The Early History of Rectifying Devices
In many ways, the search since the 1880s for better rectifier methods has grown into the entire field of power electronics. The basic form of the diode rectifier circuit was discussed in the nineteenth century, and the modern multi-kilowatt rectifiers common today use the same principles. One such example is imaged to the right. What makes the early idea significant is the recognition that the underlying process is fundamentally nonlinear, and cannot be done with any combination of linear circuit elements such as resistors, capacitors, and inductors.
One familiar nonlinear device is a rectifying diode – an element that conducts differently depending on the direction of current flow. The term diode actually refers to a generic two-terminal electronic element, but has come to be almost interchangeable with the rectification function. While the silicon P-N junction diode is the most common example today, many other technologies yield a rectifying two-terminal element. One early example is the selenium diode used by C.T. Fritts in a rectifier circuit as early as 1883. The development of the vacuum tube diode about twenty years later was essential to practical applications, but it is interesting that semiconductor rectifiers existed well before the invention of vacuum tubes.
The vacuum diode is limited in fundamental ways by the low current density possible in a vacuum system. A major improvement came when mercury was included in rectifier tubes. The mercury arc tube opened the way to multi-megawatt power levels, even at voltages as low as a few hundred volts. A 1904 paper by Charles Proteus Steinmetz considered the performance of mercury tubes for rectification. The waveforms in that paper can easily be duplicated in modern rectifier systems, and represent a broad selection of the possibilities of power rectification.
A typical rectifier system, dating from the 1940s, used mercury arc tubes to convert power from a 50 Hz 2400 V bus into 3000 V dc for a railway locomotive. Arc tubes are still used in certain specialized circumstances, such as rectification beyond 1MV.
Since before the invention of the transistor, semiconductor diodes have dominated all but the highest power levels. By the late 1930s, single devices formed from selenium, copper oxide, and other nonlinear materials were manufactured commercially. The P-N Schottky barrier diodes offer an alternative in many low-voltage situations. Modern diodes are extremely cheap – even those with ratings high enough for almost any electric load. Devices with ratings up to about 3 A and 600 V are manufactured in huge lots. Diodes rated at more than 15 kV are readily available, and currents up to about 6000 A also can be achieved (though not both simultaneously). One figure of merit is the power handling rating – the product of voltage and current ratings. Individual diodes exist with power handling capabilities above 36 MW.
The fabrication methods for diodes have evolved rapidly. Today, hundreds of diodes with power handling ratings up to perhaps 500 W each can be fabricated on a single silicon wafer. The highest power devices use the opposite method: Individual diodes formed from complete single wafers are available today, even for 20 cm wafers and larger. One of the biggest challenges with large single devices in packaging: Making a 6000 A connection to a thin, brittle disk 20 cm across in a formidable task.
Complementing the packaging challenge is the challenge to find improved materials for higher power handling. Germanium is sometimes used, but it is more sensitive to high temperatures that silicon. Gallium arsenide power rectifiers have entered the commercial arena, and other compound materials are being examined for power rectifiers. Silicon carbide and even diamond film promise new opportunities to reach extreme power levels during the coming decades. The objective for the latter materials is to increase power handling levels for a given size semiconductor by allowing devices to operate at temperatures beyond 200 degrees Celsius.