Transistors - How They Work

Transistors are present in every electronic device. So it is obvious to think about the question, how do transistors work? This article will explain exactly that.

A Few Historical Figures
Some of the historical figures who can be associated with the development of transistors can be listed as under.

• Julius Edgar Lilienfeld: A Canadian who filed for the patent, which theorized the working of transistor.
• Oskar Heil: Also filled a similar patent as that of Julius Edgar Lilienfeld.
• John Bardeen : Observed higher output on application of electrical signals to germanium crystals.
• Walter Brattain : Also, observed higher output on application of electrical signals to germanium crystals. John and Walter both worked together on crystals.
• William Shockley : Researched greatly on the semiconductor crystals, attributed as 'father of transistor'.
• John R. Pierce : Coined the term 'transistor'.
• Gordon Teal: He invented the first silicon transistor

Doping of Semiconductor
Doping is a process in which a semiconductor material is added with impurities so that its conductive properties are enhanced. Whenever charge flows through the semiconductor, molecular diffusion takes place. Again, molecular diffusion happens when charge from a highly dense area, flows to a low dense area of the same charge. Doping increases the charge density in that material. Some semiconductors are doped to charge them positively (p-type semiconductor), while some are doped for making them negatively charged semiconductor (n-type semiconductor).

Working of a Transistor
A transistor is nothing by an n-type semiconductor sandwiched in between two p-type semiconductors or a p-type semiconductor sandwiched in between two n-type semiconductors. The working of a PNP and an NPN transistor is very similar. In a pnp transistor there are three terminals. The most quarried about transistor types are pnp transistor and npn transistor which are basically the sub types of Bipolar Junction Transistor (BJT). Let us try to understand the two different types of BJTs before going into the details of the working of a bipolar junction transistor.

PNP Transistors
In both PNP and NPN transistors the letter N denotes negative and letter P denotes positive charge carriers. When the negatively charged semiconductors is embedded between two positively charged semiconductors, then such a BJT is called pnp transistor.

NPN Transistors
When the positively charged semiconductors is embedded between two negatively charged semiconductors, then such a transistor is called an NPN transistor. In both the transistors, the base is the odd one out.

Let us now, try to understand the working of a transistor using the pnp transistor as an example. In a pnp transistor, the p-type semiconductor, which is most heavily doped is the emitter, the n type semiconductor is the base and the other p-type semiconductor is the collector. The n-type semiconductor, which is sandwiched in between the two p-type semiconductors is very small in dimension. Now, in a pnp semiconductor, the emitter base junction is forward biased (p-type semiconductor is connected to the positive terminal of the battery and the n-type semiconductor is connected to the negative terminal of the battery). The collector base junction on the other hand is reverse biased (n-type semiconductor is connected to the positive terminal of the battery and p-type semiconductor is connected to the negative terminal of the battery). The forward biased emitter-base junction of a semiconductor works very similar to that of a forward biased p-n junction diode. Know more on how does a diode work.

Now the holes, or the positively charged carriers in the p-type semiconductor are repelled by the positive terminal and enter the base on increasing the emitter base voltage beyond a certain threshold level. Once these holes enter the base, they combine with the electrons in the base. Out of every 100 holes, approximately 5 holes combine with the electrons in the base of the semiconductor. The base being very thin, the remaining holes are transferred to the collector. The holes in the collector are attracted towards the negative terminal of the battery between the collector and the base. This causes the collector current. Thus, the flow of current in a pnp transistor from the emitter to the collector. Now the question is how can a transistor be used as an amplifier. A simple change in the base voltage can cause large changes of current in the transistor. The various functions of a transistor include its applications as amplifiers and switches for the very same reason.

Key Facts about Transistors
Following are a few key facts regarding transistors.

• There are more than 35 different kinds of transistor.
• Before the term transistor was coined, it was called the 'three electrode circuit element'.
• There are 8 categories of transistors.
• There are billions of transistors in a microprocessor.

After the invention of silicon transistors, the field of electronics has simply taken off. The advent of transistors has given great impetus to the understanding of science and electronics. So we must put good effort in answering the question, how do transistors work. Many scientists consider the invention of transistor as the biggest discovery in the field of electronics in the last century. Hope this article on 'transistors - how they work' proves to be of help to you.