can convert milliamperes to amperes by dividing the milliamperes by 1,000. Thus, if your desired current through the LED is 20 mA, you must use 0.02 in your Ohm’s law calculation.

3. Divide the resistor voltage drop by the current in amperes.

   This gives you the desired resistance in ohms. For example, if the resistor voltage drop is 7 V and the desired current is 20 mA, you need a resistor.

4. Round up to the nearest standard resistor value.

   The next higher resistor value from is . If you can’t find a resistor, a will do the trick.

   Note that the minor increases in resistances mean that slightly less current will flow through the resistor, but the difference won’t be noticeable. However, you should avoid going to a lower resistor value. Lowering the resistance increases the current, which can damage the LED.

Here are a few additional things to remember when you work with LEDs:

• If you’re going to place more than one LED together in series, just add up the voltage drops to calculate the size of the resistor you need. For example, if you have a 9 V battery that will supply voltage to three LEDs, each with a 2 V drop, the total voltage drop is 6 V, so the voltage drop across the resistor is 3 V. Using Ohm’s law, you can then calculate that the resistor needed to restrict the current flow to 20 mA is .
• For 9 V and less, resistors are more than adequate. If you’re applying larger voltages to the LED circuit, you may need to use resistors that can handle more power. To calculate how much power in watts the resistor should be rated for, just multiply the voltage dropped across the resistor by the current in amps. For example, if the voltage drop on the resistor is 3 V and the current is 20 mA, the power dissipated by the resistor will be 0.06 W, which is well under the limits of a resistor.
• You can get two LEDs, usually of different colors, combined into a single package. Green and red are a common combination. When two LEDs are bundled in a single package, they’re usually wired opposite of one another. That way, you can control which LED lights by changing the polarity of the voltage applied across the LED. In some cases, a third lead is used. This lead is connected to the cathode of both LEDs. The third lead enables you to light both LEDs, which yields a third color. For example, when both LEDs are lit in a green and red combination LED, the resulting color is yellow.

Using LEDs to Detect Polarity

In this section, you build a project that uses two LEDs to indicate the polarity of an input voltage. The voltage is provided by a 9 V battery connected to the circuit via a DPDT knife switch that’s wired to reverse the battery polarity. The two LEDs and their corresponding resistors are mounted on a small solderless breadboard. Figure 5-12 shows the completed circuit.

Project 17: An LED Polarity Detector

In this project, you build a polarity detector that uses LEDs to indicate the polarity of a power supply.

You need a small Phillips-head screwdriver, wire cutters, and wire strippers to assemble the circuit.