An NPN transistor is the most common type of bipolar junction transistor, or BJT for short. BJTs are often referred to simply as transistors, and come in two main types: the NPN and the PNP. The "N" represents a negatively charged layer of material and the "P" represents a positively charged layer. NPN transistors have a positive layer located in-between two negative layers. Transistors are typically used in circuits for amplifying or switching electrical signals that pass through them.
Both NPN and PNP transistors contain three leads, which are small metal pieces that connect the transistor to the circuit board. These three leads are known as the base, the collector, and the emitter. The base receives the electric signal, the collector creates a stronger electric current than the one passing through the base, and the emitter passes this stronger current on to the rest of the circuit. In an NPN transistor, the current passes through the collector to the emitter, whereas in a PNP transistor the current passes from the emitter to the collector.
What makes a bipolar junction transistor, such as the NPN transistor, stand out from other possible methods of transferring electric current is the design. The NPN transistor has a shared base used by both the collector and the emitter layers. The collector and emitter layers are also asymmetrical, meaning the amount of impurity in one layer does not match the amount in the other. Impurities are created when forming the material for the layers because these impurities create the required positive or negative charge. This process of creating a certain charge by producing impurities in the material is known as doping.
The NPN transistor is commonly used because it is so easy to produce. For a transistor to work properly, it needs to be formed from a semiconductor material. Semiconductors include materials that fall somewhere near the middle on a scale measuring good and bad conducting materials. Semiconductors can carry some electric current, but not as much as extremely conductive materials such as metal. Silicon is one of the most commonly used semiconductors, and NPN transistors are the easiest transistors to make out of silicon.
One application for an NPN transistor is on a computer's circuit board. Computers require all their information be translated into binary code, and this process is accomplished through a plethora of tiny switches flipping on and off on the computer’s circuit boards. NPN transistors can be used for these switches. A strong electric signal turns the switch on while a lack of a signal turns the switch off.