One of the main limitations of the MESFET is that although this device extends well into the mmWave range (30 to 300 GHz or the upper part of the microwave spectrum), it suffers from low field mobility due to the fact that free charge carriers and ionized dopants share the same space.
To demonstrate the need for HEMT transistors, let us first consider the mobility of GaAs compound semiconductor. As shown in the picture, with decreasing temperature, Coloumb scattering becomes prevalent as opposed to phonon lattice vibrations. For an n-channel MESFET, the main electrostatic Coloumb force is between positively ionized donor elements (Phosphorous) and electrons. As shown, the mobility is heavily dependent on doping concentration. Coloumb Scattering effectively limits mobility. In addition, decreasing the length of the gate in a MESFET will increase Coloumb scattering due to the need for a higher doping concentration in the channel. The means that for an effective device, the separation of free and fixed charge is needed.
A heterojunction consisting of n+ AlGaAs and p- GaAs material is used to combat this effect. A spacer layer of undoped AlGaAs is placed in between the materials. In a heterojunction, materials with different bandgaps are placed together (as opposed to a homojunction where they are the same).
This formation leads to the confinement of electrons from the n- layer in quantum wells which reduces Coloumb scattering. An important distinction between the HEMT and the MESFET is that the MESFET (like all FETs) modulates the channel thickness whereas with an HEMT, the density of charge carriers in the channel is changed but not the thickness. So in other words, applying a voltage to the gate of an HEMT will change the density of free electrons will increase (positive voltage) or decrease (negative voltage). The channel is composed of a 2D electron gas (2DEG). The electrons in the gas move freely without any obsctruction, leading to high electron mobility.
HEMTs are generally packed into MMIC chips and can be used for RADAR applications, amplifiers (small signal and PAs), oscillators and mixers. They offer low noise performance for high frequency applications.
The pHEMT (pseudomorphic) is an enhancement to the HEMT which feature structures with different lattice constants (HEMTs feature roughly the same lattice constant for both materials). This leads to materials with wider bandgap differences and generally better performance.