Several recent developments indicate that designers are advancing in their drive to miniaturize computers via thin films. At Hughes Aircraft Co., Culver City, Calif., tests are proceeding on an airborne computer in which thin-film capacitors and resistors operate in conjunction with conventional semiconductors of the smallest available type. Research-ers at RCA, Princeton, N.J., claim development of a thin-film cadmium-sulfide transistor operating on field-effect principles.
RCA reports it has deposited thin films of cadmium sulfide in strips so thin that electrodes can be spaced only 5 microns apart. The films are 1 micron thick. The devices use a source, drain, and gate configuration. The gate is biased positively so that the gate material, starting as an insulator, has electrons drawn into it, enhancing its conductivity. Enhancement current, rather than depletion current, is said to be the dominant factor in the device's operation.
In the laboratory transistors made so far, gain-bandwidth products of about 5,000 have been measured. RCA says the device and the techniques it has developed appear suitable for mass deposition of computer circuitry. However, speed is limited to 1 mc at present. Because only a few milliwatts are required for operation, component densities of 10,000 per sq. in. are expected.
To demonstrate feasibility of thin-film circuitry for logical processing in an airborne computer, Hughes has built a digital differential analyzer based on the circuitry of the Mark II Polaris guidance computer. The unit is said to have more than 8000 components, weighs 1 lb, occupies 20 cu. in., and requires 30 W of power. It has a core memory and logical circuitry made of thin-film capacitors and resistors deposited on top of one another and micro-transistors and diodes.
Resistors are formed by depositing a 250-Å thick layer of Nichrome on a 40-mil Fotoceram wafer and covering this layer with silicon-monoxide insulator. A layer of copper to form a capacitor plate is deposited on the insulation. The computer is divided into 25 wafers, of which 17 logic wafers are nearly identical. The wafers are hinged for accessibility so that the entire computer opens like a book (see photo). (Electronic Design, July 5, 1961, p. 4)
RCA was a leader in the development of thin-film transistors, but the devices never panned out as logic elements in computers. Probably the most important application turned out to be as the active elements in LCD panels.