4 ms·

I was expecting something a little more interesting from IEEE about characteristic time scale of transistor switching also as a function of voltage. So you can

I was expecting something a little more interesting from IEEE about characteristic time scale of transistor switching also as a function of voltage. So you can switch faster but either need to raise voltage or accept defects or errors? Then finally it makes sense to discuss heat dissipation etc.

VLM 10y ago
Part of the answer, yes no one has any idea how to improve (deg C) / (watt) which is how you measure thermal resistance. Thermodynamics was all figured out in the days of steam engines and we're kinda stuck now at a maximum. If you examine the equation there's no reason you can't dump 1000 watts if you're willing to heat the silicon die to 1000 C like a glowing vacuum tube. However (insert massive hand waving) there are characteristics of silicon doping vs temps and all this hand wavy stuff that depends on deg C such that I assure you that you can't build a junction that works with todays technology at BOTH room temp and 1000C (so you'd have to preheat your CPU, perhaps with a flame?) or worst case scenario with current chemical processes I'm not sure you can build a junction that works at 1000C at all. I'd have to think about that for awhile. Anyway pull most datasheets and even for bulk power transistors (like a RF amp or the switching transistors in your power supply) the max die temp is 125C or 150C but I don't casually remember ever seeing a transistor with a max die temp over 175C (which is hardly scientific proof there's no exotic chip out there that runs at 200C or 400C...) Your stereotypical small signal bipolar NPN like a 2N3904 might max out junction temp at 150C. Some of the graphs like collector leakage current are exponential with temp and by 150C you better design that leakage into your circuit. Note that at a couple hundred degrees per watt thermal resistance from junction to ambient you're not going to dump much power in a small signal transistor like that, but its certainly enough to run relays and LEDs and stuff, and the same general temp limits apply to all transistors. Some beastly multi-GHz multi-watt monster might have a staggeringly lower thermal resistance but the junction temp limit is still gonna be 150C or so. I guess in summary if off the shelf you could run junctions at 500C then heat wouldn't be an issue or limiter for another, I donno, decade or two. But the 150 or 175 or whatever limit is a hard physics limit with current tech, so ... If you want to impress an EE with space-ship x-files area 51 BS give him a semiconductor that works while glowing orange hot or so. That would have pretty interesting performance specs, I bet. Its also impossible AFAIK at current human technology levels. Either that or a semiconductor with room temperature superconductive bond wires. Which is more likely first? Who knows.
- the8472 10y ago
  I don't think the absolute temperature is directly related to the wattage. Heat transport also factors into it. You could always keep the chip cooled with active heat pumps or carefully applied liquid nitrogen. It's just impractical, especially getting everything cooled equally in the presence of hot/cold spots.
  - dom0 10y ago
    Heat resistance is approximately a given (surface area), so wattage depends on the permissible temperature difference between the two items (die and heatspreader). If you run hotter, you can dump more power into it. Temperature is ultimately limited by sped up diffusion of doping and such, temperature limits in surrounding materials (eg epoxy-based chip carriers or epoxy-based case material), and reduced performance due to increasing leakage currents.
- userbinator 10y ago
  TI has some interesting parts (including microcontrollers) that are rated up to 210C: http://www.ti.com/lit/sgzt009 http://www.ti.com/lit/sgzt009 Here are some MOSFETs that are rated to operation at 300C(!) http://www.x-relsemi.com/EN/Documentation/Datasheet/DS-00113-11-XTR2N0800-High-Temperature,%2080V%20N-Channel%20Power%20MOSFET%20Family.pdf http://www.x-relsemi.com/EN/Documentation/Datasheet/DS-00113... Not cheap.
- Symmetry 10y ago
  The things that change with temperature is that the number of charge carriers per volume of un-doped silicon goes up while the speed with which charge carriers move through silicon goes down. More carriers independent of doping means more current through transistors channels even when gate voltage isn't applied, but there are ways to be conservative and design around that.
- baobrien 10y ago
  Transistors built on GaN and SiC processes can operate with a junction temperature of up to around 200C. As you say, that's a pretty big deal for RF power transistors, as they can dissipate quite a lot more power without blowing up. Another issue with running nanometer-scale chips hot, from what I understand, is that the dopants will start to diffuse into the silicon, and because the feature size is so small, you eventually get transistor failures.

VLM 10y ago

Part of the answer, yes no one has any idea how to improve (deg C) / (watt) which is how you measure thermal resistance. Thermodynamics was all figured out in the days of steam engines and we're kinda stuck now at a maximum. If you examine the equation there's no reason you can't dump 1000 watts if you're willing to heat the silicon die to 1000 C like a glowing vacuum tube. However (insert massive hand waving) there are characteristics of silicon doping vs temps and all this hand wavy stuff that depends on deg C such that I assure you that you can't build a junction that works with todays technology at BOTH room temp and 1000C (so you'd have to preheat your CPU, perhaps with a flame?) or worst case scenario with current chemical processes I'm not sure you can build a junction that works at 1000C at all. I'd have to think about that for awhile. Anyway pull most datasheets and even for bulk power transistors (like a RF amp or the switching transistors in your power supply) the max die temp is 125C or 150C but I don't casually remember ever seeing a transistor with a max die temp over 175C (which is hardly scientific proof there's no exotic chip out there that runs at 200C or 400C...) Your stereotypical small signal bipolar NPN like a 2N3904 might max out junction temp at 150C. Some of the graphs like collector leakage current are exponential with temp and by 150C you better design that leakage into your circuit. Note that at a couple hundred degrees per watt thermal resistance from junction to ambient you're not going to dump much power in a small signal transistor like that, but its certainly enough to run relays and LEDs and stuff, and the same general temp limits apply to all transistors. Some beastly multi-GHz multi-watt monster might have a staggeringly lower thermal resistance but the junction temp limit is still gonna be 150C or so. I guess in summary if off the shelf you could run junctions at 500C then heat wouldn't be an issue or limiter for another, I donno, decade or two. But the 150 or 175 or whatever limit is a hard physics limit with current tech, so ... If you want to impress an EE with space-ship x-files area 51 BS give him a semiconductor that works while glowing orange hot or so. That would have pretty interesting performance specs, I bet. Its also impossible AFAIK at current human technology levels. Either that or a semiconductor with room temperature superconductive bond wires. Which is more likely first? Who knows.

the8472 10y ago
I don't think the absolute temperature is directly related to the wattage. Heat transport also factors into it. You could always keep the chip cooled with active heat pumps or carefully applied liquid nitrogen. It's just impractical, especially getting everything cooled equally in the presence of hot/cold spots.
- dom0 10y ago
  Heat resistance is approximately a given (surface area), so wattage depends on the permissible temperature difference between the two items (die and heatspreader). If you run hotter, you can dump more power into it. Temperature is ultimately limited by sped up diffusion of doping and such, temperature limits in surrounding materials (eg epoxy-based chip carriers or epoxy-based case material), and reduced performance due to increasing leakage currents.
userbinator 10y ago
TI has some interesting parts (including microcontrollers) that are rated up to 210C: http://www.ti.com/lit/sgzt009 http://www.ti.com/lit/sgzt009 Here are some MOSFETs that are rated to operation at 300C(!) http://www.x-relsemi.com/EN/Documentation/Datasheet/DS-00113-11-XTR2N0800-High-Temperature,%2080V%20N-Channel%20Power%20MOSFET%20Family.pdf http://www.x-relsemi.com/EN/Documentation/Datasheet/DS-00113... Not cheap.
Symmetry 10y ago
The things that change with temperature is that the number of charge carriers per volume of un-doped silicon goes up while the speed with which charge carriers move through silicon goes down. More carriers independent of doping means more current through transistors channels even when gate voltage isn't applied, but there are ways to be conservative and design around that.
baobrien 10y ago
Transistors built on GaN and SiC processes can operate with a junction temperature of up to around 200C. As you say, that's a pretty big deal for RF power transistors, as they can dissipate quite a lot more power without blowing up. Another issue with running nanometer-scale chips hot, from what I understand, is that the dopants will start to diffuse into the silicon, and because the feature size is so small, you eventually get transistor failures.