Actions of Transistor

A transistor is a device that functions as a switch or gate for electronic signals and regulates current or voltage flow. Three layers of a semiconductor material make up a transistor, each capable of carrying a current.

Three scientists at Bell Laboratories devised the transistor in 1947, and it quickly supplanted the vacuum tube as an electronic signal regulator. A transistor functions as a switch or gate for an electronic signal, regulating current or voltage flow. Three layers of semiconductor material make up a transistor, each capable of conducting a current. A semiconductor, such as germanium or silicon, is a substance that conducts electricity in a “semi-enthusiastic” manner.

Transistor

A transistor is created when a third doped element is added to a crystal diode, resulting in the formation of two PN junctions.

A transistor is made up of two PN junctions, which are constructed by sandwiching a p-type or n-type semiconductor between two n-type or p-type semiconductors.

There are two types of transistors, namely:

1. N-P-N transistor  

2. P-N-P transistor

Two n-type semiconductors are separated by a thin layer of p-type semiconductor in an n-p-n transistor, as shown in Fig.1(i). In a p-n-p transistor, as shown in Fig.1, two p-type semiconductors are separated by a thin layer of n-type semiconductor (ii).

Fig.1(i)

Fig.1 (ii)

Each type of transistor has the following characteristics:

1. This is a pair of PN junctions. As a result, a transistor can be thought of as a pair of diodes connected back to back.

2.  Three terminals are present, one for each type of semiconductor.

3. The middle layer is very thin. This is the most crucial aspect of a transistor’s operation.

Three pieces of doped semiconductors make up a transistor (NPN or PNP). The section on one side is known as the emitter, while the section on the other side is known as the collector. The base is the centre portion, and it connects the emitter and collector in two places.

1. Emitter: The emitter is the part on one side that supplies charge carriers (electrons or holes). To deliver a large number of majority carriers, the emitter is always forward leaning with respect to base. The emitter (p-type) of a PNP transistor is forward pushed in fig.2 (i) and supplies holes to the base junction. The emitter (n-type) of a NPN transistor has a forward bias and supplies free electrons to its junction with the base, as shown in fig.2 (ii)

Fig.2(i)

Fig.2 (ii)

2. Collector: The emitter is the part on one side that supplies charge carriers (electrons or hole). To deliver a large number of majority carriers, the emitter is always forward leaning with respect to base. The emitter (p-type) of a PNP transistor is forward pushed in fig.2 (1) and supplies holes to the base junction. The emitter (n-type) of a NPN transistor has a forward bias and supplies free electrons to its junction with the base, as shown in fig.2 (ii).

3. Base: The base is the central part between the emitter and collector that forms two PN-junctions. Because the base-emitter junction is forward pushed, the emitter circuit has a low resistance. The base-collector junction is Reverse pushed and provides high resistance in the collector circuit.

Transistor Action

Before going into detail on transistor action, it’s vital to remember the following facts about transistors:

1. The emitter, base, and collector are the three sections of the transistor. The emitter is much thinner than the base, and the collector is much wider than both. However, for simplicity’s sake, the emitter and collector are commonly depicted as being of identical size.

2. The emitter is extensively doped to allow a large number of charge carriers to be injected into the base.

3.  The base is quite thin and weakly doped. In order for less than 5% of charge carriers from the emitter to recombine in the base. It can also pass most of the charge carriers injected by the emitter to the collector.

4. The collector is doped to a degree.

5. Because the transistor contains two PN junctions, it functions similarly to two diodes. The emitter-base diode, or simply the emitter diode, is the junction between the emitter and the base. Collector-base diode or simply collector diode refers to the junction between the base and the collector.

6.  The collector diode is always reverse forced, while the emitter diode is always forward partial.

7. When compared to the collector diode, the emitter diode has a very low resistance. As a result, forward bias on the emitter diode is typically very low, whereas reverse bias on the collector diode is typically much higher.

Working of NPN Transistor

The electrons in the n-type emitter flow towards the base due to the forward bias. The emitter current IE is made up of this.

These electrons tend to join with holes when they pass through the p-type base. Because the base is lightly doped and thin, just a few electrons, less than  5%, mix with holes to form the IB base current.

More than 95% of the remaining electrons pass across into the collector region to form collector current IC.

The collector circuit receives practically all of the emitter current in this manner.

The total of the collector and base currents is the emitter current.

IE=IB+IC

NPN transistors Used

In amplifying circuit applications, NPN transistors are used. In Darlington pair circuits, NPN transistors are utilized to enhance weak signals. In applications where sinking current is required, NPN transistors are used. NPN transistors, like ‘push-pull’ amplifier circuits, are employed in some classic amplifier circuits.

Conclusion

A transistor is a device that regulates current or voltage flow by acting as a switch or gate for electronic signals. A transistor is made up of three layers of semiconductor material, each capable of conducting a current.

In 1947, three Bell Laboratories scientists invented the transistor, which swiftly replaced the vacuum tube as an electronic signal regulator. A transistor controls current or voltage flow by acting as a switch or gate for electrical signals.

When a third doped element is added to a crystal diode, two PN junctions are formed, resulting in a transistor.