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The Bipolar Junction Transistor (BJT)

Updated on December 6, 2012
A Bipolar Junction Transistor or BJT
A Bipolar Junction Transistor or BJT | Source

Transistor History and Definition

Back in 1904 to 1907 the vacuum tube was the device in which development was mostly centered to. J. A. Fleming introduced the vacuum tube in 1904. Then in 1906, Lee De Forest added a third element to the Vacuum tube – the control grid, which then resulted to the first amplifier. However, Vacuum tubes have some disadvantages, first it is huge and second it requires a lot of power and creates a lot of heat. This kept the vacuum tube from being used in smaller electronic circuits.

In December 1947, Walter Brattain and John Bardeen was able to amplify signal using what we now call as a transistor. A transistor is an electronic device that is used mainly in switching and amplification of signals.

The word transistor came from Transfer and resistor. The word means that current was transferred from a low resistance side to a high resistance side of the circuit. Transistors are more advantageous in many ways because it is smaller and does not require heat-up and needs less power.

NPN BJT Transistor
NPN BJT Transistor | Source

The Construction of a Bipolar Junction Transistor

A transistor is composed of three semiconductor materials – a p-type material sandwiched between two n-type materials or an n-type material sandwiched between two p-type materials. An N-type material is a semiconductor material where pentavalent elements are doped into the crystalline structure. A P-type material is a semiconductor where trivalent elements are doped into the crystalline structure.

Doping is a process of adding impurities to a pure substance. For semiconductors it is done to increase or decrease the material’s conductivity.

The sandwiched material is smaller than the other two and also has a lesser doping ratio. This lower doping level decreases its conductivity because the number of free carriers is reduced.

Operation of the Bipolar Junction Transistor

BJT Transistor Operation
BJT Transistor Operation | Source

A transistor has three terminals and each terminal is connected to a single semiconductor material – either p-type or n-type. Each terminal is labeled collector, base or emitter. A Bipolar Junction Transistor, from the word Bipolar, means that it relies on both minority and majority carrier flow. If it only relies on one then it cannot be called bipolar but it then will be Unipolar.

A transistor requires proper voltage polarities, or bias, in its elements for it to function properly. Consider a PNP transistor, P is the collector, N is the base and P is the emitter. A transistor functions properly when the PN junction is reverse biased. By reverse biased we mean that it is biased in such a way that it will restrict most of the current flow. The other NP junction should then be forward biased.

If we trace the flow of current we will see that it first comes from the collector which is then impeded by a high resistance. The result is lesser current through the N-type material. There will also be current in the base but that current is small. Since the NP or base-emitter junction is forward biased, the base current and what’s left of the collector current adds up then goes out of the emitter.

The combination of the flow of majority carriers (electrons) and minority carriers (holes, resulting from the reverse bias condition of the collector-base junction) causes the amplification operation of a Bipolar Junction Transistor or BJT.


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    • leakeem profile image

      leakeem 5 years ago from Earth

      It can do a lot of things and one of them is that it amplifies a signal.

    • Window Pain profile image

      Window Pain 5 years ago

      Okay, then what does it do?