At the operating wavelengths of silicon photonic chips, materials with narrower bandgaps than silicon are required for detection (absorption) purposes. Germanium, due to its ability to absorb light at telecommunication wavelengths, is a preferred material. It can be grown epitaxially on silicon to ensure compatibility with standard silicon infrastructure, which is particularly useful for long-reach telecommunication applications.
For short-reach applications, where both ends of the link are controlled, materials like bonded III/V semiconductors are also used. These materials are typically integrated near silicon waveguides, allowing for efficient light coupling into the photodetector. Evanescent coupling or butt-coupling techniques are often used to enhance light detection. Furthermore, smaller photodetector cross-sections reduce device capacitance, which in turn improves operational speed.
A state-of-the-art germanium photodetector has been demonstrated to achieve a bandwidth of 120 GHz with a responsivity of 0.8 A/W. Other experiments have demonstrated responsivities of 1.05 A/W at 20 GHz for light at a wavelength of 1550 nm, resulting in a quantum efficiency of 84%. A germanium photodetector with a capacitance of 2.4 fF has also achieved impressive results, operating at 40 GHz with an estimated quantum efficiency of 90%.
Additionally, low bias avalanche detectors have been developed by IBM, with a 10 dB gain and significantly improved noise performance, achieving operational speeds of over 30 GHz. These photodetectors, used in high-speed optical communications, are essential for achieving the high performance required by silicon photonic systems.
As silicon photonics continues to advance, photodetectors will remain critical components, especially for telecommunication applications, where low-loss, high-speed, and efficient detection is necessary.
Terms: dB gain refers to the increase or amplification of a signal’s power, expressed in decibels (dB). It measures how much stronger a signal becomes after passing through an amplifier or other device that increases signal strength. The decibel is a logarithmic unit, so even small changes in dB can represent significant changes in signal power.
Key Points:
When a device has a 10 dB gain, the output signal is 10 times stronger than the input signal in terms of power.
Gain refers to the increase in the strength or amplitude of a signal.
The decibel (dB) is a logarithmic unit used to express ratios, such as the ratio of input to output signal strength.