pHEMT

pHEMT stands for pseudomorphic high-electron-mobility transistor, a type of field-effect transistor that uses a strained, pseudomorphically grown layer to create a high-mobility two-dimensional electron gas at a heterointerface. The resulting 2DEG enables high transconductance and fast operation, offering advantages for microwave and RF applications over conventional HEMTs.

Structure and operation: A heterojunction between a wide-bandgap barrier layer and a narrow-bandgap channel layer confines electrons at the interface. Strain and band offsets produce the 2DEG, which is typically undoped near the interface. Doping is placed in the barrier away from the channel to supply carriers (remote doping). A metal gate controls the 2DEG, regulating current with high efficiency.

Materials: GaAs/AlGaAs and InP-based systems are common, using lattice-mismatched but pseudomorphic layers to induce strain. The barrier materials provide the conduction-band offset that confines the 2DEG, while the gate voltage modulates electron density in the channel.

Performance: The 2DEG yields high electron mobility and low scattering, enabling high cutoff frequencies and transconductance. pHEMTs typically offer strong RF performance, with good noise figures in suitable configurations and robust breakdown characteristics for microwave circuitry.

Applications: Used in RF amplifiers, low-noise amplifiers, and power amplifiers for cellular, satellite, radar, and microwave integrated circuits, particularly where fast switching and high-frequency operation are required.

History: PHEMT technology emerged in the late 1980s and 1990s as an evolution of HEMT technology, with maturation in GaAs and InP material systems and ongoing research into new materials and device structures to extend frequency range.