Within this region, the electrons are able to move freely, because there are no other donor electrons or other items with which electrons will collide and the mobility of the electrons in the gas is very high. These electrons in a layer that is only one layer thick, forming as a two-dimensional electron gas shown in the above figure (a). The electrons from the n-type region move through the crystal lattice and many remain close to the Hetero-junction. The operation of the HEMT is a bit different to other types of FET and as a result, it is able to give a very much enhanced performance over the standard junction or MOS FETs, and in particular in microwave RF applications. Cross-Sectional Diagrams Comparing Structures of an AlGaAs or GaAs HEMT and a GaAs HEMT operation To maintain a high-frequency performance the size of the gate should be typically 0.25 microns or less. The size of the gate, and hence the channel is very small. The thickness under the gate is also very critical since the threshold voltage of the FET is determined by the thickness only. Areas are etched as shown in the diagram below. The gate is generally made of titanium, and it forms a minute reverse biased junction similar to that of the GaAs-FET.įor the recess gate structure, another layer of n-type Gallium Arsenide is set down to enable the drain and source contacts to be made. In self-aligned ion implanted structure the Gate, Drain and Source are set down and they are generally metallic contacts, although the source and drain contacts may sometimes be made from germanium. There are two main structures that are the self-aligned ion implanted structure and the recess gate structure. The exact thickness of this layer is required and special techniques are required for the control of the thickness of this layer.

The doped layer of Aluminium Gallium Arsenide about 500 Angstroms thick is set down above this as shown in the diagrams below. This is very critical if the high electron mobility is to be achieved. The main purpose of this layer is to ensure the separation of the Hetero-junction interface from the doped Aluminium Gallium Arsenide region. After that, a very thin layer between 30 and 60 Angstroms of intrinsic Aluminium Gallium Arsenide is set down on top of this layer. The manufacture of an HEMT as follows procedure, first an intrinsic layer of Gallium Arsenide is set down on the semi-insulating Gallium Arsenide layer.

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