the material system. IBM and others use ultra-high vacuum chemical vapor deposition (UHVCVD) for SiGe; other techniques used include MOVPE for III-V systems. Normally the epitaxial layers are lattice matched (which restricts the choice of bandgap etc.). If they are near-lattice-matched the device is pseudomorphic, and if the layers are unmatched (often separated by a thin buffer layer) it is metamorphic. Limits A pseudomorphic heterojunction bipolar transistor developed at the University of Illinois at Urbana-Champaign, built from indium phosphide and indium gallium arsenide and designed with compositionally graded collector, base and emitter, was demonstrated to cut off at a speed of 710 GHz. Besides being record breakers in terms of speed, HBTs made of InP/InGaAs are ideal for monolithic optoelectronic integrated circuits. A PIN-type photo detector is formed by the base-collector-subcollector layers. The bandgap of InGaAs works well for detecting 1550 nm-wavelength infrared laser signals used in optical communication systems. Biasing the HBT to obtain an active device, a photo transistor with high internal gain is obtained. Among other HBT applications are mixed signal circuits such as analog-to-digital and digital-to-analog converters. See also High electron mobility transistor (HEMT) MESFET References External links HBT Optoelectronic Circuits developed in the Technion (15Mb, 230p) New Material Structure Produces World's Fastest Transistor 604 GHz Early 2005 Microwave technology Terahertz technology Transistor types