In the second post of Electronics 101, the basic properties of transistors will be discussed and introduced, as well as some typical applications. From the last post, it looks like I lost some images from a server restore I did a while back, so this time I’ll be sure to back up the images!
The sheer amount of transistors and their respective uses and properties is a long blog post by itself, so I will just give a brief introduction to BJTs, MOSFETs, and TTL.
The transistor is a doped semiconductor component/device used widely for amplification, switching, and eventually logic.
First, BJTs. BJT is an acronym for Bipolar-Junction Transistor, and it is a three-terminal device consisting of a collector, base, and emitter. The two main types of BJTs are NPN and PNP. The Ns and Ps describe the structure of the particular BJT, as a PNP transistor is made from an n-doped material surrounded by a p-doped material. Conversely, NPN is a p-doped material sandwiched by n-doped material. In both cases, the middle-doped material is the base in the BJT. For a PNP BJT, the collector is where the current leaves the component, and the emitter is where the larger current enters the component. An NPN BJT is the exact opposite — the larger current enters the collector and leaves the emitter. This can be seen in the photo below.
The difference between these two transistors is, on a general level, the polarity of each terminal. However, both share the property of essentially being two diodes with a common anode or cathode depending on whether it’s P-doped or N-doped (anode, in the case of diodes, is the negatively polarized side of the component, and the cathode is the positive).
A BJT has a smaller current which controls a larger current via the base terminal. In an NPN BJT, the larger current comes into the collector, and leaves through the collector. Conversely, in a PNP BJT, the larger current comes into the emitter and exits through the collector. As mentioned before, the BJT may act like a switch. This is true such that the base having no current is analogous to an open switch (called cut-off mode), and supplying the base terminal with current will result in either two modes: active or saturation. Saturation is when the base current is high enough such that the maximum amount of current flow possible from emitter to collector is present. Active mode is an in-between mode where the current provided through the base is less than saturation but higher than cut-off. The amount of current flowing from emitter to collector is proportional to the base current, but amplified by some amount of gain.
Next is MOSFETs. MOSFET is an acronym for Metal-Oxide Semiconductor Field-Effect Transistor. A MOSFET is a type of FET, which is simply a unipolar transistor, as opposed to the bipolar BJT. The MOSFET, and all FETs in general, differ from the BJT such that they rely on an electric field to conduct electricity essentially from source to drain. The MOSFET is composed of three terminals (sometimes four), just like the BJT; however, the terminals have different names and different applications. The terminals on a MOSFET include a drain, source, and gate (and sometimes a body).
The MOSFET, and its variations – pMOS and nMOS, are very commonly used in the field of digital logic. Also, like the BJT, the MOSFET is commonly used in amplification of signals and in switching applications. Using only a few MOSFETs, creating logic AND, OR, and NOT gates are possible. Greater combinations result in NAND, XOR, and NOR gates.
Well that’s it for now! Check back on the next Electronics 101 post to learn about capacitors and inductors!
Tweet This!