A mysterious transparent chip has been found in an HP computer, dating back to 1977. Investigations have concluded that the custom chip was formed on a sapphire substrate, and thus, we are seeing a silicon-on-sapphire IC. What was it for? Its function was far more mundane than you might expect – it was a support component on a floppy disk controller.
Computer historian and IC reverse engineering enthusiast Ken Shirriff uncovered this interesting chip by de-capping a broken interface chip in a vintage HP computer. However, the interface wasn’t from some futuristic photonic supercomputer server experiment from the company; it was found in a humble floppy drive controller PCB sitting between HP’s interface bus (HP-IB) and a Z80 processor.
Shirriff explains that the silicon-on-sapphire IC was found beneath the cap of a chip called the PHI (Processor-to-HP-IB Interface) and was used in multiple HP products back in the day. “It handles the bus protocol and buffered data between the interface bus and a device’s microprocessor,” according to historians’ investigations.
Silicon-on-sapphire may sound somewhat futuristic, but Shirriff’s blog highlights that ICs made this way have been around since 1963 or earlier. A notable example of a silicon-on-sapphire IC is the RCA 1802 processor used on the Galileo space probe that studied Jupiter and its moons.
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The IC reverse engineering enthusiast also detailed some of the peculiar features of silicon-on-sapphire, such as the sapphire substrate being an insulator, effectively isolating the various silicon regions upon it. Apparently, this type of construction “reduces the capacitance between transistors, improving performance.” Moreover, sapphire’s insulation properties can prevent stray currents and protect against low-impedance short circuits and the effects of radiation – hence its use in the aforementioned space electronics applications.
Shirriff’s blog goes into a deep dive with a look inside the HP PHI chip, its construction, and die. He even examines its logic gates, first-in-first-out buffers (FIFOs), and address decoder. Please check out the blog for all these finer details and more.
In conclusion, the computer historian echoes our initial thoughts that this silicon-on-sapphire IC is “interesting as an example of a ‘technology of the future’ that didn’t quite pan out.”
Shirriff also contrasts late 70s era processors built on silicon-on-sapphire vs regular silicon in terms of energy consumption and clock speeds. Would you be surprised to hear that silicon-on-sapphire ICs were far superior using these metrics? Things might have panned out differently if these transparent ICs had been mass-produced at better yields and lower manufacturing costs. A frightening statistic highlighted by Shirriff is that HP’s silicon-on-sapphire yields were a mere 9%.