Welcome to Semiconductor Physics, Part 2. In the first part of our series, we talked about how silicon bonds together with other atoms to form a crystal lattice, and introduced the concept of energy bands and bandgaps. We used these concepts to define conductors, insulators, and semiconductors, and showed one means of altering the conductivity of semiconductors.

If you haven’t read Part 1 yet, it would be prudent to do so before continuing on with Part 2.

Before we go on with Part 2, though, we’ll take a moment to clarify an issue that’s almost always a source of confusion. When scientific pioneers first discovered the idea of electricity and current years ago, they did not have available the other pertinent details we have today. Primarily, they did not have a model of the atom as we have today, and thus did not have an accurate concept of protons, neutrons and electrons, and their properties.

It was identified that current was the movement of charge, and that it involved the movement of tiny charged particles through a medium. What was not known was what those particles actually were, as we know today (electrons). They surmised that current was the flow of tiny, positively charged particles, through a wire. Thus, if current was moving from left to right, for example, that meant that positive particles were moving from left to right.

As we know today, that assumption was wrong. The particles that move are electrons, and they carry a negative charge. The positively charged protons do not move through the material. In the same scenario as above, their definition of current as consisting of positive particles moving from left to right is, in reality, negative particles moving from right to left. By the time the current atomic model was discovered, and this error realized, conventions were already widely established, and could not be easily changed. Thus, it was left alone, and the actual definition of the direction of current is opposite the real direction of electron flow. So when you see a schematic showing current flowing in a certain direction, know that, in actuality, electrons are moving in the opposite direction.

http://media.hardwareanalysis.com/articles/small/10647.gif" alt="Semiconductor Physics">Fig. 1 - The direction of current flow versus current flow. Due to an early scienfitic error, current is actually defined as flowing in the opposite direction of electrons.

We know it’s confusing, but keep that in mind when reading through the rest of this series.