The name resistor comes from its property that resists the current flow. The resistor is undoubtedly the oldest electrical device. The concept of resistance dated all the way back to 1826 when Ohm’s law was conceived. The first resistors made before the turn of the century were of carbon composition and metal wire. Thick-film resistor, also made from carbon at the time, started around 1930. Thin-film resistor based on sputtering technology began around 1960. All these resistors were obviously discrete components until integrated-circuit technology emerged around 1970, when diffused or ion-implanted resistors were built monolithically inside the chip. Rather surprisingly, resistors in all the above forms are still in use for special requirements. This oldest electrical device is still one of the most common components in both integrated circuits and discrete circuits.
Structurally, resistors can be divided into two groups, those fabricated in integrated circuits, and those as discrete components. Integrated components can be further classified into: (1) diffused or ion-implanted resistor, (2) thin-film resistor, (3) epitaxial resistor, and (4) pinch resistor. Most of these resistors are isolated by a p-n junction except for the thin-film resistor which is deposited on an insulating layer. In semiconductor resistors, p-type material is more common because of the higher resistance as a result of lower hole mobility. For integrated thin-film resistor, poly-Si, amorphous Si and metal alloys such as silicides are common. The pinch resistor is a by-product of the bipolar technology (or JFET technology) where the base-layer resistance is “pinched” by the top emitter diffusion.
Discrete resistors can be classified into (1) thin-film resistor, (2) thick-film resistor, (3) metal wire-wound resistor and (4) bulk resistor. Discrete thin-film resistors are made from vacuum deposition, mostly sputtering, of materials such as Та, TaN, Ni-Cr, C, and Sn02. Common substrate materials are alumina and glass. Thick-film resistors, typically in the order of 25 |xm thick, are fabricated by a different process involving coating of a resistive paste, followed by firing. Examples of materials are ruthenium and palladium alloys. Wire-wound metal resistors are mainly for high-precision applications, but they have higher inductance. Bulk resistors are made of carbon composition, cermet (the name comes from combination of ceramic and metal), and metal oxide. Film resistors often require trimming to fine-tune the resistance value. This can be done by laser trimming. An alternative is abrasive trimming for thick-film resistors, and oxidation for thin-film resistors. One special kind of discrete resistor is called the chip resistor for ease of insertion or soldering to circuit boards. These are usually film resistors, and the packages have end caps instead of open wires.