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InP-Si sandwich chips combine the best of both worlds

The integration of an indium phosphide chip with a silicon chip could be the key to faster and more powerful terahertz devices for high resolution and mobile applications
Two Leibniz institutes have broken new technological ground; they have successfully combined their up to now, separate technologies.

Due to their high performance, the novel chips developed within the HiTeK project promise to open the door to new applications.

Wolfgang Heinrich and Bernd Tillack are convinced of holding the key to faster and more powerful terahertz (THz) chips. The two scientists and their teams come from the Berlin-based Ferdinand-Braun-Institut (FBH) and from the IHP-Leibniz-Institut für innovative Mikroelektronik in Frankfurt/Oder.

FBH is one of the leading institutes in developing III-V semiconductors, while IHP is specialised in silicon-based systems and circuits. Both Leibniz institutes joined forces within the HiTeK project to combine the advantages of silicon-based CMOS (Complementary Metal Oxide Semiconductor) circuits from the IHP with those of InP circuits from the FBH.

The partners have taken an important step within the project by successfully integrating both circuits onto a semiconductor wafer, with experimental results demonstrating their high performance.

With the integration on one chip, new ambitious applications in the THz range are within reach. These include high-resolution imaging systems for medical and security technology as well as ultra-broadband mobile communication applications.


InP / Silicon sandwich wafer

For such applications, high output powers along with faster computer processors are needed, offering enhanced computer operation per second. In order to achieve this, circuits on the chips need to become smaller to boost miniaturisation in the semiconductor industry.

If the frequency range around 100 gigahertz (GHz) and beyond is to be covered, however, the breakdown voltage in the CMOS switching circuits decreases significantly. As a consequence, the available output power of the chips declines. This implies that the capability of generating sufficiently strong signals to establish a radio link and to detect material defects becomes insufficient.

To find a solution to this problem, IHP conducted research on bipolar CMOS based on SiGe, enhancing the breakdown voltages at high speed compared to pure CMOS. By combining a standard CMOS circuit with a second InP circuit promised further improvement.

Both circuits are realised in a “sandwich-like” structure and lie one on top of another. Where the traditional silicon-based CMOS technology reaches its limits, this novel material combination delivers the desired properties; high output powers at high frequencies. The sandwich chips enable a high level of production and integration of CMOS circuits - particularly regarding the fact that 95 percent of all digital and analogue-digital circuits are based on this technology.

“It was particularly challenging to make both technologies compatible at the interfaces”, underlines Wolfgang Heinrich from the FBH. To achieve this, the whole development environment of both processes as, for example, the software for the circuit layout had to be merged in a first step.

Subsequently, both layers had to be dimensioned so that they reach the essential good transmission properties for frequencies around 200 GHz. Precision work was also highly demanded to adjust the circuits precisely to each other with an accuracy of less than 10µm.

Heinrich is especially proud of the friction-less cooperation, “We managed to align both technology worlds so smoothly that the circuits deliver fully the specified high-frequency performance. This also demonstrates what added value can be created by bundling the competencies of two institutes like IHP and FBH”.

The next steps are to further stabilise the process and to optimise the circuits. A follow-up project has already been granted. In this way, the potential of the hybrid chips will be exploited fully to reach the borders of what is feasible. This will set the stage for the novel sandwich circuits to be integrated in sophisticated applications in the near future.

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