Basically, a MOS transistor works because a semiconductor like silicon doesn't conduct electricity very well, because there are not enough free electrons. Free electrons are electrons that are free to move if you apply a voltage. In the MOS transistor shown, there aren't enough free electrons in the green area called "p-type Si" to conduct electricity between the two yellow areas labelled "source" and "drain". If we apply a positive voltage - like the + pole on a flashlight batter - to the blue area labelled the "gate", then that + voltage attracts negatively (-) charged electrons between the source and drain. Those negatively (-) charged electrons are free electrons, free to move, free to carry electricity between the source and the drain. So - if we apply a positive voltage to the gate, the MOS transistor conducts electricity between the source and the drain. If we don't apply a positive voltage to the gate, the MOS transistor cannot conduct electricity between the source and the drain. We use this ability to change how well transistors conduct electricity to build logic circuits that do things like: * Conduct electricity only if positive voltages are applied to two gates of two transistors at the same time * Conduct electricity if a positive voltage is applied to the gate of one or the other of two transistors, or to both. These logic circuits are used to build more complex logic circuits that process information - like a microprocessor, microcomputer, and such. I have an explanation of this at this website on logic circuits Some companies do use copper in some of our recent semiconductor processes - the copper is used to connect the transistors together. The copper always conducts electricity, so it cannot be used as a semiconductor as described above, but it can be used to conduct electricity between semiconductor transistors.