Creating Current Sources with Power MOSFETs

Although the first metal-gate MOSFET had been developed at Bell Labs in 1960, the FET was considered all but impossible to make by those who tried, because it was so unstable. So it was not until 1963 when two young RCA researchers, Steve Hofstein and Fred Heiman, made the first stable and working devices. By the mid-1960s, RCA had some of the first small, metal-can MOSFETs in production and in its own consumer products. In the early 1970s, DMOS, which had been developed in Japan, became available in the United States. It was to become the foundation for subsequent power MOSFET development for the next three decades. Then in 1976, Siliconix announced MOSPOWER | the world's first commercially available vertical power MOSFETs with their distinctive V-groove, which was etched into the chip's structure. Up until then, MOSFETs had a lateral structure and were limited to power levels of less than 1 Watt. In comparison, with this new type, current flowed through the device vertically. It was capable of faster switching and at power levels of several amps. Unfortunately, the V-groove design was found to create a high-energy field at its point, which was hard to control, and it created stability problems. This in turn led to a truncated structure (without a sharp tip at the bottom of the V-shape), which improved operation somewhat. Although these devices were difficult and expensive to build, as well as requiring an ultra-clean processing environment, they revolutionized power MOSFET technology from then on. The same year, Japan's Hitachi introduced the world's first complementary power MOSFETs on the same chip, using a proprietary planar, lateral structure. Some of these were used in Hitachi's own consumer audio amplifiers. These breakthroughs were followed in 1979 by International Rectifier, which introduced a competing MOS structure that it dubbed the HEXFEi "~. This design used thousands of paralleled, hexagonally shaped cells on a single FET chip. This dramatically reduced the on-resistance, RDS(on), and enabled switching even higher currents. Since those early days, continued R&D of silicon power MOSFET technology has been taking place around the world, and enhancement-mode devices have become increasingly popular. Common applications that have driven further improvements in power MOSFET technology have included switch-mode power supplies (SMPS), as well as power switches, disconnect safety switches in battery chargers, and amplifiers. The evolution of the personal computer (both desktop and laptop models) has created an enor203