Transistors are semiconductor devices that are used in modern electronic equipment to switch or amplify electronic signals. They are also used for the same purpose in electrical power machinery. Transistors are widely accepted to be the basic building blocks of modern electronics machinery. They are manufactured from semiconductor materials.
Semiconductor definition: Semiconductors are naturally occurring materials that exhibit the behaviour of both conductors and insulators. However, they are neither good conductors nor good insulators. This property of semiconductors can be exploited by means of doping to achieve controlled current flow that is very important for the functioning of a transistor.
A transistor has three terminals that connect it to any external circuit. The voltage and current at any two terminals can be used to control the voltage and current at the other two terminals. Today, transistors are packaged individually. But many more transistors are found embedded in IC (integrated circuits).
Transistors are important components of almost all modern electronic devices. From supercomputers used in advanced labs to the smartphone seen in everybody’s hands, transistors are used in all the major electronics devices we see around us. As a result, transistors are considered by many to be one of the most important inventions of the 20th century.
MOSFET transistors, also called MOS transistors, are the most widely used transistors. They are used more than any other transistor in existence currently. The first MOS transistor was invented at Bell laboratories in 1959. The inventors were Mohamed Atalla and Dawon Kahng. Bell Labs is a very prominent research institution in the US.
With the invention of the MOS transistors, we entered the digital age. MOS transistors are used extensively in digital electronics. The processors we use in our personal computers have MOS transistors as a very important component. Today, MOS transistors are mass-produced using a highly automated process.
When a small signal is applied between one pair of terminals in a transistor, this signal can be manipulated to control a much larger signal at the other pair of terminals. This property of a transistor is called gain because the signal is acquiring strength in the process of switching or amplification. A strong output is therefore produced at the other pair of terminals. This output can be voltage, current or an electronic signal. The output is always proportional to the input signal. Due to this property, the transistor can act as an amplifier.
Transistors can also be used to turn current on and off in a circuit. The use of the transistor makes the circuit electrically controllable. The amount of current that flows through the transistor is determined by other circuit elements.
Bipolar transistors have base, emitter and collectors as the three terminals. A small current between base and emitter controls and switches a much larger current between the collector and emitter.
Transistor switches exist in either on or off state. Transistor switches are used in both high powered appliances and low powered appliances. One example of use in a low powered appliance is the use of switch transistors in logic gates. Logic gates are the building blocks of the processors used in the computers that we use.
Important parameters of a switch transistor are the magnitude of current switched, the voltage handled, and the speed at which the switching takes place. This is described by the phenomenon of rise and fall times.
Grounded emitter transistor circuits are most commonly used as switches in circuits. If the base current rises even slightly in such transistors, the emitter and the collector current show a dramatic increase. The collector voltage drops because of reduced resistance between the collector and the emitter. If the voltage difference between the collector and the emitter is zero, the collector current only depends upon load resistance and the supply voltage. This condition is called saturation, and the transistor is said to be in an on state.
An emitter amplifier transistor is designed in such a way that a small change in voltage changes the small current that is transitioning through the base of the transistor. The current amplification at the base configures the transistor in such a way, combined with the properties of the circuit that a small change in the input voltage produces a proportionately large change in the output voltage.
Various configurations are possible for the amplifying transistor. In some configurations, voltage gain is achieved, in other configurations, the current gain is achieved. The amplifying transistors are used in a variety of devices-- from mobile phones to televisions. They amplify and reproduce sound, are used in radio transmission and also help in signal processing.