Although the transistor was invented in the Bell Laboratories in 1947, it and its progeny remain a mystery to most mechanics. These components are used in all alternators to rectify alternating current to direct current and can be found in current regulators produced by CAV, Lucas, Bosch, and Delco. In the future they will have a much wider use and possibly will substitute for relays.
Solid-state components are made of materials called semiconductors, which fall midway in the resistance spectrum between conductors and insulators. In general these components remain nonconductive until subjected to a sufficient voltage.
Silicon and germanium are the most-used materials for semiconductors. Semiconductor crystals for electronics are artificial crystals, grown under laboratory conditions. In the pure state these crystals are electrically just mediocre conductors. But if minute quantities of impurities are added in the growth state, the crystals acquire special electrical characteristics that make them useful for electronic devices. The process of adding impurities is known as doping.
When silicon or germanium, is doped with arsenic or phosphorus, the physical structure remains crystalline. But for each atom of impurity that combines with the intrinsic material, one free electron becomes available. This material has electrons to carry current and is known as n-type semiconductor material.
On the other hand the growing crystals might be doped with indium, aluminum, boron, or certain other materials and result in a shortage of electrons in the crystal structure. “Missing” electrons can be thought of as electrical holes in the structure of the crystal. These holes accept electrons that blunder by, but the related atom quickly releases the newcomer. When a voltage is applied to the semiconductor, the holes appear to drift through the structure as they are alternately filled and emptied. Crystals with this characteristic are known as p-type materials, because the holes are positive charge carriers.
When p-type material is mated with n-type material, we have a diode. This device has the unique ability to pass current in one direction and block it in the other. When the diode is not under electrical stress, p-type holes in the p-material and electrons in the n-material complement each other at the interface of the two materials. This interface, or junction, is electrically neutral, and presents a barrier to the charge carriers. When we apply negative voltage to the n-type material, electrons are forced out of it. At the same time, holes in the p-type material migrate toward the direction of current flow. This convergence of charge carriers results in a steady current in the circuit connected to the diode. Applying a negative voltage to the n-type terminal so the diode will conduct is called forward biasing.
If we reverse the polarity—that is, apply positive voltage to the n-type material—electrons are attracted out of it. At the same time, holes move away from the junction on the p side. Because charge carriers are thus made to avoid the junc- tion, no current will pass. The diode will behave as an insulator until the reverse- bias voltage reaches a high enough level to destroy the crystal structure.
Diodes are used to convert alternating current from the generator to pulsating direct current. An alternator typically has three pairs of diodes mounted in heat sinks (heat absorbers).
Diodes do have some resistance to forward current and so must be protected from heat by means of shields and sinks. In a typical alternator the three pairs of diodes that convert alternating current to pulsating direct current are pressed into the aluminum frame. Heat generated in operation passes out to the whole generator.
As useful as solid-state components are, they nevertheless are subject to certain limitations. The failure mode is absolute. Either the device works or it doesn’t. And in the event of failure, no amount of circuit juggling or tinkering will restore operation. The component must be replaced. Failure might be spontaneous—the result of manufacturing error compounded by the harsh environment under the hood—or it might be the result of faulty service procedures. Spontaneous failure usually occurs during the warranty period. Service-caused failures tend to increase with time and mileage as the opportunities for human error increase.
These factors are lethal to diodes and transistors
High inverse voltage resulting from wrong polarity Jumper cables connected backward or a battery installed wrong will scramble the crystalline structure of these components.
Vibration and mechanical shock Diodes must be installed in their heat sinks with the proper tools. A 2 X 4 block is not adequate.
Short circuits Solid-state devices produce heat in normal operation. A transistor, for example, causes a 0.3–0.7V drop across the collector-emitter terminals. This drop, multiplied by the current, is the wattage consumed. Excessive current will overheat the device.
Soldering The standard practice is for alternator diodes to be soldered to the stator leads. Too much heat at the connection can destroy the diodes.