MODFET are similar to MESFET, but with the difference that MODFET’s are made of different kind of semiconductors (heterojunction). The two different semiconductors create at the interface a very good conductive channel (2DEG).

The mobility of the electrons in this channel is very high, which is one of the reasons they are very fast. To deplete the channel a negative voltage must be applied at the gate. The voltage needed to deplete the channel is called the threshold voltage.

A heterojunction is a junction between two semiconductors with a different bandgap.

In case of a MODFET the channel is undoped to avoid scattering at impurities.

At the junction the bands bend and because of the differed bandgaps the conduction band gets bend below the Fermi energy and creates a low density metal. The idea is to use this metal as the conductive channel.

The advantage is that it’s possible to deplete the channel with the gate and that the channel has a very high mobility.

MODFET’s can be found in high frequency preamplifier like in cell phones, electronic warfare systems, microwave and millimeter wave communications, radar, and radio astronomy. They are very fast and can be used at frequencies up to 600 GHz and more. They also have a high gain and low noise.