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I am not actually talking here about SiGe(Silicon-Germanium hybrids) but an integrated circuit that uses entirely Germanium(doped of course in certain areas)? It is my understanding that even though Germanium was used to make the first discrete transistors, there are many problems with using it as a substrate for IC fabrication. Such as the fact that Germanium Dioxide is water soluble.

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  • \$\begingroup\$ Interesting Question +1 .When I was a teenager there were Si and Ge transistors in about equal numbers .All the ICs were Si .I asked the same question then .Ge had lower VCE sat and VBE and more gain .I was told by people long dead that thermal drift would be really bad .This seemed believable because you had to be careful when designing direct coupled amplifiers .I was also told that there was a limited amount of Ge and an abundance of Si and that in the future everything would be electronic . \$\endgroup\$ – Autistic Jun 24 '17 at 23:32
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    \$\begingroup\$ What would be the advantage of Ge semiconductors? I fail to see that. \$\endgroup\$ – Marcus Müller Jun 24 '17 at 23:33
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    \$\begingroup\$ @MarcusMüller - Lower characteristic voltages means less power. Note that Ge was popular for some power products for quite a while, but the development of Schottky diodes effectively killed the class. Intrinsic electron mobility is higher as well, so Ge was faster. \$\endgroup\$ – WhatRoughBeast Jun 25 '17 at 0:51
  • \$\begingroup\$ @Mr X .Ge does have some downsides so there must be a compelling upside to make this a runner . \$\endgroup\$ – Autistic Jun 25 '17 at 0:53
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    \$\begingroup\$ I have access to germanium wafers that we use for power applications. It's direct bandgap, so that's nice for as you do not have that lattice vibration when due to the shift. Generally, all of these are cost considerations. It's 1k for a Si wafer, and 10k for a sapphire wafer. I have no idea how expensive the Germanium wafers are. \$\endgroup\$ – b degnan Jun 25 '17 at 2:33
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We are not close to making pure Ge ICs, and likely never will be. The odd diode or power device where the low voltage wins, yes, but the process technology for significant integration, no.

The technology to be able to process silicon into ICs as we do today has been won over 40 to 60 years (depending on your start date) of painstaking incremental investment. Given a choice between a tweak that uses the existing silicon processes (continue to reduce dimensions, alloy Ge with it, put it on an insulating sapphire substrate) and a totally new material to learn how to process and handle the chemistry, the choice will be for silicon, unless there is some huge advantage for the new material.

A big, big drawback for straight Ge is the low temperature performance of the material. You can't solder ICs by bringing the whole board up to temperature as you can with silicon, it's hand solder leads or clamp only, and then use excellent heatsinking. What the market wants in higher temperature performance, that's one of the things driving SiC for instance.

IC require much more than discretes, every aspect of the physics and chemistry, the resistors, the well capacitance, the oxide stability, the interconnect metals, the lattice sizes to mention a few from a long list, all form part of a system within which the IC designers work.

Discretes are a little more forgiving, and here new materials do get used at a low level of integration. GaN is making higher speed FETs, SiC is making higher voltage higher temperature transistors, SiGe first appeared in discretes for a decade or two where the teething troubles were sorted out, then there's InGaP, and AlGaAs, and a host of others, and Ge too for some specialist diodes and power transistors.

Given how long Ge has been around, it's obvious that there is not a sufficient performance advantage to make it worth trying to work with the material's problems on any significant scale.

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  • \$\begingroup\$ So what you're saying is there is no incentive to learn how to fabricate Germanium ICs. But that it is not impossible. \$\endgroup\$ – Mr X Oct 3 '18 at 22:41
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    \$\begingroup\$ @MrX My answer left out one of the most important drawbacks of Ge which is the low temperature, I can toast a germanium device at lower than 100C (ask me how I know!) So germanium devices are unusable on modern boards, only working when you can clamp them, like power diodes, or use hand assembly with leaded devices. So still no incentive. Germanium as a component material yes, look at SiGe, but it runs through a silicon process. \$\endgroup\$ – Neil_UK Oct 4 '18 at 5:09
  • \$\begingroup\$ Good point. Germanium is only useful for low voltage devices but it's greater electron mobility and low barrier potential might make it very useful for high GHz/THz applications. So a Germanium IC would need more extensive shielding. \$\endgroup\$ – Mr X Oct 5 '18 at 16:14

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