Introducing the MOSFET
Chapter 2, Digital Logic, introduced the NPN transistor, a type of bipolar junction transistor (BJT). The NPN transistor is called bipolar because it relies on both positive and negative charge carriers to function.
In semiconductors, electrons serve as the negative charge carriers. There are no physical particles with positive charge involved in a semiconductor operation; rather, the absence of a normally present electron in an atom exhibits the same properties as a positively charged particle. These missing electrons are referred to as holes. Holes function as the positive charge carriers in bipolar junction transistors.
The concept of holes is so fundamental to semiconductor operation that William Shockley, one of the inventors of the transistor, wrote a book entitled Electrons and Holes in Semiconductors, published in 1950. We'll next examine the behavior of positive and negative charge carriers in unipolar transistors.
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