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What are transistors?


Transistors are active components that control the flow of electrons. They can switch electronic signals from one type to another and they can also amplify the signal. A typical transistor is made up of semiconducting material with three pins to connect to external circuits, but they can also be found embedded in integrated circuits, millions at a time.


How is a transistor made and how does it work?


Bipolar Junction Transistor:

The most common type of transistor, a bipolar junction transistor, is constructed from P-type and N-type semiconductors that were discussed in the Diodes MathApp. While a diode is typically a P-type semiconductor bonded with an N-type semiconductor (P-N), a junction transistor differs slightly consisting of a layer of P-type sandwiched between two N-types (N-P-N) or vice versa (P-N-P). As a result, instead of a single junction, there are two junctions. The three pins of a junction transistor are called the emitter, the base, and the collector.  Each one corresponds to a layer of the semiconducting material. The collector can be thought of as the electrical supply, the emitter as the exit or outlet for the supply, and the base as the gate that controls the flow of the supply. The amount of current that flows through the transistor can be regulated by varying the amount of current that is applied to the base. A small current at the base can be used to regulate a much larger current flowing through the transistor. Due to this property, the transistor is characterized as an amplifier. Furthermore, a transistor can act as a switch because there is a threshold of voltage that must be applied at the base in order to make the transistor work. If the applied voltage is not above the threshold, then the transistor is OFF, otherwise, it is ON. As such, a transistor plays a very important role in integrated circuits as it is able to model a binary function through its role as a switch. Moreover, the use of a transistor as a switch has various advantages, such as: no moving parts, it can be switched very rapidly, the switching action can be controlled without the need of a human controller, and the entire device can be miniaturized to a very small size.


The animation below helps show how an (N-P-N) bipolar junction transistor works. The (P-N-P) transistor also functions in a similar manner. Initially when you click start, there is no voltage applied to the base, the electrons from the N-type semiconductors are diffusing into the P-type layer to fill the holes. This process creates a depletion zone (D-zone) near the junctions, which has an overall negative charge due to the electrons migrating to the P-type region. As a result,  the electric charge that tries to flow through the transistor is repelled by this D-zone and the current is not able to flow through. When you apply a small positive voltage to the base, the excess electrons in the D-zones are attracted towards this positive voltage effectively shrinking the depletion zone and allowing incoming charge to overcome the repulsive force. As a result, current can now flow through the transistor.



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