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| Why is electron mobility so important for microwave circuits? |
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| fonograph:
Why its so important when making super fast transistors? I read that semiconductor resistivity depends on charge mobility and carrier concentration.From that it seems to me that it should not matter if you have many slow electrons or lesser amount of fast electrons yet the super fast circuits all use the fast electron,not many,type of semiconductor like InP and GaAs. Is this just coincidence or is carrier,especially electron mobily somehow more special that it would seem from resistivity calculations? |
| Kleinstein:
In a crude approximation the input current to a FET or BJT provides a limited number of carriers. So for a given input current / power the carrier mobility is what determines the possible higher frequency gain. Using more carriers is not a real option as it takes more input current. |
| fonograph:
Yeah but when there is more carriers,wouldnt it take less voltage to get that current than in high mobility low carrier concentration material? If you have more current but less voltage,power stays same. |
| jpb:
I used to work in modelling microwave GaAs FETs and HEMTs. The electrons at operating potentials generally are moving at their saturated velocity so mobility doesn't affect this. These devices are complicated in real operation. Even with HEMTs where the 2D electron gas is supposed to moving away from the donor ions, in real devices they get heated and scattered . A lot of the high gain comes from high doping. I was part of a research project that looked at comparing HEMTs and FETs and found that a suitably doped FET could be as low noise as a HEMT (at room temperature - a cooled HEMT at low current might be a different case). For microwave circuits, a big advantage of GaAs over Si is that it has a semi-insulating state which makes for low loss transmission lines (for monolithic microwave integrated circuits). |
| fonograph:
jpb Your claiming electron moblity doesnt matter much becose they are moving at saturation velocity? Tell me about that GaAs semi insulating state,never heard about that before... :popcorn: Also,how is carrier saturation velocity different from carrier mobility? When some material have high carrier mobility,it must automatically mean it also have high saturation velocity... or is that wrong? Are carrier moblity and saturation velocity unrelated? Can there exist a high mobility low saturation velocity material? |
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