be made smaller than p-channel MOSFETs, due to p-channel charge carriers (holes) having lower mobility than do n-channel charge carriers (electrons), and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, neglecting leakage current, unlike CMOS logic, NMOS logic consumes power even when no switching is taking place. Mohamed M. Atalla and Dawon Kahng, after they invented the MOSFET, fabricated both pMOS and nMOS devices with a 20 µm process in 1960. Their original MOSFET devices had a gate length of 20µm and a gate oxide thickness of 100 nm. However, the nMOS devices were impractical, and only the pMOS type were practical working devices. A more practical NMOS process was developed several years later. NMOS was initially faster than CMOS, thus NMOS was more widely used for computers in the 1970s. With advances in technology, CMOS logic displaced NMOS logic in the mid-1980s to become the preferred process for digital chips. Emitter-coupled logic (ECL) ECL uses an overdriven bipolar junction transistor (BJT) differential amplifier with single-ended input and limited emitter current. The ECL family, ECL is also known as current-mode logic (CML), was invented by IBM as current steering logic for use in the transistorized IBM 7030 Stretch computer, where it was implemented using discrete components. The first ECL logic family to be available in integrated circuits was introduced by Motorola as MECL in 1962. Transistor–transistor logic (TTL) In TTL logic, bipolar junction transistors perform the logic and amplifying functions. The first transistor–transistor logic family of integrated circuits was introduced by Sylvania as Sylvania Universal High–Level Logic (SUHL) in 1963. Texas Instruments introduced the 7400 series TTL family in 1964. Transistor–transistor logic uses bipolar transistors to form its integrated circuits. TTL has changed significantly over the years, with newer versions replacing the older types. Since the transistors of a standard TTL gate are saturated switches, minority carrier storage time in each junction limits the switching speed of the device. Variations on the basic TTL design are intended to reduce these effects and improve speed, power consumption, or both. The German physicist Walter H. Schottky formulated a theory predicting the Schottky effect, which led to the Schottky diode and later Schottky transistors. For the same power dissipation, Schottky transistors have a faster switching speed than conventional transistors