In a npn transistor, holes are the minority carriers.
- Free electrons are the majority carriers in a n-type semiconductor.
- Holes are the absence of an electron in a covalent bond. They can be thought of as positive charges that move through the semiconductor.
- Donor ions are positively charged atoms that have been added to a semiconductor to create a n-type semiconductor.
- Acceptor ions are negatively charged atoms that have been added to a semiconductor to create a p-type semiconductor.
In a npn transistor, the base region is made of p-type semiconductor, while the emitter and collector regions are made of n-type semiconductor. The emitter and collector regions are heavily doped with n-type impurities, while the base region is lightly doped with p-type impurities.
When a small voltage is applied to the base-emitter junction, a large number of holes are injected into the base region. These holes diffuse into the emitter region, where they combine with free electrons. This recombination process releases energy in the form of heat.
The electrons that are released from the recombination process are then accelerated by the electric field in the collector region. They flow through the collector region and out of the transistor.
The current that flows through the collector is much larger than the current that flows through the base. This is because the collector region is much larger than the base region.
The ratio of the collector current to the base current is called the current gain of the transistor. The current gain of a transistor can be very high, typically in the range of 100 to 1000.
Transistors are used in a wide variety of electronic devices, including amplifiers, oscillators, and switches.