Before we discuss semiconductors and how they operate, it is important to understand current flow theory. There are two different theories of how current flows: electron current flow and conventional current flow (sometimes referred to as "hole flow").
The electron current flow theory says that current flow in a circuit is the movement of electrons through the conductors. Since the electrons have a negative charge and unlike charges attract each other, the electrons move from the negative terminal of the battery to the positive terminal. So the electron theory says that current flows from negative to positive.
The conventional current flow theory, which has been accepted for many years, says that current flows from the positive terminal of the battery to the negative terminal. The conventional current flow theory is sometimes called the hole flow theory because this theory says that when an electron moves, an empty hole is left behind. The holes are said to travel in the opposite direction from the electrons in the conductor. To understand how this could work think of a line of cars stopped at a stop sign. As one car pulls away from the stop sign a hole is left and the next car in line moves forward to fill the hole. Now the hole has moved back to where the second car was and the third car moves forward to fill it. As each car in turn moves forward to fill the hole, the hole moves to the rear. The cars move one direction and the holes move the other, just like electrons and holes in a circuit.
When looking at an electrical circuit, either the electron current flow theory or conventional current flow theory can be applied because the circuit operation and the schematic will be the same. When dealing with diagrams that use electronic symbols, such as diodes and transistors, the arrow in the symbol always points in the direction of conventional current flow. Because the conventional current flow theory is widely accepted in the automotive industry, it is used throughout this book.
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