By Richard Stevenson, from compound semiconductor.net.

TriQuint, Anadigics, Skyworks SOlutions and Win Semiconductors have developed different technologies to monolithically integrated an HBT with a pHEMT or a FET. TriQuint says that its BiHEMT process is deisigned to address the needs of internal product designs and foundry customers. As a result, a premium is placed on process flexibility, which measn that E- and D-mode gate pHEMT devices are included in the process. (For applications that don't require both E- and D-mode devices, some process steps can be omitted.)

The key difference between Skyworks and Anadigics, the FET/HBT integrators, is their approach to the FET/HBT epitaxial structure.

Skyworks integrates its MESFET into its HBT emitter structure, which has the benefits of straightforward epitaxial growth and processing. However, this comes at the expense of FET performance, which is limited by the poor RF isolation of the MESFET.

Anadigics, in constrast, places AlGaAs/InGaAs layers underneath the HBT's subcollector - a relatively thick layer that doubles as the pHEMT's heavily doped n-type ohmic contact. This approach produces a pHEMT with good RF isolatoin at the penalty of a very deep, wide recess and a presumed increase in gate-source spacing, along with a higher drain/source resistance that results from the thick pHEMT contact layer.

TriQuint's approach differs from that of both Skyworks and Anadigics. The epitaxial layers of the pHEMT and the HBT are completely separated, and there is no compromise in pHEMT epitaxial design, according to the company. Instead, compromises are required in the actual epitaxy growth; growing an entire HBT structure on top of a pHEMT forces additional thermal cycling compared with a standalone pHEMT. There are also process trade-offs, due to the patterning of submicron gates on wafers with tall HBT mesas.

Win Semiconductors, a Taiwanese foundry with standalone power HBT and E/D pHEMT processes, has pursued a similar HTB/pHEMT integration approach to TriQuint. The process technology is clearly different but the chips produced also feature E/D-mode pHEMT integration, with the HBT and pHEMT separately optimized. The HBT is grown within the BiHEMT structure and the two types of transistor are completely separated to the extent that they share no epitaxial layers.