Intrinsic Silicon

Many people already understand, that two types of silicon exist, N+ -Doped, and P -Doped.

Well I’ve known for some time, that another type of silicon which exists, is called ‘Intrinsic Silicon’. This is a form of silicon, which theoretically contains no dope at all, and which is therefore non-conductive. It’s not even a semiconductor in that state.

This type of silicon might be of some interest in the design of modern Integrated Circuits, especially in the reduction of the capacitance of individual transistors. But there areĀ  essentially two problems with its use:

  1. It’s practically impossible for the silicon to be perfectly pure. The concentrations of Dope, in the N+ or P -Doped silicon, are already extremely low. The concentration of impurities in Intrinsic Silicon is simply lower, industrially, than in the intentionally-doped silicon, not truly zero. And what this means in practice, is that ‘larger pieces’ of Intrinsic Silicon are still partially conductive. In fact, how low the concentrations of N+ or P -Dope can be brought in the industrial process, depends on how low the level of impurities is, in the silicon, to begin with. In either type of intentionally-doped silicon, the concentration of dope must still be at least one order of magnitude greater, than the level of impurities was.
  2. Actually, I think that Intrinsic Silicon is more expensive in bulk, than either type of intentionally-doped silicon, which means, that if the entire wafer needed to be made out of it, since the substrate of the wafer is meant to provide mechanical support as well, then the cost of the manufacturing process would increase.

Yet, small pieces of Intrinsic Silicon, as the following image shows, can still be used to provide lateral insulation, between the P -Doped and the N+ -Doped wells of individual transistors, where a “buried oxide layer” provides vertical insulation between those wells, and the actual wafer:

https://upload.wikimedia.org/wikipedia/commons/e/ee/Cmos-chip_structure_in_2000s_%28en%29.svg

And, it would be my expectation that because Intrinsic Silicon is ‘non-conductive’, larger pieces of it should also be optically transparent, which means that some people might mistake it for glass.

By definition, glass would be ‘amorphous’, which means ‘not crystalline’, which would make actual glass useless as a semiconductor. However, amorphous forms of silicon can readily be used in the design of wafers, as long as they do not need to participate in the actual semiconductive behavior between N+ and P -Doped silicon.

Dirk