MESFET- The High Electron Mobility Transistor (HEMT)
MESFET is constructed with the channel along the GaAs well, we can take advantage of this reduced scattering and resulting higher mobility. The effect is especially strong at low temperature where lattice (phonon) scattering is also low. This device is called a modulation doped field-effect transistor (MODFET) and is also called a high electron mobility transistor (HEMT).
Operation of HEMT:
- In Fig.(a) we have left out the band-bending expected at the AlGaAs/GaAs interfaces. we expect the electrons to accumulate at the corners of the well due to band-bending at the heterojunction.
- In fact, only one heterojunction is required to trap electrons, as shown in Fig.(b) Generally, the donors in the AlGaAs layer are purposely separated from the interface by -100 A.
Using this configuration, we can achieve a high electron concentration in the channel while retaining high mobility, since the GaAs channel region is spatially separated from the ionized impurities which provide the free carriers.
- In Fig.(b), mobile electrons generated by the donors in the AIGaAs diffuse into the small band gap GaAs layer, and they are prevented from returning to the AIGaAs by the potential barrier at the AIGaAs/GaAs interface.
- The electrons in the (almost) triangular well form a two-dimensional electron gas. Sheet carrier densities as high as 1012 cm-2 can be obtained at a single interface such as that shown in Fig.(b).
- Ionized impurity scattering is greatly reduced simply by separating the electrons from the donors. Also, screening effects due to the extremely high density of the two-dimensional electron gas can reduce ionized impurity scattering further.
- In properly designed structures, the electron transport approaches that of bulk GaAs with no impurities, so that mobility is limited by lattice scattering. As a result, mobilities above 250,000 cm2/V-s at 77 K and 2,000,000 cm2/V-s at 4 K can be achieved.
Advantages of HEMT:
- The advantages of a HEMT are its ability to locate a large electron density (~1012 cm-2) in a very thin layer (<100 A thick) very close to the gate while simultaneously eliminating ionized impurity scattering.
- The AIGaAs layer in a HEMT is fully depleted under normal operating conditions, and since the electrons are confined to the heterojunction, device behavior closely resembles that of a MOSFET.
- The advantages of the HEMT over the Si MOSFET are the higher mobility and maximum electron velocity in GaAs compared with Si, and the smoother interfaces possible with an AIGaAs/GaAs heterojunction compared with the Si/Si02 interface.
- The high performance of the HEMT translates into an extremely high cutoff frequency, and devices with fast access times.