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

[Randron]

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?

 

[Bobbywhy]

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”
http://arxiv.org/abs/1009.3670

Hope this helps.
Bobbywhy

 

[Randron]

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.

 

[jsgruszynski]

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.

 

[alphysicist]

Yes, you are correct. The higher mobility of the electrons or holes in a transistor allows them to move more quickly through the device, which in turn allows for higher frequency operation. This is because the speed at which a transistor can switch or amplify a signal is limited by the time it takes for the carrier concentration to reach steady state in the device. This is known as the transit time or the lag time.

In a BJT, the concentration profile in the base region needs to reach steady state for the transistor to accurately amplify a signal. This is dependent on the diffusion constant, which is directly related to the mobility of the carriers. As the mobility increases, the carriers are able to move through the device more quickly, reducing the lag time and allowing for faster switching and higher frequency operation.

In the case of High Electron Mobility Transistors (HEMTs), the 2DEG is formed at the interface between two semiconductor materials with different bandgaps. This creates a very high mobility channel for carriers to move through, resulting in faster switching and higher frequency operation.

In summary, the higher mobility of carriers in a transistor allows for faster switching and higher frequency operation by reducing the lag time for the concentration profile to reach steady state. This is why higher mobility is essential for the operation of transistors, especially at high frequencies.