attracted attention due to their high-power perfor- mance. The HEMTs are used in integrated circuits as digital ON/OFF switches. HEMT transistors are able to operate at higher frequencies than ordinary transistors, up to mmWave frequencies. Ideally, the two different materials used for a heterojunction would have the same lattice constant (spacing between the atoms). In prac- tice, the lattice constants are typically slightly different (e.g., AlGaAs on GaAs), resulting in crystal defects. A HEMT where this rule is violated is called a pHEMT or pseudo-morphic HEMT. This is achieved by using an extremely thin layer of one of the materials SO thin that the crystal lattice simply stretches to fit the other mate- rial. This technique allows the construction of transistors with larger bandgap differences than otherwise possi- ble, giving them better performance. Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the mHEMT or metamorphic HEMT, an advancement of the pHEMT. The buffer layer is made of AllnAs, with the indium concentration graded SO that it can match the lat- tice constant of both the GaAs substrate and the GaInAs channel. A 2DEG is a scientific model in solid-state physics. It is an electron gas that is free to move in two dimen- sions, but tightly confined in the third dimension. This tight confinement leads to quantized energy levels for motion in the third direction, which can then be ignored for most problems. Thus, the electrons appear to be a 2D sheet embedded in a 3D coordinate system. The analogous construct of holes is called a two- dimensional hole gas, and such systems have many useful and interesting properties. New Radio Access RF and Transceiver Design Considerations 735 high-frequency applications. For low-voltage applications, InP HBT is expected to replace GaAs HBT. InP HBT is made of narrow bandgap material InGaAs. The bias voltage is as low as 0.5-0.6 V. In addition, InP HBT also exhibits better heat conduction characteri