InP Heterobipolar Transistors for THz Power Electronics
The terahertz range (300 GHz – 3 THz) of the electromagnetic spectrum is largely unused for the lack of solid-state terahertz electronic components. Exciting applications can be found in many areas including communications, navigation, security, material engineering, and medical and life sciences, owing to the characteristics of terahertz electromagnetic radiation. Applications include high-bandwidth (≥ 100 Gb/s) wireless communication links, high-resolution RADAR for automotive and robotic applications, imaging of concealed objects, and low radiation dose medical imaging.
To date, the realization of solid-state terahertz devices with significant output power which may be monolithically integrated into compact circuits remains an unsolved technological challenge. Thus, faster transistors capable of delivering significant power are needed. Indium phosphide heterobipolar transistors (InP HBT) can reach unity power gain frequencies above 1 THz with emitter widths around 100 nm, owing to the high electron mobility and velocity in this semiconductor material. The high breakdown field in InP allows to operate scaled HBT with relatively high collector voltage (4 V), leading to significant RF power output.
At FBH, the unity power gain frequency of InP HBT shall now be increased within the framework of the four-year EU research project "THzPowerElectronics" by lateral and vertical dimensional scaling with simultaneous reduction of contact resistance and vertical epitaxial structure optimization. The goal is to reach transistor unity gain frequencies in excess of one terahertz, and circuit fundamental operating frequencies of 300 to 500 GHz, with useable output power in the range of a few milliwatts.
In a first step, using electron beam lithography instead of the previously employed optical stepper lithography, the width of the emitter will be shrunk from 800 nm down to 500 nm, and then to 200 nm, which will boost the cutoff frequencies from now 350 GHz to exceed 700 GHz. Proximity dose-corrected electron beam lithography allows the realization of single and double emitters of varying sizes on the same wafer. In a second step, changes to the process architecture, such as lateral isolation spacers between emitter and base contact, are needed when reducing the emitter width further towards 50 nm aiming to surpass one terahertz unity power gain frequency fmax.
In June 2014, a first run of electron beam lithography defined wafers was completed, yielding working devices with emitter widths between 500 nm and 300 nm. These HBT showed a slightly improved fmax compared to the 800 nm wide InP HBT. To reach the targeted 700 GHz, the vertical epitaxial structure was changed according to 2D device simulation of the InP HBT. These wafers are currently being processed in FBH’s cleanroom.
The research leading to these results has received funding from the European Union Marie Curie Career Integration Grant "THzPowerElectronics", Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 2012-333858.