Q Ken, KE6ZWN, asks, “The material in an NPN transistor consists of a positive layer (the base) surrounded by two
negative layers (the emitter and collector). How do the manufacturers build transistors so that the current flows correctly
between layers while at the same time keeping them insulated
from each other?”
A See Figure 1. Transistors are made up of layers of material
that are “doped” with impurities so that they are either “P” type (positive charge) or “N” type (negative charge). Now, these charges aren’t quite like the ones in a battery— they exist in the form of an occasional extra electron or positive ion (an atom with an electron deficit) that are available as “current carriers.”
Where two different layers of material touch, an exchange of current carriers takes place and a neutral (or near neutral)
“depletion region” forms (an area where the net charge is depleted).
In a bipolar transistor, the base-emitter junction depletion region is fairly thin, whereas the region between the base and the collector is rather thick.
From QST June 2001