The positive holes in the P-type silicon get attracted to the negative terminal of the battery. The negative electrons in the N-type silicon get attracted to the positive terminal of the battery. When you put N-type and P-type silicon together as shown in this diagram, you get a very interesting phenomenon that gives a diode its unique properties.Įven though N-type silicon by itself is a conductor, and P-type silicon by itself is also a conductor, the combination shown in the diagram does not conduct any electricity. A diode is a one-way turnstile for electrons.

You may have seen turnstiles at a stadium or a subway station that let people go through in only one direction. A diode allows current to flow in one direction but not the other. That's what happens in a diode.Ī diode is the simplest possible semiconductor device. N-type and P-type silicon are not that amazing by themselves but when you put them together, you get some very interesting behavior at the junction. P-type silicon is a good conductor.Ī minute amount of either N-type or P-type doping turns a silicon crystal from a good insulator into a viable (but not great) conductor - hence the name 'semiconductor.' A hole happily accepts an electron from a neighbor, moving the hole over a space. The absence of an electron creates the effect of a positive charge, hence the name P-type. When mixed into the silicon lattice, they form 'holes' in the lattice where a silicon electron has nothing to bond to. Boron and gallium each have only three outer electrons. P-type - In P-type doping, boron or gallium is the dopant.Electrons have a negative charge, hence the name N-type. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. The fifth electron has nothing to bond to, so it's free to move around. Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. N-type - In N-type doping, phosphorus or arsenic is added to the silicon in small quantities.A pure silicon crystal is nearly an insulator - very little electricity will flow through it.īut you can change all this through a process called doping. All of the outer electrons in a silicon crystal are involved in perfect covalent bonds, so they can't move around. While silicon crystals look metallic, they are not, in fact, metals. Metals tend to be good conductors of electricity because they usually have 'free electrons' that can move easily between atoms, and electricity involves the flow of electrons. This makes a silicon crystal an insulator rather than a conductor. In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current. In silicon, the crystalline form is a silvery, metallic-looking substance. In carbon, we know the crystalline form as diamond. The four electrons form perfect covalent bonds with four neighboring atoms, creating a lattice. Carbon, silicon and germanium (germanium, like silicon, is also a semiconductor) have a unique property in their electron structure - each has four electrons in its outer orbital.

Microchip transistor diagram