A transistor is an electronic component that uses semiconductors to amplify and switch electronic signals and power. Transistors are composed of a semiconductor material with a minimum three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Transistors replaced vacuum tubes and are more efficient. Today, some transistors are packaged individually as shown in the picture above, but more often they are embedded in integrated circuits.
A single model of transistor can be made in several different packaging materials which are usually made of glass, metal, ceramic, or plastic. The only thing that the package material often dictates the power rating and frequency characteristics of that particular transistor. Power transistors have larger packages with a hole so that they can be bolted to heat sinks to enable cooling.
There are two types of transistors, which have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing from the base to the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain. The collector in some power transistors may be connected directly to the metal of the enclosure.
Transistors are commonly used as electronic switches, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates. Transistors are commonly used to amplify a signal. The design of a transistor uses its common-emitter amplifier to allow a small change in voltage connected to the base of the transistor to produce a larger change in the output of the transistor.
There are many different types of transistors and most look virtually the same making it impossible to tell the difference between two types without looking them up. The two most common types of transistors are NPN and PNP. These two types represent the two basic types of setups for transistors.
In a PNP transistor, the current passes through the emitter before it reaches the base, and the current leaving the base passes through the collector; for example, "emitter-base-collector." PNP is an acronym for the polarity of the transistor; the emitter is positive, the base is negative, and the collector is positive. The majority current carrier in a PNP transistor are holes, or a lack of electrons, rather than electrons.
In a NPN transistor, the emitter and collector reverse roles, so the current passes through the collector before reaching the base and leaves through the emitter; for example, "collector-base-emitter." An NPN transistor's polarity is negative-positive-negative, and the majority current carriers are electrons instead of holes. Because NPN and PNP have opposite charges, the battery's bias, or polarity powering the transistor, is reversed.
The labeling methods for transistors can sometimes be inconsistent but there is a general rule for naming them that can be used to find out more about a transistor based on the information provided on its label.
The types of some transistors can be determined based on the information provided in the label. There are three major semiconductor labeling standards. In each standard there is an alphanumeric prefix that provides clues to type of the device. The three labeling standards are include the Japanese Industrial Standard, Pro Electron Standard, and the JEDEC EIA370 Standard. There are also other naming conventions that are standard to specific companies which only adds to the confusion and may not always provide a reliable source of determining the transistor's identity. These issues that occur in manufacturing labels are only added to when yo consider how some transistors have duplicate labels such as the J176 low-power Junction FET and the higher-powered MOSFET 2SJ176 (both of which are labeled J176).
The first category of transistors is based on the date that the type of semiconductor material was first used to make transistors. The
metalloid germanium (1947) and silicon (1954)— in amorphous, polycrystalline and monocrystalline form; the compounds gallium arsenide (1966) and silicon carbide (1997), the alloy silicon-germanium (1989), the allotrope of carbon graphene (research ongoing since 2004), etc.
The other categories are as follows:
The Second category is structure: BJT, JFET, IGFET (MOSFET), IGBT, and "other types"
Electrical polarity (positive and negative) : NPN, PNP (BJTs); N-channel, P-channel (FETs)
Maximum power rating: low, medium, high
Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for transition frequency—the frequency of transition is the frequency at which the transistor yields unity gain)
Application: switch, general purpose, audio, high voltage, super-beta, matched pair
Physical packaging: through-hole metal, through-hole plastic, surface mount, ball grid array, power modules.
Amplification factor hfe or βF (transistor beta).
Using this classification taxonomy a particular transistor can be described as something along the lines of: silicon, surface mount, BJT, NPN, low power, high frequency switch.
Types of transistors: (including some more unusual types)
- Bipolar junction transistor
- Heterojunction bipolar transistor
- Schottky transistor
- Avalanche transistor
- Darlington transistor
- Insulated gate bipolar transistor
- Photo transistor
- Multiple-emitter transistor
- Multiple-base transistor
- Field-effect transistor
- Carbon nanotube field-effect transistor (CNFET)
- Junction gate field-effect transistor (JFET)
- Metal semiconductor field effect transistor (MESFET)
- High Electron Mobility Transistor (HEMT, HFET, MODFET)
- Metal–oxide–semiconductor field-effect transistor (MOSFET)
- Inverted-T field effect transistor (ITFET)
- Fin field-effect-transistors (FinFET)
- Fast-recovery epitaxial diode field-effect transistor (FREDFET)
- Thin film transistor
- Organic field-effect transistor (OFET)
- Ballistic transistor
- Floating-gate transistor
- Field-effect transistor (FET)
- Ion-sensitive field effect transistor
- Electrolyte-oxide-semiconductor field effect transistor (EOSFET)
- Deoxyribonucleic acid field-effect transistor (DNAFET)
- Diffusion transistor
- Unijunction transistors
- Single-electron transistors (SET)
- Nanofluidic transistor
- Multigate transistors
- Tetrode transistor
- Pentode transistor
- Trigate transistors
- Dual gate FETs
- Junctionless Nanowire Transistor (JNT)
Resources: What Is a PNP Transistor? | eHow.com http://www.ehow.com/info_10026494_pnp-transistor.html#ixzz21B92sdPW, http://en.wikipedia.org/wiki/Transistor 
