We have studied semiconductor devices. Firstly, we understand what semiconductors in terms of energy band diagrams are. Then we understand what is doping of semiconductors, what are p-type and n-type semiconductors. Diodes, two terminal semiconductor devices, are studied in terms of its I-V characteristics as well as Zener effect in semiconductor diodes and use of Zener diodes. Transistors, three terminal semiconductor devices, are the most important components of today's digital integrated circuits. A detailed explanation on Bipolar Junction Transistor (BJTs), MOSFET, JFET has been given, particularly their operational principles, biasing and small signal equivalent circuits. Next we have investigated the power semiconductor devices. In first section on switches, we have studied how a diode, transistor and MOSFET can be used as a switch in integrated circuits. We have also introduced to other power semiconductor devices by drawing its structure and symbol, explaining operational principles with the help of equivalent circuits viz. SCRs, TRIACs and IGBTs.