I am a Navy trained Electronics Technician. Semiconductors are materials such as Silicon and Germanium that can function both as a conductor or an insulator and are doped with impurities to produce Pentavalent and Trivalent materials known as "N" type materials and "P" type materials respectively.
When N and P type materials are fused together in "junction Recombination" to make semi-conductor devices known as Diodes they have what are known as a PN junction. The N type material is used as the Cathode or Negative pole and the P type material is used as the anode or positive pole. This fusion causes a "Depletion region" to form where the two types of material meet.
The Depletion region is the area where the electrons have gone to fill up the holes and the majority carriers have been repelled. N type materials have a majority of electrons while P type materials have a majority of holes. Everyone knows what electrons are but holes are a bit difficult to explain. suffice it to say that they are the gaps where electrons aren't.
By Biasing the device(applying voltage) in the forward direction one can decrease the depletion region to the point that current can flow through the device or one can reverse bias(apply voltage in the other direction) the device which has the effect of increasing the depletion region to the point where current can not flow through the device. This is what makes the diode so special in an alternating current circuit a diode can prevent current from flowing in one direction while allowing the current to flow in the other direction. Diodes are the main component in limiters which takes alternating current and limits either the positive alternations to zero or the negative alternations to zero. By adding a resistor (piece of carbon) in parallel with a diode one can create a clamper the effects of which is to take a square wave and clamp it above or below zero amplitude.
Transistors are another semi-conductor device very similar to the diode except it has two Depletion regions and can be made of either two N type materials and a P type material between them or it can be made with two P type materials and an N type material between them and are called NPN transistors and PNP transistors respectively.
Transistors are regularly used in Regulators, Amplifiers and switches. Transistors replaced vacuum tubes and do pretty much the same thing.
Almost all integrated circuit chips are semi-conductors and contain millions of transistors.
Contrary to popular belief Semi-conductors have nothing to do with inductor coils, capacitors, resistors, or transformer coils. Most switches are used to create an open or a short in a circuit depending on whether you need to turn on the current or turn the current off. If I have time I may write articles on each of these components describing what they are how they are made and there common usages.