The semiconductor revolution that we have witnessed in the past few decades is perhaps best illustrated by the invention that fundamentally change the way we communicate. Telephone, when it was first invented some 100 years ago, looked like this. Nowadays, it of course looks like this. Mr. Bell will not recognize the present-day telephone as his own invention, because it is actually a completely different device operating on a completely different set of principles. If you look inside your telephone, you will find something like this. Inside which, you will find semiconductor devices. Semiconductor devices are what enable this revolutionary device. Semiconductor devices are at the heart of the technological revolution that we have witnessed and enjoyed during the past few decades. Semiconductor revolution has been so successful that we find semiconductor devices everywhere. Not only do we find them in computers and tablets and phones, we find them in cameras, TV sets, solar panel, room lighting, even in appliances, and in your car. You find semiconductor devices at home, on the road, at grocery stores, and at the gym. They are truly ubiquitous, and we can only expect that they would become even more so in the future. In this context, it is an excellent time to learn how semiconductor devices work, which is what our series on semiconductor devices is designed to accomplish. Our series on semiconductor devices consists of three courses: first, on semiconductor physics; next, on p-n junction and metal semiconductor contact; and then third, on bipolar junction transistor and field-effect transistor. In the fourth course on semiconductor physics, we start with quantum theory of solids. Here, we discuss energy band structure, what gives rise to energy band structure, and how they dictate materials properties, especially semiconductor materials property. Next, we discuss how electrons are distributed in the energy bands of a semiconductor and how they lead to electrical current. Finally, we discuss how carriers can be generated in an isolated inside the semiconductor, and develop a theoretical framework in which we describe the current behavior of a semiconductor under conditions that can be readily applied to a lot of the operating conditions of real semiconductor devices. Building on this theoretical framework, in course two, we discuss diodes. First, p-n junction. What is p-n junction? What is the device structure? What are their key characteristics? Then we discuss metal semiconductor contact, and how they are made, and how they behave under various operating conditions, and how they are similar and different from p-n junction and other related devices. Finally, we will discuss optoelectronic devices based on the p-n junction and metal-semiconductor contexts, such as the LED, light-emitting diodes shown here. LEDs is fast becoming the mainstream technology in the multi-billion dollar lighting industry. In addition, we will discuss semiconductor lasers, photo detectors, and solar cells. Next, our course on transistor will discuss two types of transistors, bipolar junction transistor, and the field-effect transistor. First, we start with metal-oxide-semiconductor device, and how they behave under various biasing conditions. Then, we discuss the field-effect transistors using the MOS device as their gate as shown here. This MOSFET is now a mainstream technology in the integrated circuit technologies. Finally, we will discuss bipolar junction transistor, and we will discuss the key characteristics of bipolar junction transistors, and discuss the similarities and differences between the bipolar junction transistors and the field-effect transistors. This course will provide a solid foundation on semiconductor devices, so that you can deal with semiconductors or develop semiconductors effectively at your workplace. Also, this course will serve as a gateway course for more advanced topic courses on advanced semiconductor devices.