Ultra-sensitive and fast infrared imaging has become increasingly important in applications that require high frame rates at low light levels, such as exoplanet imaging. The sensitivity of conventional short-wave infrared cameras is limited by their readout noise level. This limitation can be addressed by the internal gain of the sensors, but only if fast response time and low dark current are achieved simultaneously. Recent theoretical predictions suggested that reducing the internal capacitance of detectors with internal gain can increase their sensitivity. Here, we show the experimental validation of this prediction for III–V heterojunction phototransistors. We have fabricated a 320 × 256 array of InGaAs/InP infrared phototransistors integrated with a conventional silicon readout circuit. The array is made of two groups of pixels: 50% are devices with a 1 μm base diameter and the other 50% with a 2 μm base diameter. Characterization of a large number of pixels shows that 1 μm devices have significantly higher sensitivity than 2 μm devices. These have an average noise equivalent photon sensitivity of about 20 photons at a camera frame rate of ∼500 frames per second, which is better than the best existing infrared cameras with a similar cutoff wavelength and frame rate. Interestingly, the processing variation in the 1 μm devices resulted in variation in sensitivity, and a good number of devices show sensitivity to less than 10 photons. These results suggest that the proposed phototransistors are promising for ultra-sensitive short-wavelength infrared cameras.
Nanowire photodetectors are attractive for their high speed and responsivity, enabled by small junction capacitance and high internal gain. However, their effectiveness is hampered by a low quantum efficiency due to poor light coupling to their intrinsically small size. The optically sensitive area can be increased by connecting arrays of standing nanowires (pillars) in parallel under a single readout, but the increase in dark current and total capacitance might reduce pixel sensitivity. The net effect has not yet been thoroughly investigated. In this work, we prove that such multipillar architecture indeed improves effective pixel sensitivity without reducing speed. Our theoretical analysis reveals that the pixel response time is dominated by the constituent nanowires rather than by the global capacitance, resulting in improved quantum efficiency for equivalent speed. We simultaneously characterize different pixel designs on a single focal plane array, demonstrating the viability of multipillar architectures for large-area detectors and imagers.
The sensitivity of infrared (IR) focal plane arrays (FPAs) is often limited by a low pixel fill factor. Solid immersion microlens arrays address this problem by focusing the light reaching each pixel into the most sensitive part of that pixel. This strategy is used in CMOS image sensors but has not been industrially adopted for IR FPAs due to significant difficulties in integration with compound semiconductors. Here, we present an all-in-one solution for producing solid immersion microlens arrays compatible with various IR FPAs regardless of their substrate material. Our strategy is to use refractive lenses made of SiO 2 and Si 3 N 4 with very broad-band and efficient focusing abilities. Notably, our strategy works across a broad range of wavelengths with little performance degradation, meaning it is scalable to various applications. We implemented our method in short-wavelength IR FPAs and demonstrated 7.4 times improvement in quantum efficiency. This is the first demonstration of an immersion microlens array in a non-silicon infrared FPA.
Photodetectors with internal gain are of great interest for imaging applications, since internal gain reduces the effective noise of readout electronics. High-gain photodetectors have been demonstrated, but only individually rather than as a full array in a camera. Consequently, there has been little investigation of the interaction between camera complementary metal oxide semiconductor (CMOS) electronics and the slow response time that high-gain photodetectors often exhibit. Here we show that this interaction filters shot noise and causes noise statistics to differ from the common Poisson distribution. As an example, we investigate a 320 × 256 array of InGaAs/InP high-gain phototransistors bonded to a CMOS readout chip. We demonstrate the filtering effects and discuss their consequences, including new (to the best of our knowledge) methods for extracting gain and increasing dynamic range.
Reconfigurable detectors with dynamically selectable sensing and readout modes are highly desirable for implementing edge computing as well as enabling advanced imaging techniques such as foveation. The concept of a camera system capable of simultaneous passive imaging and dynamic ranging in different regions of the detector is presented. Such an adaptive-autonomous detector with both spatial and temporal control requires programmable window of exposure (time frames), ability to switch between readout modes such as full-frame imaging and zero-suppressed data, modification of the number of pixel data bits and independent programmability for distinct detector regions. In this work, a method is presented for seamlessly changing time frames and readout modes without data corruption while still ensuring that the data acquisition system (DAQ) does not need to stop and resynchronize at each change of setting, thus avoiding significant dead time. Data throughput is maximized by using a minimum unique data format, rather than lengthy frame headers, to differentiate between consecutive frames. A data control and transmitter (DCT) synchronizes data transfer from the pixel to the periphery, reconfigures the data to transmit it serially off-chip, while providing optimized decision support based on a DAQ definable mode. Measurements on a test structure demonstrate that the DCT can operate at 1 GHz in a 65 nm LP CMOS process.
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