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Semiconductors
A semiconductor starts out as a crystalline structure comprised of a single type of atom. Each of these atoms is locked in place, sharing electrons with each of its neighbors in a uniform pattern that is repeated billions of times. This material is then baked in an oven that contains a carefully selected gas. A few of the gas atoms replace a few of the crystal atoms. If the gas atoms contain one more electron than the crystal atoms, there will be a few free electrons in the material and the material will be called an "N-type" semiconductor. If the gas atoms have one less electron than the crystal atoms, there will be places in the material that are missing an electron and the material will be called a "P-type" semiconductor. These holes, where electrons should be, move freely about the material.
Diodes
If a P-type semiconductor is placed next to an N-type semiconductor, there will a slight concentration of electrons and holes near the junction. If a positive potential is applied to the N-type semiconductor and a negative potential is applied to the P-type semiconductor, the holes and electrons are pulled away from the junction and no current flows. If the potentials are reversed, the holes and electrons are pushed together and current flows. A diode is an electrical component that allows current to flow in one direction but not in the opposite direction.
Transistors
A transistor is somewhat like two diodes connected in opposite directions. There are actually three segments of semiconductor where the two outer segments are the same and the middle segment is different. The arrangement can be either PNP or NPN. The three parts are called the emitter, the base and the collector. These names reflect the way transistors were originally used. Signals started at the emitter and travelled across the transistor to the collector. A small voltage on the base controlled the signal going across the transistor. The central slice (the base) is thinner than the other two slices.
PNP
The collector and emitter of a PNP transistor are both P-type semiconductors. They have holes that can act as carriers of electricity. The thin slice of N-type semiconductor between the collector and emitter means that a signal going in either direction will have to go across a junction in the "wrong" direction, i.e., the direction that makes it look like a diode biased in the wrong direction. When a voltage is applied to the base, i.e., the N-type slice, it controls how much effect this N-type slice interferes with the emitter and collector flow.