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Electrical design seems to have a lot of dead ends where we can't go any further. Does anyone have any insights as to what the current status is for transistors

Electrical design seems to have a lot of dead ends where we can't go any further. Does anyone have any insights as to what the current status is for transistors based on light? I would like to start with chip design (did microchip programming as CS), is it even a feasible to achieve something in that field? Is there any good places / papers to start with?

smaddox 8y ago
Optical switches are not a contender for general computing. Scaling beyond the diffraction limit (approximately half the wavelength) requires plasmonics, which is arguably more lossy than traditional transistors. If anything ever subplants CMOS, it will still likely be based on electron transport. Electrons have a lot of great properties for switching. They are massive enough to be localizable into small spaces, but light enough to be cheap (energy-wise) to accelerate up to high speeds. They are ubiquitous, and they interact strongly with matter, electric fields, and magnetic fields. Light is great for signal transport over long distances in a large part because it interacts so weakly with matter. But that same property makes it difficult to switch.
jpmattia 8y ago
> Does anyone have any insights as to what the current status is for transistors based on light? It's a good question, but there is something often ignored in press-releases about photonic transistors: How exactly does a speedup occur? For example: Photons do not interact with each other, so a photonic transistor will rely on electrons or holes to effect some sort of interaction between photons. That interaction might occur in a photorefractive material, but ultimately the photorefraction is a result of photons interacting with electrons of the underlying material. So why are the electrons in the photorefractive material faster than the electrons in a conventional transistor? It also might be worth noting that the fastest fT in conventional transistors is around 0.5 THz, so the bar is not particularly low. FD: I have a patent on photonic transistors from Bell Labs days. FD2: I have become an old cynic on photonic transistors. FD3: The above is really a rant on high-speed transistors. Photonic transistors might well have a superiority in different areas, eg quantum computing.
- petra 8y ago
  fT for the fastest transistor - probably isn't the parameter limiting electronic circuits - it's more about wire length and capacitance, and power density of a circuit. It's both true inside circuits, but also outside, at the "memory wall" - which on the surface at least, seems particularly fitting for photonics.
  - jpmattia 8y ago
    > it's more about wire length and capacitance At the fastest speeds, interconnect on crucial lines is engineered as a transmission line, where the inductance balances out the capacitance. When you look at the propagation in the transmission line, the energy of the traveling wave is dominated by the electric and magnetic fields outside the metal. So it is not generally appreciated that transmission-line interconnect is already photonic, and moving at photonic speeds. As you might expect, for most transmission-line geometries, the speed of the signal is not terribly far away from the speed-of-light in the material. Moreover, the intuition that "metal wires" are slowing down the signal is not taking into account that an "all photonic transmission" still requires some sort of confinement. If you were to eliminate the metal, you would still need some sort of waveguide to confine the photonic energy. Inevitably, the waveguide will have some region of a higher dielectric, which as you know will slow down the propagation and also have some loss. As it turns out, the final propagation speed is close to a transmission line. So again, it's important to quantify exactly how the speedup occurs by switching to "photonic interconnect", because the reality is that it is already photonic. Where photonic interconnect might have had a use is where the lines are very long, and the loss associated with a transmission line gets impractically large due to the confining metals. At that point, it's an engineering tradeoff: Converting to and from photons at each end of the line is non-trivial, and not all materials lend themselves to converting photons to electrons. There have been efforts since the 80s to put GaAs on silicon just to address this issue. (I was part of a team that got to 100MHz for GaAs-on-Si. The fact that GaAs-on-Si photonic transmission is still pretty much confined to the lab tells you everything you need to know about the manufacturability.)
    - Ice_cream_suit 8y ago

smaddox 8y ago

Optical switches are not a contender for general computing. Scaling beyond the diffraction limit (approximately half the wavelength) requires plasmonics, which is arguably more lossy than traditional transistors. If anything ever subplants CMOS, it will still likely be based on electron transport. Electrons have a lot of great properties for switching. They are massive enough to be localizable into small spaces, but light enough to be cheap (energy-wise) to accelerate up to high speeds. They are ubiquitous, and they interact strongly with matter, electric fields, and magnetic fields. Light is great for signal transport over long distances in a large part because it interacts so weakly with matter. But that same property makes it difficult to switch.

jpmattia 8y ago

> Does anyone have any insights as to what the current status is for transistors based on light? It's a good question, but there is something often ignored in press-releases about photonic transistors: How exactly does a speedup occur? For example: Photons do not interact with each other, so a photonic transistor will rely on electrons or holes to effect some sort of interaction between photons. That interaction might occur in a photorefractive material, but ultimately the photorefraction is a result of photons interacting with electrons of the underlying material. So why are the electrons in the photorefractive material faster than the electrons in a conventional transistor? It also might be worth noting that the fastest fT in conventional transistors is around 0.5 THz, so the bar is not particularly low. FD: I have a patent on photonic transistors from Bell Labs days. FD2: I have become an old cynic on photonic transistors. FD3: The above is really a rant on high-speed transistors. Photonic transistors might well have a superiority in different areas, eg quantum computing.

petra 8y ago
fT for the fastest transistor - probably isn't the parameter limiting electronic circuits - it's more about wire length and capacitance, and power density of a circuit. It's both true inside circuits, but also outside, at the "memory wall" - which on the surface at least, seems particularly fitting for photonics.
- jpmattia 8y ago
  > it's more about wire length and capacitance At the fastest speeds, interconnect on crucial lines is engineered as a transmission line, where the inductance balances out the capacitance. When you look at the propagation in the transmission line, the energy of the traveling wave is dominated by the electric and magnetic fields outside the metal. So it is not generally appreciated that transmission-line interconnect is already photonic, and moving at photonic speeds. As you might expect, for most transmission-line geometries, the speed of the signal is not terribly far away from the speed-of-light in the material. Moreover, the intuition that "metal wires" are slowing down the signal is not taking into account that an "all photonic transmission" still requires some sort of confinement. If you were to eliminate the metal, you would still need some sort of waveguide to confine the photonic energy. Inevitably, the waveguide will have some region of a higher dielectric, which as you know will slow down the propagation and also have some loss. As it turns out, the final propagation speed is close to a transmission line. So again, it's important to quantify exactly how the speedup occurs by switching to "photonic interconnect", because the reality is that it is already photonic. Where photonic interconnect might have had a use is where the lines are very long, and the loss associated with a transmission line gets impractically large due to the confining metals. At that point, it's an engineering tradeoff: Converting to and from photons at each end of the line is non-trivial, and not all materials lend themselves to converting photons to electrons. There have been efforts since the 80s to put GaAs on silicon just to address this issue. (I was part of a team that got to 100MHz for GaAs-on-Si. The fact that GaAs-on-Si photonic transmission is still pretty much confined to the lab tells you everything you need to know about the manufacturability.)
  - Ice_cream_suit 8y ago