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Alternating Current (AC):
A type of electrical current in which the direction of the flow of electrons switches back and forth. In the US, the current that comes from a wall outlet is alternating; it cycles back and forth sixty times each second. The current that flows in a flashlight, on the other hand, is direct current (DC), which does not alternate.

A device which takes in a weak electric signal and sends out a stronger one. Amplifiers are used to boost electrical signals in many electronic devices, including radios, televisions, and telephones. Both vacuum tubes and transistors can be amplifiers, though today vacuum tubes are rarely used for this purpose.

The smallest possible piece of any pure element that still has the properties of that element. Atoms are made of smaller particles including electrons, protons, and neutrons. Differences in the numbers of these particles create the differences between the elements. An atom is about 500-billionths of an inch, or a hundred millionths of a centimeter across.

A special kind of vacuum tube which can be used to amplify weak electric signals. The audion was used in the Bell phone system, as well as in early radios and computers. It was eventually replaced in most applications by the transistor.

A device that stores electrical charge, using a positively charged surface and a negatively charged surface with a gap between them. The Leyden jar, used by early electrical experimenters (including Benjamin Franklin) was a form of capacitor. A smaller kind of capacitor is often used in electrical circuits.

Cat's Whisker:
A tiny metal wire used in early home radio kits. Radio listeners carefully moved the wire tips around on the surface of a crystal to just the right spot to allow an electrical signal to travel down one wire, through the crystal and out the other wire. If the wires were positioned on the right spot, and it was often tough to find the right spot, the radio signal could be heard through the earphones.

An electrical property of particles, such as electrons and protons, which causes them to attract and repel each other. A material with an excess of electrons is defined to have a "negative" charge; material with an absence of electrons (or an excess of protons) is defined as "positive." Materials with a balanced number of electrons and protons are called "neutral." Positive and negative charges attract each other. That attraction can cause interesting effects at the junction between positive and negative semiconductors. This special junction is what makes the right configuration of semiconductors work as a transistor.

A string of electronic devices such as transistors, resistors, capacitors, and diodes connected by wires so that current can run through it in a complete loop. Circuits can be simple or complex. The wiring connecting a switch to a light to the power source and back to the switch is a simple circuit; opening the switch breaks the circuit and stops the current flow. Even a computer chip is simply an extremely complicated network of circuits.

Any material that easily allows the flow of electricity. Metals are good conductors. Such materials conduct electricity because electrons can move from one atom of a conductor to the next, forming an electric current.

A chunk of solid material in which all the atoms are lined up in an orderly pattern like rows of oranges in a grocery store. Transistors are made out of semiconductor crystals. Growing perfect germanium and silicon crystals with no defects or unwanted impurities is key to building a working transistor.

The flow of charge carriers (holes or electrons) through a conducting wire or crystal.

Rectifier. An electronic device with two wires or terminals. A rectifier allows electrical current to flow through in only one direction and is used for converting alternating current into direct current. Rectifiers were important for use in radios, which required direct current to power the amplifiers driving speakers or headphones.

Direct Current (DC):
Current which moves in a single direction in a steady flow. Normal household electricity is alternating current (AC) which repeatedly reverses its direction. However, many electronics devices require DC, and therefore must convert the current into DC before using it. Diodes are used to convert AC to DC.

Deliberately adding a very small amount of foreign substance to an otherwise very pure semiconductor crystal. These added impurities give the semiconductor an excess of conducting electrons or an excess of conducting holes (the absence of conducting electrons) which is crucial for making a working transistor.

Electric Field:
The region around any electrically charged material contains an electric field that affects other charged objects. The field around a negatively charged material pulls positively-charged objects in toward the material, while negatively-charged objects are pushed away. Around a positively charged material, on the contrary, negative objects are attracted and positive objects are repelled. The strength of the field gets rapidly weaker as one moves further away from the charged material.

Electric Signal:
Information expressed through changes in an electric current. Information can be encoded in on-and-off switches of current, in the amplitude of the current, or in other easily-detectable changes. Sound waves, for example, can be converted to electricity by a microphone and sent as an electrical signal through the wires of a stereo to the speakers.

An electrical lead or wire attached to any electronic device or circuit through which current may flow in or out.

Electrons are one of the particles that makes up an atom and particles that, in motion, can form an electric current. An electron is 2000 times lighter than the lightest atom.

Any of over a hundred fundamental materials containing only one kind of atom. Some common elements are oxygen, gold, hydrogen, and silicon. All other materials are made of compounds or mixtures of elements. Water, for example, is made of two hydrogen atoms attached to one oxygen atom.

Field-effect transistor:
The most common type of modern transistor, and the type of transistor used in integrated circuits. The field-effect transistor is so named because an incoming weak electrical signal creates an electrical field across a section of semiconductor. This field causes a second electrical current to flow across the semiconductor, identical to the first weak signal, but stronger.

Four-layer diode:
A semiconductor device which can take alternating current and turn it into direct current but which also stops letting any current through once a certain voltage is reached. The diode is made of four layers of alternating types of semiconductor.

A chemical element that acts as a semiconductor, meaning sometimes it conducts electricity and sometimes it doesn't. The first transistors were all made out of germanium.

When an array of atoms in a crystal is missing a conducting electron, it's said to have a "hole." Since conducting electrons are negative, holes are positive. Even though holes are an absence of a conducting electron, scientists often talk about holes flowing as if they were real particles.

Any material that does not conduct electricity. Glass and rubber are common insulators. These materials are made of atoms which don't allow electrons to move freely, which means there can be no electric current.

Integrated Circuit:
A collection of transistors and electrical circuits all built onto a single crystal. Today's integrated circuits are no more than a centimeter long, and they can carry millions of microscopic transistors. All computers have integrated circuits inside.

Junction Transistor:
The second type of transistor built, and a direct ancestor of modern day transistors. It consists of two sections of one type of semiconductor (N- or P-) around a middle slab of the other type. The junctions between the semiconductor sections cause an incoming weak electrical signal to be amplified.

A circuit of transistors and other electrical components on a chip that can process programs, remember information, or perform calculations.

N-type semiconductor:
A semiconductor which has an excess of conduction electrons. A semiconductor can be made into N-type by adding trace amounts of another element to the original semiconductor crystal. Today's transistors all require sections of both N-type and P-type semiconductors.

P-type semiconductor:
A semiconductor which has an excess of conducting holes. It is created by adding trace amounts of other elements to the original pure semiconductor crystal. Today's transistors all require sections of both N- type and P-type semiconductors.

Point-contact transistor:
The very first type of transistor ever made, invented by John Bardeen and William Brattain. So-called because it consists of two metal points making light contact with the surface of a germanium crystal.

See Diode

A material that conducts more than an insulator but less than a conductor. Some semiconductors conduct at some times but not at others. Some common semiconductors are silicon and germanium. Transistors are made out of semiconductor crystals.

Silicon is one of the most common elements on Earth in the Earth's crust, it's second in mass only to oxygen and can be found in any quartz crystal. Beach sand is largely silicon. Silicon is also the semiconductor material out of which almost all modern transistors are made.

Solid State:
Relating to the properties of solid, usually crystalline, materials including semiconductors. Solid state science is often a mixture of physics, and chemistry, and materials science.

Transistors are tiny electrical devices that can be found in everything from radios to robots. They have two key properties: 1) they can amplify an electrical signal and 2) they can switch on and off, letting current through or blocking it as necessary.

Type-A transistor:
What Bell called the first point-contact transistors they built. These were installed into the Bell Phone system, and given to the military and scientists for other purposes as well -­ but the Type-A transistor was never widely used.

Voltage is a measure of the energy required to move a charge from one point to another. A difference in the amount of electric charge between two points creates a difference in potential energy, measured in "volts," which causes electrons to flow from an area with more electrons to an area with fewer, producing an electric current.

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