Enhancing InP HBTs with transferred substrate technology
Optimising high frequency and power performance, the 3" wafer-level process enables lithographic access to both the front- and backside of an Indium phosphide HBT. The vital step in gaining access to both sides is to completely remove the supporting substrate
Research on high-speed transistors is driven by applications for imaging and wide band communications.
Recent technical advances of InP-based transistors with several hundred gigahertz (GHz) operating frequencies, together with their outstanding material properties qualify them as key components in such systems.
At the Ferdinand-Braun-Institut (FBH), a transferred substrate (TS) technology has been established to optimise high frequency and power performance of InP heterojunction bipolar transistors (HBTs). The 3" wafer-level process enables lithographic access to both the front- and backside of the HBT aligned to each other.
The resulting linear device set-up, shown in Fig. 1, eliminates dominant transistor parasitics and relaxes design trade-offs. The essential step for gaining access to both sides of the epitaxial structure is to completely remove the supporting substrate.
Fig. 1: FIB cross-section of a TS-HBT with emitter (e), base (b) and collector (c)
Therefore, a robust adhesive wafer bonding procedure via benzocyclobutene (BCB) has been developed. It yields a homogenous, crack- and void-free composite matrix of transistors transferred on a wafer-level scale.
The optimised device topology enhances the HBT performance dramatically. Transistors with 2 × 0.8 × 5 μm2emitter area, as shown in Fig. 2, feature fT= 376 GHz and fmax= 385 GHz at breakdown voltages BVCEO > 4.5 V.
Fig. 2: Extrapolated fTand fmax of a TS-HBT
They combine high frequency performance with saturated output power Pout> 14.2 dBm @ 77 GHz and an inherently good matching to 50 Ω. As the device architecture is highly scalable, the researchers at FBH say they may be able to further enhance the high frequency as well as power performance in the future.
In the study, the researchers designed and produced power amplifiers using TS technology for 90 GHz operation. The S-parameter measurements shown in Fig. 3 agree well with modelling.
Fig. 3: S-parameter measurements of a 90 GHz power amplifier in TS technology
Currently, the innovative transistor set-up is utilised in an ongoing project to integrate InP-based circuits on top of BiCMOS wafers heterogeneously.
This research has been described in detail in the paper, "W-Band Amplifier with 8 dB Gain Based on InP- HBT Transferred-Substrate Technology," by T. Al-Sawaf et al, Proc. German Microwave Conference, Ilmenau, Germany, 12--14 March 2012.