Transistors 101: A Detailed Introduction to Using Transistors

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Transistors are one of the most commonly used elements in electronic circuits. Their ease of use and simple working principle is what makes them popular among electronic designers. They mainly perform two functions: switching and amplification. You just need a few calculations to introduce this three-legged device into your next project and enable it to function correctly. So, let’s look at transistors in detail and see how you can use one in your upcoming electronic project. By the end of this blog post, you will have a solid understanding of the internal structure of a transistor, its different types, and how to incorporate them in electronic circuits.

How important are transistors, anyway?

Transistors are used in almost every electronic circuit. Moreover, they are used inside Integrated Circuits (IC), logic gates (AND, OR, NOT, XOR, etc.), and many other electronic components. On average, an IC contains 42 million transistors, and an iPhone 11 houses 8.5 billion transistors.

What does a transistor’s internal structure look like?

Transistors are made of a semiconductor material such as silicon, germanium, and others. The addition of impurities into semiconductor wafers allows transistor manufacturers to create n-type and p-type regions. This process is called doping. 

Doping allows a semiconductor wafer, such as silicon, to divide into two regions; n-type and p-type. What are these regions, and what is the difference between them? The number of positive and negative charges present in that region is what sets them apart. Negatively charged particles are called electrons, and the positively charged areas are called holes because the absence of an electron creates a “hole.” In an n-type region, the majority carriers are electrons, while in a p-type region, the majority carriers are holes. 

A transistor is made by sandwiching a p-type region between two n-type regions, and vice versa. NPN and PNP are the two types of transistors, based on their internal structure. The three terminals of a transistor originate from each of the three doped regions present inside it. The middle zone is the base terminal, while the other two are emitter and collector terminals. 

How Do Transistors Work?

Transistors work as either amplifiers or switches. When working as an amplifier, a transistor takes a small input current and amplifies it to produce a larger output current. On the other hand, while working like a switch, a low input current at the input terminal switches on and drives a larger current at the output terminal. Both transistor configurations are advantageous, which makes them very popular in electronic circuit design. 

If we connect the negative terminal of a battery to the n-type region (emitter), and the positive terminal to the base (p-type region), a current will flow from the base to the emitter. Similarly, if we put the collector (n-type region) at a higher positive potential than the base and emitter, an emitter current will generate and flow towards the collector. The collector-emitter current, ICE, is controlled through the base voltage.

The switching and amplification modes are achieved by providing the right base, collector, and emitter voltages. Let’s look at some basic transistor circuits to understand how amplification and switching occurs. 

Transistor Configurations

There are three main transistor configurations that are widely used in electronic circuit design: 

Common Emitter
The common-emitter configuration works

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