thin ice
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2015-06-01 |
| Journal | Electronics Letters |
| Authors | Anonymous |
Abstract
Section titled âAbstractâDiamond films provide a way to more than double the power output achievable with âTerahertz gapâ transistors, in research from Germany. The films are used to provide much better heat management in InP DHBTs for more reliable and higher power operation, using a process compatible with existing transistor production technologies. A whole processed wafer in transferred-diamond technology made the lab at the Ferdinand-Braun-Institute Leibniz-Institut fĂŒr Höchstfrequenztechnik, Berlin The electromagnetic spectrum between 300 GHz and 3 THz, in the so-.called Terahertz gap, is largely unused owing to a lack of suitable electronic components. Many applications could be opened up with this region in fields ranging from satellite communication to medical imaging. But for this to happen, suitable transistors are needed with high RF output power and efficiency at these frequencies. Indium phosphide double-heterobipolar transistors (DHBTs) can provide multiple times the RF output power of silicon-based technologies between 300 GHz and 1 THz. This is because InPâs material properties allow for fast electron transport paired with a high breakdown field. However, an important limit on InP DHBT device operation is thermal resistance. Heat generated in the device needs to be effectively removed to keep the junction temperature inside the transistor acceptable for reliable operation. Junction temperature also needs to be controlled as higher temperatures mean reduced RF gain and transistor efficiency. In this issue of Electronics Letters, researchers from the Ferdinand-Braun-Institut, Leibniz-Institut fĂŒr Höchstfrequenztechnik (FBH) present an InP DHBT design incorporating a thin-film diamond layer that can cut the thermal resistance of a DHBT by around 75%. Diamond has 5-6 times higher thermal conductivity than silicon and the thin layer acts as an effective heat spreader, dissipating the heat produced in the active region of the transistor to the heat sink on the back of the circuit board. âIn principle, this advance would allow us to triple the power density of the DHBT devices while maintaining the same junction temperature,â explains FBH team member Ksenia Nosaeva. âHigher output power of a single transistor device is a significant enabler of efficient high RF power circuits. Although circuit output power can be increased by putting many transistors in parallel, RF power combining schemes are usually not very broadband and introduce passive losses. For example, this technology would enable the realisation of broadband high-power RF transmitters above 300 GHz for very high resolution radar applications in robotics and automotive systems. The higher efficiency would translate into lower system weight and power usage, enabling mobile applications to use this technology, as well.â The addition of the diamond film to the completed InP DHBT circuit layer stack was the most challenging part of the work. Internal stresses in the very hard diamond film and the difference in the thermal expansion coefficients can lead to film delamination and cracks in the resulting sandwich. The diamond film is transferred from its silicon handle wafer onto the finished InP circuit layers in a wafer bond process. Many process parameters besides the actual wafer bonding process, e.g. BCB bond layer thickness and application, had to be optimised to achieve acceptable yields of defect-free wafer area. High quality via connections are essential for efficient thermal transport and low-loss electrical connections to the contact pads on the surface of the wafer from the active device layers below. So the teamâs second challenge was creating deep vertical vias through the diamond and the BCB glue layer, followed by a through-via gold metallisation step through these high aspect ratio structures. The teamâs work resulted in the development of an additional technology module in FBHâs InP DHBT integration that can be added to the baseline InP DHBT process for high RF output power applications. This diamond module is also compatible with FBHâs InP DHBT-SiGe BiCMOS process, capable of monolithic integration of InP DHBT subcircuits with SiGe BiCMOS, enabling complex system-on-a-chip sub-mm-wave RF sources with high output power. The output power of both these technologies can be more than doubled immediately with the inclusion of the diamond heat spreading process module and the diamond heat spreading film concept could also be be applied to other high-power semiconductor technologies, such as gallium nitride (GaN). Left: A close-up of microwave integrated circuits and process control structures on the processed wafer with transferred diamond layer. Right: A single chip with microstrips clearly visible under the transparent diamond layer The FBH researchers are now in the process of applying this work to make high-power integrated circuits at mm-wave and sub-mm-wave frequencies and intend to publish results very soon, including amplifier circuits with much improved RF output power at and above 100 GHz. Their goal is to enable circuit and subsystem applications that are not accessible with existing technologies and they believe heat spreading technology will become even more important as HBT dimensions are further reduced in efforts to close the Terahertz gap.