The researcher discovered a transparent microchip in the HP computer 1977

The researcher discovered a transparent microchip in the HP computer 1977

Computer historian and chip reverse-engineering enthusiast Ken Shirriff found a mysterious transparent chip inside a 1977 HP computer. He found out that the microcircuit is silicon-on-sapphire and serves as an auxiliary component of the floppy disk controller.

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Schirriff discovered this chip in the floppy drive controller circuit board, located between the HP interface bus (HP-IB) and the Z80 processor. According to him, the silicon-on-sapphire chip was once used in many HP products. “It processes the bus protocol and buffers data between the interface bus and the device’s microprocessor,” explained the researcher.

In this chip, a thin layer of silicon is deposited on a sapphire substrate to form a circuit. Silicon belongs to the N-type and, if necessary, is converted to the P-type by ion implantation. A layer of metal wiring is formed on top, as well as transistors with a metal gate. The diagram below shows a cross section of the circuit.

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Shirriff points out that chips made this way have been around since 1963 or earlier. A clear example of “silicon-on-sapphire” is the RCA 1802 processor used on the Galileo space probe, which studied Jupiter and its moons.

HP MC2 / Hewlett-Packard Journal, 1977

The enthusiast also detailed some of the features of silicon-on-sapphire, such as the sapphire substrate as an insulator. All in all, this design “reduces the capacitance between transistors, improves performance.” Moreover, the insulating properties of sapphire can prevent stray currents and protect against low-impedance short circuits and radiation exposure.

Shirriff studied the chip’s logic elements, FIFO buffers, and a decoder that selects a particular register based on address lines.

Composite image of the PHI chip crystal taken with a microscope / righto.com

A computer historian concludes that this chip is “interesting as an example of future technology that didn’t quite live up to its promise.”

Schirriff also compares late 1970s silicon-on-sapphire processors with conventional silicon in terms of power consumption and clock speed. He notes that the former are superior to the latter. However, silicon-on-sapphire had a low yield (only 9%) and a high cost.

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