to name a few. By far, however, the two most important ingredients that are of fundamental importance to the semiconductor-device designer are boron and phosphorus. When a silicon wafer is doped with either boron or phosphorus, its electrical conductivity is altered dramatically. Normally, a pure silicon wafer contains no free electrons; all four of its valence electrons are locked up in covalent bonds with neighboring silicon atoms (see Fig. 4.5). Without any free electrons, an applied voltage will have little effect on producing an electron flow through the wafer.A silicon wafer in pure form doesn’t contain any free charge carriers; all the electrons are locked up into covalent bonds between neighboring atoms. However, if phosphorus is added to the silicon wafer, something very interesting occurs. Unlike silicon, phosphorus has five valance electrons instead of four. Four of its valance electrons will form covalent bonds with the valance electrons of four neighboring silicon atoms (see Fig. 4.6). However, the fifth valance electron will not have a “home” (binding site) and will be loosely floating about the atoms. If a voltage is applied across the silicon-phosphorus mixture, the unbound electron will migrate through the doped silicon toward the positive voltage end. By supplying more phosphorus to the mixture, a larger flow of electrons will result. Silicon that is doped with phosphorus is referred to as n-type silicon, or negative-charge-carrier-type silicon.A Note to Avoid Confusion Boron atoms have three valance electrons, not four like silicon. This means that the combined lattice structure has fewer free valance electrons as a whole. However, this does not mean that a p-type silicon semiconductor has an overall positive charge; the missing electrons in the structure are counterbalanced by the missing protons in the nuclei of the boron atoms. The same idea goes for n-type silicon, but now the extra electrons within the semiconductor are counterbalanced by the extra protons within the phosphorus nuclei. Another Note to Avoid Confusion (Charge Carriers) What does it mean for a hole to flow? I mean, a hole is nothing, right? How can nothing flow? Well, it is perhaps misleading, but when you hear the phrase “hole flow” or “flow of positive charge carriers in p-type silicon,” electrons are in fact flowing. You may say, doesn’t that make this just like the electron flow in n-type silicon? No. Think about tipping a sealed bottle of water upside down and then right side up (see Fig. 4.8). The bubble trapped in the bottle will move in the opposite direction of the water. For the bubble to proceed, water has to move out of its way. In this analogy, the water represents the electrons in the p-type silicon, and the holes represent the bubble. When a voltage is applied across a p-type silicon semiconductor, the electrons around the boron atom will be forced toward the direction of the positive terminal. Now here is where it gets tricky. A hole about a boron atom will be pointing toward the negative terminal. This hole is just waiting for an electron from a neighboring atom to fall into it, due in part to the lower energy configuration there. Once an electron, say, from a neighboring silicon atom, falls into the hole in the boron atom’s valance shell, a hole is briefly created in that silicon atom’s valance shell. The electrons in the silicon atom lean toward the positive terminal, and the newly created hole leans toward the negative terminal. The next silicon atom over will let go of one of its electrons, and theelectron will fall into the hole, and the hole will move over again—the process continues, and it appears as if the hole flows in a continuous motion through the p-type semiconductor
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