Semiconductors

The term semionductor applies to any components made from one of a series of semiconductive metals, most commonly silicon and germanium. The most basic semiconductors are diodes and transistors, which are usually made from a single crystal of silicon with junctions to control the flow of current.

Diodes

A diode is made up of a silicon crystal with a single point contact, which allows current to flow in only one direction, kind of like an electrical version of a check valve. Diodes have an anode and a cathode, with the cathode lead marked by a white or black band. An easy ay to remember this is that the line on the diode's casing is the same end as the line  through the diode's schematic symbol.

Probably the most common usage of diodes are in rectifiers, where they're used to separate the positive and negative components of AC to provide DC. A single diode placed in series with an AC current is what is referred to as a half-wave rectifier, meaning that it only separates either the positive or negative component, depending on which direction it's placed. Since this isn't all that efficient, most rectifiers are what are called full-wave rectifiers. When using a transformer with a center tap, a full-wave rectifier is made by connecting the cathodes of two diodes together. The outer taps of the transformer are connected to the diodes' anode leads, and the transformer's center tap is used as a ground. When using a transformer without a center tap, you can still have a full-wave rectifier, but it requires four instead of two diodes arranged as shown in the graphic. This type of rectifier is called a bridge rectifier or diode bridge. Aside from being used as a rectifier, a diode bridge can be found in battery powered electronics when its necessary to make sure the batteries aren't installed backwards. You can make your own bridge or full wave rectifier out of discrete diodes, or you can use one of a huge variety of integrated circuits made for this purpose.

The current that flows out of the rectifier is separated into positive and negative, but still has the peaks and valleys of the original AC current, referred to as ripple. You'll also notice that if you measure the voltage here, it will be less than the AC voltage by about 30%.  Unless the rectifier will be driving something like a lamp or certain types of motors, this choppy power will need to be smoothed by an appropriately sized filter capacitor to come up with true DC. As mentioned in the page on capacitors, the capacitor stores the DC charge during the peaks, and releases it during the valleys. If you measure the voltage here, without any load on the power supply, you'll notice that it's gone up to about 30% higher than the original AC voltage. Larger loads require larger filter capacitors to minimize ripple and maintain a steady voltage.

Common types of rectifier diodes are available in sizes from 1A up to hundreds of amps. The most common diodes used when building electronics are probably the general purpose 1N400x series, with the last number indicating the voltage the diode is capable of handling. For example, 1N4001 diodes are only rated to 50V, while 1N4007 diodes are rated to 1kV. The size of the diode is dependent on the current that will be flowing through them. Common 1A diodes are about the size and shape of a resistor, while larger diodes over 5A may come in epoxy cases with a metal heatsink.

A second usage for diodes is separating the positive an negative components of signals. Such small signal diodes can be used as detectors in radio sets. If you'd like to try this yourself, try connecting a small germanium diode to a piezoelectric earphone, and the other end to a long wire. If you have a strong local AM radio station, you will be able to hear it playing faintly through the earphone.

Other small diodes like the 1N4148 are what are called switching diodes. These diodes are used to protect switching transistors driving a relay coil from the voltage spike created by the relay turning off. Instead of going through the transistor, the voltage is shunted to the ground and dissipated as heat.

Zener diodes are a special class of diode, which has its own schematic sysmbol. Zener diodes are installed in reverse, compared to other diodes. A zener diode will have a specific breakdown voltage, meaning that it will conduct, in reverse, but only if the voltage is higher than the breakdown voltage. The higher the voltage above the breakdown voltage, the more the zener diode will conduct. This property makes zener diodes useful as a voltage reference. A common application is to connect a zener diode in series with a resistor across a power supply. The steady voltage across the zener can then be used to power small loads, or large loads when combined with a transistor. Zener diodes are available with a wide range of breakdown voltages, from 1V to hundreds of volts, with 5, 6 and 12V being the most common.

Transistors

There are two main types of transistors, bipolar junction transistors (BJTs) and field effect transistors (FETs). The differences between them involve a lot of really complicated physics that even I don't really understand fully. What's important is that you can sort of think of transistors as the electrical equivalent of a gate valve. Transistors can be used in amplifiers, where they take a small alternating voltage and modulate a larger DC voltage to create an amplified version of the original signal. Combined with a voltage reference like a zener diode, a high current transistor can be used to supply a steady voltage in a power supply. In digital circuitry, transistors are used as simple on/off switches, and are usually operated at 5V.

Bipolar Junction Transistors

This type of transistor consists of three layers, either an N type layer sandwiched between two P type layers in what's called a PNP transistor, or a layer of P type material sandwiched between two N type layers in what's called an NPN transistor. The position of the N and P layers determine the transistor's polarity, and which way around it should be connected in a circuit.

There are three leads to a BJT, referred to as the emitter, collector, and base. The base controls the flow of electrons between the collector and emitter. For now, let's look at how common switching transistors behave in a circuit.

To use an NPN transistor as a switch, you would connect its emitter to the negative supply, and the negative lead of the component you're switching to its collector. Increasing the positive voltage on the base causes the transistor to conduct more and more, until it's saturated and fully on. To use a PNP transistor as a switch, you do the opposite. Connect the transistor's emitter to the positive supply, and the collector to the positive lead of the component being switched. Increasing the negative voltage on the base causes the transistor to switch on. The operating principal is the same when using a BJT as an amplifier or for controlling a voltage, although the details may differ.

Field Effect Transistors

Like BJTs, there are two different types of  FETs, N-channel and P-channel. Further subtypes of FETs are based on how they operate or what they're made of. Examples include; joint field effect transistors (JFETs), metal oxide semiconductor field effect transistors (MOSFETs) and gallium-arsenide field effect transistors (GaAsFETs). Field effect transistors have fast switching speeds, and are most commonly found in digital integrated circuits like computer processors, which may contain billions of MOSFETs. GaAsFETs are most common in high frequency radio circuits like LNBs.

Field effect transistors have three leads, labeled source, gate, and drain. The channel is composed of the source and drain, with the voltage applied to the gate being used to control the flow of current. To use an N-channel FET as a switch, connect the source to the negative supply, the negative pole of the device being switched to the drain, and apply the control voltage to the gate. To use a P-channel FET as a switch, do the opposite, the source is connected to the positive supply, the drain connected to the positive pole of the device being switched, and the control voltage is applied to the gate. The voltage across the FET is directly proportional to the amount of control voltage applied to the base.

Types and Packages

Commercially, transistors are available in a variety of sizes and styles. Different types of cases are referred to by TO followed by a number. Small signal transistors and small switching transistors usually come in a TO-92 package, a small black epoxy tube with one flat side. Larger high current transistors usually come in a TO-220 package, which is physically larger and has an integral copper heatsink that can be bolted to a larger aluminum heatsink if necessary. Radio frequency transistors and older power transistors are sometimes enclosed in a metal can, such as TO-3. Surface mount transistors are available for soldering directly to PCBs. Transistors usually have a voltage and current rating, and are available in ratings from a few milliamperes at 50V up to hundreds of amps and thousands of volts. Higher current transistors are physically larger in order to dissipate more heat.

Each transistor will have a datasheet available from the manufacturer with the pinout and operating characteristics. Don't assume that all transistors have the same pinout, even if they're the same type. One manufacturer may use pin one for the base, while another may use pin two or three. When possible, always get the datasheet from the manufacturer of your transistor.

Integrated Circuits

There are hundreds of thousands of different integrated circuits for performing every function imagineable. An integrated circuit is a combination of transistors, resistors, diodes, and capacitors etched onto a single silicon crystal with photolithography. Integrated circuits can contain just a few components, such as the linear voltage regulators used in power supplies, or billions of components as in advanced CPUs and memory arrays. For all intents and purposes, you can categorize integrated circuits into three broad categories; linear, digital, and mixed signal. All ICs have a model number stamped on the case. You can find the datasheet for the IC, which will always include the pin assignments and maximum ratings by searching the manufacturer's website, or visiting one of the online datasheet catalogs.

Linear ICs

The most common type of linear IC is probably the amplifier. Amplifier ICs are available in a wide range of types and sizes. Small signal amps are used extensively in most consumer electronics, a single TV may contain several dozen for amplifying, inverting, and comparing audio, video and sync signals. Larger amplifier ICs with low output impedances are available for driving things like louspeakers and headphones. Most of these amps will require at least a few external resistors and capacitors, some of which may be used to change the amount of amplification or the frequencies to be amplified.

Another very common linear IC is the voltage regulator, specifically the L78xx series. A voltage regulator combines a voltage reference like a zener diode, several transistors and some form of feedback into a single chip. These come in a three lead package, TO-92 for low current models and TO-220 for 1A and higher. The last two digits in the model number indicate the voltage the circuit will output, for example, L7812 is a 12V regulator. The three leads are usually labeled input, output, and ground, with the output voltage being referenced to the ground. In most cases, the only external components required are a single capacitor. Adjustable regulators like LM317 use an external potentiometer connected to the ground lead to allow fine adjustment of the output voltage.

Digital ICs

These use different voltage levels to represent the binary values 0 and 1, commonly 5V for 1 or true, referred to as logical high and 0V for 0 or false, referred to as logical low. Digital circuits have a clock signal, which moves the bits through the system, one step at a time. The faster the clock signal, the faster the circuit will operate.

The most common digital ICs are probably logic gates. There are a variety of logic gates available, referred to by the logical function they perform. Logic gates commonly available include AND, OR, XOR, NAND, NOR, XNOR, inverters and buffers. Each type of logic gate has a truth table or state chart, which tells what the logic gate will output for a given input. For example, an AND gate will output true only if both inputs 1 AND 2 are true, while an OR gate will output true if input 1 OR 2 is true. Gates that have an N in front of their name invert their output. Inverters, which are sometimes called NOT gates, have a single input and a single output, with the output being the inverse of the input. Buffers, like inverters, have a single input and output, with the output being the same as the input. Buffers and inverters are commonly used to boost a digital signal before it travels down a line. Most logic gates will pass some of the voltage applied to their inputs, even if the voltage is between high and low. In situations where this might cause a problem, logic gates with schmidt trigger inputs can be used. This type of trigger will only change state when a suitable voltage is applied.

Part numbers for logic gates can get a little weird. In the olden days, logic gates had four or five digit part numbers starting with eirther 74 or 40. For example, 7420 is a Dual 4-input NAND Gate, 4066 is a Quad Bilateral Switch, etc... For each generation they've added more and more letters, but the only ones that really matter are the ones starting with 74 or 40. For example, 74LHCT20-T can be used in place of 7420, 74HCT4066D can be used instead of 4066, and so on. For the most part, logic gates are forward compatible. The new parts behave the same way but operate faster or on less power or something.

Other useful types of digital ICs include multivibrators and flip-flops. A flip-flop changes the state of its output whenever its input is brought high. A multivibrator can oscillate at a frequency determined by the external components attached to it, producing a square wave. Timer ICs like the NE555 are commonly employed wherever something needs to happen at certain intervals, or after a certain amount of time. Multiplexers, encoders, and decoders can be used when you need to select between different inputs.

The most complicated types of digital circuits are things like microprocessors, microcontrollers, PGAs, and memory. A microprocessor is a collection of thousands or even millions of logic gates, along with some high speed memory and control logic. Common examples of microprocessor architectures include Intel IA32, AMD 64, Motorola 68k, POWER, ARM, z80, and MIPS. A microprocessor interfaces to a memory array via an external data bus. Memory chips of the type used in modern PCs are arrays of millions of capacitors connected through multiplexers and selectors. The microprocessor moves the position in memory that it wants to read to its address bus, and the memory returns the value stored at that address to the CPU through the data bus. The CPU can then perform operations on that data in its arithmetic logic unit and then store the result back in memory.

A microcontroller is a computer on a chip that combines a microprocessor, memory, and program storage in a single package. Microcontrollers are used in almost all electronic appliances to select appropriate actions to perform based on user input. Any appliance with a keypad and/or display will have at least one microcontroller. The microcontroller generally has a number of general purpose I/O pins that can be used to turn LEDs on and off, check the status of buttons or sensors, or communicate with other integrated circuits. Common types of microcontrollers include Microchip's PIC, Atmel's AVR and various designs based on ZiLOG's z80 and Motorola's 6800 8-bit microprocessors, which were used as the CPU for home computers and video games in the 80's.

A more recent innovation is the programmable gate array, sometimes called a field programmable gate array or FPGA. These are essentially a collection of thousands of logic gates that can be programmed to operate in a certain way. Common uses for FPGAs are in hardware based cryptography, and in other applications where a lot of resource intensive computation will be done, such as video encoding.

Mixed Signal ICs

This is a term used for ICs that have both digital and analog parts on the same chip, or for analog parts with a digital control. The most prevalent type of mixed signal IC are digital-to-analog and analog-to-digital converters. D/A converters take a digital input, and output an analog voltage, whereas A/D converters do the opposite, taking an analog voltage and quantifying it as a number. Turning a continuous analog signal such as sound into digital data requires the A/D converter to sample the sound repeatedly. The number of times the voltage is sampled is referred to as the sampling frequency. CD quality audio is sampled 44,100 times per second, a sampling frequency of 44.1kHz. Each sample is a certain number of bits long, referred to as its resolution. CD audio samples are 16 bits long, meaning that there are 65,536 possible voltage levels. An eight bit sample only provides 256 possible voltage levels, whereas a 24 bit sample will provide 16,777,216 possible values. The larger the sample, the more accurate it will be.

D/A converters perform the opposite function. The circuit is given a number, and it uses a multiplexer to select one of a series of resistors arranged in what's called a ladder network. Each number will have its own voltage value, for example, an 8-bit DAC with a 1V power supply would interpret the number 125 as 0.488V, 126 as 0.492V, and so on. If you want to know more about DACs, you may be interested in the hi-fi audio DAC that I made.

Logic gates that do not have schmidt trigger inputs are sometimes used in analog circuits. Specifically, things like multivibrators are sometimes used to provide an oscillating tone, multiplexers and switches are sometimes used to select between different analog inputs, and things like AND an OR gates may be useful for comparing two or more analog voltages.

Packages and types

Integrated circuits come in a wide variety of packages. The most common type of package for hobby use is the dual-inline package or DIP/DIL. These ICs have two parallel rows of pins, and are very versatile as they can be plugged into a socket, soldered to a PCB, wire-wrapped, or wired point-to-point. DIPs and their sockets are available with from 8 to around 64 pins, with the higher pin count versions being longer, and usually much wider as well. Single inline package or SIP/SIL has the same pin spacing as DIP, but with all of the pins on one side. Both DIPs and SIPs have a dot, notch or other marking to indicate pin 1. The pin numbers run down that side, and up the other side.

Certain high powered ICs like power amplifiers and voltage regulators come in packages similar to those used for power transistors, which is officially referred to as TO-220, but some manufacturers use alternate names like pentawatt. These ICs generate a lot of heat, which the external cooling fin helps dissipate. If the fin by itself isn't enough, heatsinks with additional aluminum cooling fins can be bolted on. The ground, or one of the other pins may be internally connected to the metal fin, so be careful with what you allow to come in contact with it.

More recent ICs are being sold in surface mount, or SMD packages. This type of package is physically smaller than DIPs, with gull-wing leads that are meant to be soldered to the surface of a PCB. Unfortunately, this makes them fairly hard to use if you won't be soldering them to a PCB. The good news is that most ICs are also available in DIPs, and when they aren't there are inexpensive adapter boards that allow you to use them like DIPs. To further complicate matters, there are different sizes of surface mount chips; small outline integrated circuit or SOIC, small outline package or SOP, thin small outline package or TSOP, thin shrink small outline package or TSSOP, and quad flat package or QFP. By far the easiest to deal with are the SOICs. The SOP, TSOP, TSSOP and QFP are increasingly tiny and require a lot of patience, a very small soldering iron, and a steady hand to properly use. Some, like the TQFN package are virtually impossible to solder by hand.

You can usually tell which manufacturer made an IC by the first few letters of the part number; here are some common ones:
AN: Panasonic
CXD: Sony
LA: Sanyo
CS: Crystal/Cirrus Logic
LM: ST Microelectronics
KA: Samsung
BA: Rohm