ABSTRACT
Figure 15.1 Choice of insertion of FET epilayers into the HBT structure. Courtesy of Pete Zampardi, Skyworks Solutions. The original merged epi-FET process developed at Rockwell and UCSD [1] used AlGaAs/GaAs commercial metal-organic chemical vapor deposition (MOCVD) wafers, with the difference that a highly doped channel layer was inserted between the AlGaAs emitter and the top heavily doped cap layers. See Fig. 15.2 [15]. The FET uses the top cap layers for source and drain. Unique features of the FET are that the AlGaAs emitter layer is fully depleted, thus confining the channel and the p+ GaAs base layer is used as the back gate. In this original form there was no etch stop for channel etching; instead the channel current was monitored during the etch step. After gate deposition, the rest of the processing was all common to HBTs. Good
FET (0.5 mm gate) and HBT characteristics were achieved, and circuit functionality of complex circuits (like static random access memory [SRAM]) was demonstrated.
