Microprocessor chronology

The first microprocessors were designed and manufactured in the 1970s. Intel's 4004 of 1971 is widely regarded as the first commercial microprocessor. Designers predominantly used MOSFET transistors with pMOS logic in the early 1970s, switching to nMOS logic after the mid-1970s. nMOS had the advantage that it could run on a single voltage, typically +5V, which simplified the power supply requirements and allowed it to be easily interfaced with the wide variety of +5V transistor-transistor logic (TTL) devices. nMOS had the disadvantage that it was more susceptible to electronic noise generated by slight impurities in the underlying silicon material, and it was not until the mid-1970s that these, sodium in particular, were successfully removed to the required levels. At that time, around 1975, nMOS quickly took over the market. This corresponded with the introduction of new semiconductor masking systems, notably the Micralign system from Perkin-Elmer. Micralign projected an image of the mask onto the silicon wafer, never touching it directly, which eliminated the previous problems when the mask would be lifted off the surface and take away some of the photoresist along with it, ruining the chips on that portion of the wafer. By reducing the number of flawed chips, from about 70% to 10%, the cost of complex designs like early microprocessors fell by the same amount. Systems based on contact aligners cost on the order of $300 in single-unit quantities, the MOS 6502, designed specifically to take advantage of these improvements, cost only$25. This period also saw considerable experimentation with various word lengths. Early on, 4-bit processors were common, like the Intel 4004, simply because making a wider word length could not be accomplished cost-effectively in the room available on the small wafers of the era, especially when the majority would be defective. As yields improved, wafer sizes grew, and feature size continued to be reduced, more complex 8-bit designs emerged like the Intel 8080 and 6502.