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Why does higher mobility = higher frequency of operation (transistors)

  1. Feb 17, 2013 #1
    I am going to give a presentation on High Electron Mobility Transistors (the ones that use a 2 dimension electron or hole gas), and although I can find many detailed descriptions of the band structures and what causes the 2DEG's high mobility, I haven't found a clear description of the process by which field effect transistors (or any transistor) start to fail to accurately amplify a signal at higher frequencies (or switch faster).

    For the BJT, it seems that the source of this failure could be the lag time for the concentration profile in the base to reach steady state, which would be related to the diffusion constant, which is related to the mobility. Am I correct about this?
  2. jcsd
  3. Feb 17, 2013 #2


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    Re: Why does higher mobility = higher frequency of operation (transist

    I am no expert but I found this which may illuminate for you some reasons for the high performance of these 2DEG devices:

    “Unexpected features of branched flow through high-mobility two-dimensional electron gases”

    Hope this helps.
  4. Feb 17, 2013 #3
    Re: Why does higher mobility = higher frequency of operation (transist

    While this was a very interesting description of the electron flow in 2DEGs, I don't think it deals with my original question about transistor performance.
  5. Feb 22, 2013 #4
    Re: Why does higher mobility = higher frequency of operation (transist

    Mobility --> Transit Time across the base --> upper bound on BJT Ft

    However PN-junction BJTs are primarily bandwidth limited by junction capacitance (with BC capacitance multiplied by Miller effect). This why you see cascodes: feeding into a common base helps to reduce the effective RC time constant which determines the bandwidth.

    Where HBTs enter the picture is to eliminate conventional PN junction capacitance (and increase injection efficiency). HEMTs are majority carrier devices plus the nature of the 2DEG (lack of scattering from dopant atoms) gives better mobility than possible with minority carrier currents or majority currents with dopants.
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