The CMOS image sensor is widely used in digital multimedia applications. Its performance ramps up every year with denser integration, better noise suppression, adjustable dynamic range, and lower power ^[1] . However, the band gap of silicon fundamentally limits the absorption spectrum to be in the visible and near-infrared light (λ < 1100nm) ^[2] . In our research, we propose an integrated CMOS imager with a graphene photodetector. With the zero band gap, the graphene photodetector absorbs long-wavelengths (λ > 1μm) and enables integrated circuits for a wide spectrum of imaging applications such as thermal and terahertz imaging.

In the design of the graphene photodetector, graphene’s high conductivity is a key problem. When a bias voltage is applied to a graphene photodetector, the resulting leakage current easily dominates the photocurrent. Also, the low output impedance severely limits the output voltage of the graphene photodetector up to only a few microvolts, which degrades the signal-to-noise ratio (SNR). To resolve the leakage issue and improve SNR, we propose to use a modulated input source. The mechanical shutter working at around 1 kHz shifts the input signal out from the DC region where the leakage component dominates. The modulation also suppresses flicker noise in the photodetector and the readout circuits, by a band-pass filter followed by a multi-stage low-noise amplifier.

Figure 1 shows the pixel design of the photodetector. Unlike p-n junction photodiodes ^[3] , the graphene photodetector is placed on top of the silicon substrate, requiring no extra area; as a result, higher integration can be achieved in each pixel. The amplitude of the photocurrent is measured by an 8b-resolution ADC in a column-parallel architecture as shown in Figure 2.  By multiplexing each pixel, this architecture gives a good trade-off between frame rate and power consumption.

1. A. Fish and O. Yadid-Pecht, “Low power CMOS imager circuits,” in Circuits at the Nanoscale: Communications, Imaging, and Sensing, K. Iniewski, Ed. Boca Raton: CRC Press/Taylor & Francis, 2009.
2. R. Kaufmann, “Near infrared image sensor with integrated germanium photodiodes,” Journal of Applied Physics, vol. 110, pp. 023107-6, July 2011.
3. M. Perenzoni, N. Massari, D. Stoppa, L. Pancheri, M. Malfatti and L. Gonzo, “A 160×120-pixels range camera with in-pixel correlated double sampling and fixed-pattern noise correction,” IEEE Journal of Solid-State Circuits, vol. 46, pp. 1672-1681, July, 2011.