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Transistors - FAQS

Transistors

Transistors

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

 

Transistors are tiny switches that can be triggered by electric signals. They are the basic building blocks of microchips, and roughly define the difference between electric and electronic devices. They permeate so many facets of our daily lives, in everything from milk cartons to laptops, illustrating just how useful they are.

Characteristics and Materials
Transistors are composed of three parts: a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts ON, less than five volts OFF.
Semi-conductive materials are what make the transistor possible. Most people are familiar with electrically conductive and non-conductive materials. Metals are typically thought of as being conductive. Materials such as wood, plastics, glass and ceramics are non-conductive, or insulators. In the late 1940 a team of scientists working at Bell Labs in New Jersey, discovered how to take certain types of crystals and use them as electronic control devices by exploiting their semi-conductive properties.Most non-metallic crystalline structures would typically be considered insulators. But by forcing crystals of germanium or silicon to grow with impurities such as boron or phosphorus, the crystals gain entirely different electrical conductive properties. By sandwiching this material between two conductive plates (the emitter and the collector), a transistor is made. By applying current to the semi-conductive material (base), electrons gather until an effectual conduit is formed allowing electricity to pass The scientists that were responsible for the invention of the transistor were John Bardeen, Walter Brattain, and William Shockley. Their Patent was called: Three Electrode Circuit Element Utilizing Semiconductive Materials.

 

 


Transistors are devices that control the movement of electrons, and consequently, electricity. They work something like a water faucet -- not only do they start and stop the flow of a current, but they also control the amount of the current. With electricity, transistors can both switch or amplify electronic signals, letting you control current moving through a circuit board with precision.

Bipolar Junction Transistors

The most common type of transistor is a bipolar junction transistor. This is made up of three layers of a semi-conductor material in a sandwich. In one configuration the outer two layers have extra electrons, and the middle layer has electrons missing (holes). In the other configuration the two outer layers have the holes and the middle layer has the extra electrons.

 

 

Transistor Types

Description

Applications


Bipolar Junction Transistors

Bipolar Junction Transistors are transistors which are made up of 3 regions, the base, the collector, and the emitter. Bipolar Junction transistors, unlike FET transistors, are current-controlled devices. A small current entering in the base region of the transistor causes a much larger current flow from the emitter to the collector region.
Bipolar junction transistors come in two main types, NPN and PNP. A NPN transistor is one in which the majority current carrier are electrons. Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The other type of charge, holes, are a minority. PNP transistors are the opposite. In PNP transistors, the majority current carrier are holes.

   

Field Effect Transistors

Field Effect Transistors are transistors which are made up of 3 regions, a gate, a source, and a drain. Unlike bipolar transistors, FETs are voltage-controlled devices. A voltage placed at the gate controls current flow from the source to the drain of the transistor.

   

Small Signal Transistors

small signal transistors are used primarily when amplifying small signals, such as a few volts and only when using milliamperes of current. When using larger voltage and current (larger power), using many volts or amperes of current, a power transistor should be used.

Small Signal Transistors are transistors that are used primarily to amplify low-level signals but can also function well as switches.

Small Switching Transistors
Small Switching Transistors are transistors that are used primarily as switches but which can also be used as amplifiers. Typical hFE values for small switching transistors range from 10 to 200, with maximum Ic ratings from about 10 to 1000mA. They come in NPN and PNP forms.

Power Transistors
Power transistors are suited for applications where a lot of power is being used- current and voltage.

High Frequency Transistors
High Frequency (RF) Transistors are transistors that are used for small signals that run at high frequencies for high-speed switching applications.

Phototransistors
Phototransistors are light-sensitive transistors.

Unijunction Transistors
Unijunction transistors are three-lead transistors that act exclusively as electrically controlled switches; they are not used as amplifiers.