In this topic we are going to cover semiconductors and their applications. Chapters starts with introduction to solids and their classification based on conductivity and energy bands. Solids are classified into conductors, insulators and semiconductors. Here we are interested to study semiconductors. They are further divided into two groups- intrinsic and extrinsic, based on number of holes and electrons in it. Extrinsic is also classified into p-type and n-type semiconductor based whether number of holes is greater or electrons. Here we get equation of concentration of holes and electrons in thermal equilibrium.

Then comes the process of formation of P-N junction when p and n type of semiconductor are brought together. There are two types of connection while connecting it in circuit- forward and reverse bias. It’s important to understand working of diodes in both bias and their related terms like drift current, threshold voltage. Then we study about application of diodes as rectifier, voltage regulator (zener diode). These are used in optoelectronic devices like photodiode, LED and photovoltaic devices.

Transistor is made of three chips of semiconductors alternatively placed- n-p-n or p-n-p transistor. It has three parts- emitter, collector and base. Each one has its own characteristics and properties. There are three configuration in which transistor can be connected- common emitter, base and collector from which we are studying common emitter here. In this we study characteristics of n-p-n transistor in different range of input voltage and based on our input, it has different application like switch, amplifier, oscillator. There are also terms like current amplification factor, input-output resistance which need to understand clearly. Further, chapter moves to digital electronics and logic gates. There are 5 main logic gates which have specific relationship between input and output. They are NOT, OR, AND, NAND and NOR gates. Chapter ends with introduction to Integrated Circuits.

Typical mistakes:

  1. In calculating total current in semiconductors, we have to consider current of hales and electrons both, not only of one.
  2. Don’t confuse in elements which make n-type and p-type semiconductor.
  3. Drift and diffusion are different concepts. Don’t misuse them.
  4. Don’t confuse in symbols of diodes or n-p-n or p-n-p transistors and also their circuit configurations.
  5. Don’t mess up in direction of currents in transistors in different configurations.


  1. It’s good to have idea about common values like energy gap in silicon etc.
  2. Focus on characteristics in forward-reverse bias, terms and graphs (V vs. I) related to it.
  3. Understand the working of each application of diodes and transistors and circuit diagram also.
  4. In transistor, focus on small things like how currents add up or properties of base, emitter and collector.
  5. Also how equations/properties changes from n-p-n to p-n-p transistor.
  6. Solve some problems on CE configuration to get clear idea.
  7. Remember different configurations for different applications of transistor like switch, amplifier etc.
  8. Get the idea of feedback amplifier and oscillator.


Chapter can be scoring one if you understands basic concepts. Focus on basic working of diodes and transistors, numerical on CE configuration of transistor, logic gates and their applications.

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