AC Electrics - Semiconductors — Page 259, Lesson 273

I want to walk you through the bipolar junction transistor — a fundamental building block in aircraft electrical systems. Let's start with its construction. The bipolar transistor, also called the junction transistor, is built from two junction diodes placed together. It consists of either a thin layer of P-type semiconductor — typically just 25 micrometres thick — sandwiched between two N-type semiconductors. That configuration is called an N-P-N transistor. Alternatively, you can have a thin layer of N-type semiconductor sandwiched between two P-type semiconductors, and that is called a P-N-P transistor. You can see both structures laid out in Figure 16.8. Now, regardless of whether it's N-P-N or P-N-P, the transistor has three distinct regions. They are named the Collector, the Base, and the Emitter. The Base is that thin middle layer — only 25 micrometres — and it's critical to how the device works. The circuit symbol for each type of transistor looks almost identical, but there is one key difference: the direction of the arrow drawn between the Base and the Emitter. That arrow always represents conventional current flow. For an N-P-N transistor, the arrow points from the Base to the Emitter. For a P-N-P transistor, the arrow points from the Emitter to the Base. So the arrow direction tells you which type you're dealing with. Let's look at how the N-P-N transistor operates. Imagine we apply an electromotive force — an EMF — across the Collector-to-Emitter region. If we do that alone, no current flows. The transistor is essentially switched off. But now, if we add a second EMF across the Base-to-Emitter region, something changes dramatically: a large current flows from the Emitter to the Collector. The full theory of current flow inside a transistor is complex and beyond the scope of this course, but I can give you a simple explanation of what happens. When you apply that bias voltage — the EMF — between the Base and Emitter of an N-P-N transistor, the junction becomes forward biased. That means a large number of free electrons are attracted into the Base region. Here's the clever part. The Base region is very thin — only 25 micrometres — so very few holes are produced there for those free electrons to combine with. The surplus electrons that don't find a hole simply diffuse right across the Base and into the Collector region. Once in the Collector, they migrate toward the applied positive potential on the Collector side. Meanwhile, any holes that have combined with free electrons are replaced. How? An electron leaves the Base region and goes to the positive terminal of the bias supply, which regenerates the hole. So the Base current is small, but it controls a much larger flow. The net result is this: a relatively small Base-to-Emitter current flow produces a large Emitter-to-Collector current flow. That's the core principle of transistor amplification and switching — a small signal at the Base controls a much larger current through the main path.