Transistors
A transistor is a semiconductor, meaning that sometimes it conducts electricity, and sometimes it doesn’t. Its internal resistance varies, depending on the power that you apply to its base.
NPN and PNP transistors are bipolar semiconductors. They contain two slightly different variants of silicon, and conduct using both polarities of carriers—holes and electrons.
The NPN type is a sandwich with P-type silicon in the middle, and the PNP type is a sandwich with N-type silicon in the middle. If you want to know more about this terminology,
and the behavior of electrons when they try to cross an NP junction or a PN junction, you’ll have to read a separate source on this subject. It’s too technical for this book. All you need to remember is:
• All bipolar transistors have three connections: Collector, Base, and Emitter, abbreviated as C, B, and E on the manufacturer’s data sheet, which will identify the pins for you.
• NPN transistors are activated by positive voltage on the base relative to the emitter.
• PNP transistors are activated by negative voltage on the base relative to the emitter.
In their passive state, both types block the flow of electricity between the collector and emitter.
You can think of a bipolar transistor as if it contains a little button inside, as shown in Figures 2-89 and 2-90. When the button is pressed, it allows a large current to flow. To press the button, you inject a much smaller current into the base by applying a small voltage to the base. In an NPN transistor, the control voltage is positive. In a PNP transistor, the control voltage is negative.
