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Resources >> Transistor Basics

Transistor Basics

History of Transistors
A transistor is a small electronic device that can cause changes in a large electrical output signal by small changes in a small input signal. Simply put, transistors amplify current. Transistors can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage. Alternatively, a transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

The transistor can be said to be one of the most important inventions of the 20th century. The transistor as we know today was developed by Bell Labs in 1948. At that time, it received little attention. However, people gradually realized the potential of the transistor and by 1960, most electronic devices (like radios, computers, etc.) used them in place of vacuum tubes and valves.

Working of a Transistor
Transistors work on the principle that certain materials are conductive in proportion to the number of  "free"? electrons that are available to conduct the electric charge. For example, silicon crystals have very free electrons in its natural state. By adding some impurities to it in a controlled manner, the conductivity of silicon can be increased. Such a material is then called as semi conductor. Semiconductor material which conducts by virtue of excess free electrons is called n-type material while material which conducts by virtue of electron deficiency is called p-type. If gallium is added to silicon, it reduces the number of free electrons and becomes p-type. If arsenic is added, there is a surfeit of electrons and the silicon then becomes n-type.
A standard transistor consists of three layers of silicon or germanium semiconductor material. Impurities (as mentioned above) are added to each layer to create a specific electrical positive or negative charged behavior. Depending upon the impurities added, the transistor can be classified into two types: NPN and PNP. The 'P' stands for a positively charged layer and the 'N' stands for negatively charged layer. There is no particular difference between PNP and NPN transistors except the polarity of voltages that need to be applied to make the transistor operate. 

Transistors are useful as they have the ability to use a small signal at one terminal to control a much larger signal at the other. This property is called the gain of the transistor. Transistors used in this manner are called as 'amplifiers'. A transistor can also be used to turn current on or off like a 'switch'.

The weak input signal (that gets amplified by the transistor) is applied to the center layer called the base and usually referenced to ground which is also connected to the bottom layer. The bottom layer is called the emitter. The larger output signal is take from the collector also referenced to ground and the emitter. Additional resistors and capacitors are required along with at least one DC power source to complete the transistor amplifier.

Transistor Types
Out of the transistors mentioned above, the more common transistor is the bipolar NPN transistor. There is one more type of transistor, called the FET transistor. A FET transistor is high input impedance transistor, Generally transistors fall into the category of bipolar transistor, either the more common NPN bipolar transistors or the less common PNP transistor types. There is a further type known as a FET transistor which is an inherently high input impedance transistor. Modern field effect transistors or FET's including JFETS and MOSFETS now have some very rugged transistor devices.

Transistors have replaced vacuum tubes and valves almost everywhere, and are nowadays manufactured as handy modules.