{"id":5423,"date":"2012-07-05T14:20:43","date_gmt":"2012-07-05T14:20:43","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2012\/?p=5423"},"modified":"2012-07-18T22:53:54","modified_gmt":"2012-07-18T22:53:54","slug":"graphene-based-cmos-infrared-imaging-system","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2012\/graphene-based-cmos-infrared-imaging-system\/","title":{"rendered":"Graphene-based CMOS Infrared Imaging System"},"content":{"rendered":"

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<\/a>] <\/sup>. However, the band gap of silicon fundamentally limits the absorption spectrum to be in the visible and near-infrared light (\u03bb < 1100nm) [2<\/a>] <\/sup>. In our research, we propose an integrated CMOS imager with a graphene photodetector. With the zero band gap, the graphene photodetector absorbs long-wavelengths (\u03bb > 1\u03bcm) and enables integrated circuits for a wide spectrum of imaging applications such as thermal and terahertz imaging.<\/p>\n

In the design of the graphene photodetector, graphene\u2019s 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.<\/p>\n

Figure 1 shows the pixel design of the photodetector. Unlike p-n junction photodiodes [3<\/a>] <\/sup>, 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. \u00a0By multiplexing each pixel, this architecture gives a good trade-off between frame rate and power consumption.<\/p>\n\n\t\t