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A*STAR photodiode breaks broadband speed limits

Posted: 18 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:photodiode? photodetector? transimpedance amplifier?

A research team at the A*STAR Institute of Microelectronics has created what it says is an ultrafast photodetector that can be efficiently fabricated onto silicon chips, eradicating the limit for broadband speeds. Photodiodes, or photodetectors, are key components of fibre-optical broadband networks: they convert light to electrical signals. Using thin films of germanium and silicon, the detector can be deposited on a chip using cost-efficient thin-film deposition or growth process that is compatible with electronic components.

Ning Duan and his co-workers developed their photodetector-known as an avalanche photodiode-using one of the fastest photodetector designs available, they stated. The device is made from semiconductors that operate under an applied electrical field at high voltage. In this field, arriving light excites electrons. They gain so much energy that it leads to an 'avalanche' of electrons, which can be easily detected. Since silicon is not suitable for infrared light detection, the related element germanium is typically used instead.

A*STAR photodecetor

Electron microscope image of avalanche photodetector. The device measures 30 micrometers in diameter.

The team succeeded in depositing germanium thin films on silicon by developing a so-called epitaxial process. During the deposition of the silicon and germanium layers, electrical conductivity was enhanced by selectively implanting into the films atoms from other elements such as arsenic and boron. The deposition worked at only a few hundred degrees Celsius, which is low enough to be compatible with industrial silicon fabrication techniques.

The detectors fabricated from these structures are designed for operation at wavelengths of about 1,550nm, which is the spectral region used in telecommunications. Compared to a normal germanium photodetector, the avalanche design has enhanced the detected signal by a factor of 30. The gain-bandwidth product, which characterizes both the detector enhancement as well as operation speed, is as high as 31GHz. This level is twice that of the traditional avalanche photodetectors based on compound semiconductors such as galliumCarsenide, the team indicated.

Further research is needed so that these detectors can be integrated with other electronic components on the same chip. A transimpedance amplifier, which converts electrical current from the photodetector into an electrical voltage, is an important component still lacking on the chips, said Duan."

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