Electrical resistance is measured in units called ohms. The higher the value inohms, the greater the resistance, and the more difficult it becomes for current to flow.For wires, the resistance is sometimes specified in terms of ohms per foot or ohms perkilometer. In an electrical system, it is usually desirable to have as low a resistance, orohmic value, as possible. This is because resistance converts electrical energy into heat.Thick wires and high voltages reduce this resistance loss in long-distance electricallines. This is why such gigantic towers, with dangerous voltages, are necessary in largeutility systems.SemiconductorsIn a semiconductor, electrons flow, but not as well as they do in a conductor. You mightimagine the people in the line being lazy and not too eager to pass the balls along. Somesemiconductors carry electrons almost as well as good electrical conductors like copperor aluminum; others are almost as bad as insulating materials. The people might be justa little sluggish, or they might be almost asleep.Semiconductors are not exactly the same as resistors. In a semiconductor, the ma-terial is treated so that it has very special properties.The semiconductors include certain substances, such as silicon, selenium, or gal-lium, that have been “doped” by the addition of impurities like indium or antimony.Perhaps you have heard of such things as gallium arsenide, metal oxides, or siliconrectifiers. Electrical conduction in these materials is always a result of the motion of electrons. However, this can be a quite peculiar movement, and sometimes engi-neers speak of the movement of holes rather than electrons. A hole is a shortage of anelectron—you might think of it as a positive ion—and it moves along in a direction opposite to the flow of electrons (Fig. 1-6).14 Basic physical concepts1-6Holes move in the opposite direction from electrons in a semiconducting material.