Hole flow

Chapter Hole flow

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • electron flow all the way back in chapter 1. It would be a good idea to turn back for a mo-ment and review this material, because it will help you understand the concept of hole flow.Hole flowIn a P-type semiconductor, most of the current flows in a way that some people find pe-culiar and esoteric. In a literal sense, in virtually all electronic devices, charge transferis always the result of electron movement, no matter what the medium might be. Theexceptions are particle accelerators and cloud chambers—apparatus of interest mainlyto theoretical physicists.The flow of current in a P-type material is better imagined as a flow of electron ab-sences, not electrons. The behavior of P-type substances can be explained more easilythis way. The absences, called “holes,” move in a direction opposite that of the electrons.Imagine a sold-out baseball stadium. Suppose 19 of every 20 people are randomlyissued candles. Imagine it’s nighttime, and the field lights are switched off. You stand atthe center of the field, just behind second base.The candles are lit, and the people passthem around the stands. Each person having a candle passes it to the person on theirright if, but only if, that person has no candle. You see moving dark spots: people with-out candles. The dark spots move against the candle movement. The physical imageyou see is produced by candle light, but the motion you notice is that of candles ab-sences.Figure 19-2 illustrates this phenomenon. Small dots represent candles or electrons.Imagine them moving from right to left in the figure as they are passed from person toperson or from atom to atom. Circles represent candle absences or holes. They “move”from left to right, contrary to the flow of the candles or the electrons, because the can-dles or electrons are being passed among stationary units (people or atoms).This is just the way holes flow in a semiconductor material.Behavior of a P-N junctionSimply having a semiconducting material, either P or N type, might be interesting, anda good object of science experiments. But when the two types of material are broughttogether, the P-N junction develops properties that make the semiconductor materialstruly useful as electronic devices.Figure 19-3 shows the schematic symbol for a semiconductor diode, formed byjoining a piece of P-type material to a piece of N-type material. The N-type semicon-ductor is represented by the short, straight line in the symbol, and is called the cathode.The P-type semiconductor is represented by the arrow, and is called the anode.In the diode as shown in the figure, electrons flow in the direction opposite the ar-row. (Physicists consider current to flow from positive to negative, and this is in thesame direction as the arrow points.) But current cannot, under most conditions, flowthe other way. Electrons normally do not flow in the direction that the arrow points.If you connect a battery and a resistor in series with the diode, you’ll get a currentflow if the negative terminal of the battery is connected to the cathode and the positiveBehavior of a P-N junction363