Semiconductor Class 12 Physics Notes, Definition, Working Principle, Formula & Real-Life Applications

Think of a clean and pure crystal of silicon with no additives. That is what we call an intrinsic semiconductor. It is as pure as the material can get. But in its natural state, this kind of semiconductor isn’t very useful at room temperature. Why? Because its electrons need a boost to break free and carry current. As the temperature rises, some electrons get excited enough to jump into the conduction band. When they do, they leave behind holes—empty spaces that act like positive charges. So now, two types of charge carriers exist: electrons and holes. Both contribute to the overall current. You can think of this process like popcorn kernels in a pan. Nothing happens at first. But once the heat kicks in, a few kernels start popping randomly, but enough to make a difference.

Now let us say we want to make that silicon crystal a bit more practical. We do that by adding something extra. This process is called doping, and it gives us what’s known as an extrinsic semiconductor. Here is how it works. If we introduce an element with five electrons in its outer shell, like phosphorus, we get an n-type semiconductor. These extra electrons are free to move, making the material more conductive. On the other side, adding an element like boron, which has only three outer electrons, creates a p-type semiconductor. Now we have got more holes than electrons, and holes become the primary charge carriers. This is not just theory. Doping is what makes the silicon chip inside your phone or laptop actually work. Without it, transistors and other components would be nearly impossible to build.

Now for a bit of chemistry meets engineering: what happens when you join a p-type and n-type semiconductor? You get a p-n junction, a boundary that forms the core of almost every modern electronic device. Right at the junction, electrons from the n-side meet holes from the p-side. They recombine, and in doing so, they cancel each other out. This creates a small depletion zone which a region with no free charge carriers. An internal electric field appears, acting like a gate that controls the flow of electricity. Try this: Apply a voltage so the p-side is positive and the n-side is negative. This forward bias reduces the barrier, and current flows. Reverse the voltage, and the junction tightens its grip, blocking current in what's called reverse bias. It is a smart, simple trick and it is the basis for diodes, solar cells, and transistors.

Semiconductors devices is not only for important for JEE mains exam aspirants but also for  Class 12th Board Exam students and NEET exam students.

A semiconductor diode is perhaps the simplest device built using a p-n junction, but don’t let that simplicity fool you. It has one job: allow current to flow in only one direction. Yet that function is vital across all electronics. In forward bias, the diode conducts. It is like opening a gate to let the charge carriers move freely. But flip the voltage, and the gate shuts. The depletion region grows, and current drops to near zero, except for a tiny leakage. This characteristic of blocking one direction, conducting the other, is why diodes are used for rectification, signal detection, and even voltage regulation when paired with designs like Zener diodes.

Let us bring it all together in one practical application: rectification. Most household electricity is AC (alternating current) that switches direction. But electronics usually need DC (direct current) that flows steadily in one direction. That is where diodes come in. In a half-wave rectifier, a single diode blocks the negative half of the AC signal. Only the positive pulses get through. It is simple but not very efficient. A full-wave rectifier, however, uses multiple diodes in a clever setup called a bridge circuit. It flips the negative cycle to positive, allowing current to flow during both halves of the AC cycle. Add a smoothing capacitor, and you get a clean, usable DC output. This is the technology behind chargers, adapters, and power supplies in virtually every digital device.

Semiconductors is an important chapter for those students who are preparing for the JEE exam, which makes it necessary to practice NCERT solutions of semiconductors for a in-depth knowledge of important concepts.