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

In 1926, Julius Edgar Lilienfeld, an Austro-Hungarian physicist, proposed the concept of a field-effect transistor. However, it was not possible to actually construct a working device at that time on the given concept. In 1947, the first working device to be built was a point-contact transistor invented by American physicists named Walter Brattain and John Bardeen while working under William Shockley at Bell Labs. In 1956, the three shared the Nobel Prize in Physics for their achievement in the most widely used transistor which is the MOSFET: Metal–oxide–semiconductor field-effect transistor, which is also known as the MOS transistor. The MOSFET was known as the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses.

Silicon is the substance which is used to make the transistors. Some are also made from germanium. Certain other semiconductor materials are sometimes used to make transistors. In a field-effect transistor, a transistor may have only one kind of charge carrier or may have two kinds of charge carriers in bipolar junction transistor devices. Transistors are generally smaller as compared with the vacuum tube and require less power to operate. Certain vacuum tubes have advantages over transistors at high operating voltages or very high operating frequencies. There are many types of transistors which are made to standardized specifications.

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The key active components are known as Transistors in practically all modern electronics. In the 20th century, many transistors are thus considered to be one of the greatest inventions.

The metal–oxide–semiconductor field-effect transistor, also known as the MOS transistor, is by far the most widely used transistor. This is used in applications ranging from electronics and computers to communications technology such as smartphones. The MOSFET has been considered to be the most important transistor, possibly the most important invention in the field of electronics. It has led to the birth of modern electronics. Since the 20th century, the MOS transistor has been considered as the fundamental building block of modern digital electronics, paving the way for the digital age. The Trademark Office and US Patent calls it a "groundbreaking invention that transformed culture and life around the world". The first transistor invention at Bell Labs was named an IEEE Milestone in 2009. In 1948, the list of IEEE Milestones also included the inventions of the junction transistor and the MOSFET in 1959.

Transistor as a Switch

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Transistors are commonly used as electronic switches in digital circuits which can be either in an "on" or "off" state, both for high-power applications such as power switched-mode and the supplies of power are there and for low-power applications such as logic gates. Important parameters for this application include the voltage handled, the current switched and the switching speed, characterised by the rise and fall times.

In a circuit of grounded-emitter, such as the light-switch circuit, as the base voltage rises, the emitter and collector currents rise exponentially. The voltage collector drops the voltage because of reduced resistance from collector to emitter. If the voltage difference between the emitter and collector were zero or even near zero, the collector current would be limited only by the load resistance light bulb and the supply voltage. This process is called saturation because the current is flowing from collector to emitter and that too very freely. The switch when saturated is said to be on.

Transistor as Amplifier

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The amplifier which is also known as a common-emitter is designed so that a small change in voltage -Vin changes the small current through the base of the transistor whose current amplification combined with the properties of the circuit means that small swings in- Vin produce large changes in Vout, where V- voltage in and out.

In single transistors, various configurations of amplifiers are possible, with some providing voltage gain, some current gain and some both. From television to mobile phones, vast numbers of products include amplifiers for radio transmission, sound reproduction and signal processing. The amplifiers which have audio of the first discrete-transistor are barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture was evolved. Audio amplifiers of the Modern transistor up to a few hundred watts are common and relatively inexpensive.

FAQ (Frequently Asked Questions)

Q1. List the most common use of Transistors.

Ans: In digital circuits, the transistors are used as electronic switches which can be either in an on/off mode both for the high power applications such as the switch-mode power supplies and for the low power applications such as logistic gates.

Q2. List the three main components of a Transistor.

Ans: There are three most important parts of a transistor namely base, emitter and the collector or in the modern switching applications the gate, the source and the drain.

Q3. What is highly dropped in a Transistor?

Ans: The emitter is heavily dropped in most of the transistors. Its job is to reject or emit electrons into the base. These bases are very lightly doped and are also very thin and it possesses most of the emitter electrons injected on the collector.

Q4. Explain the PNP Transistor.

Ans: An PNP transistor is the just opposite of the NPN transistors. The PNP transistor uses the base current for controlling much large emitter-collector current.