Significance The HgCdTe linear avalanche focal plane detector has the characteristics of high gain, high bandwidth and low excess noise, and has shown great application potential in the field of aerospace, astronomical observation, military equipment and geological exploration. Based on their own HgCdTe infrared FPA detector technology, Leonardo, Raytheon, DRS and Sofradir have developed HgCdTe APD focal plane devices. The demonstration of active gating imaging, active/passive dual-mode imaging and 3D imaging have been completed, showing attractive application prospect of HgCdTe APD. However, the research on HgCdTe APD detector technology is still at the initial stage in China, and its application is still in the exploration stage due to the lack of evaluation method.
Progress The parameters of the HgCdTe infrared focal plane array cannot completely cover the characterization of HgCdTe APD. According to the characteristics and application requirements of HgCdTe APD, in order to accurately characterize the performance of HgCdTe APD focal plane devices, it is necessary to introduce parameters such as gain, excess noise factor, noise equivalent photon number and time resolution. The gain of the APD is used to measure the amplification ability to the input, which is defined as the ratio of the response of the device with gain to the response without gain. The test method of the gain is given and the gain for an APD FPA prepared by Kunming Institute of Physics is shown (Fig.1, Fig.2). The average gain of the APD FPA has an exponential relationship with the bias. When the bias is −8 V, the gain of the FPA is 166 and the gain nonuniformity does not exceed 3.4%. The randomness of the carrier multiplication of the APD introduces excess noise, which makes the SNR of the output deteriorate when the input is amplified. Usually, excess noise factor is used to describe the deterioration of SNR, which can be calculated by the ratio of the device output SNR without gain to the device output SNR with gain. It's worth noting that the conditions need to be consistent during the test, otherwise, the change of the bandwidth will cause the test data not to reflect the true excess noise factor level of the device. The result is shown (Fig.1, Fig.3). Similar to noise equivalent temperature difference, noise equivalent photon number (NEPh) is used to evaluate the sensitivity of APD device in active imaging mode, which is mainly determined by the device gain, dark current level, background flux and readout circuit noise. Generally, NEPh refers to the limiting performance of the device, which is generally tested under the non-background limit (the optical current caused by the background flux should be less than the dark current). In the same conditions, the NEPh of APD device in high gain state decreases with the decrease of integration time (Fig.4). Coupling the APD device with the ROIC with timing function, the distance information can be obtained, which can be evaluated by time resolution. The time resolution reflects the minimum time interval of the pulse laser reaching the focal plane which can be distinguished by the APD, representing the minimum distance that can be distinguished. Finally, combined with the application of HgCdTe linear avalanche device and its characteristics, its application in active/passive infrared imaging and fast infrared imaging is discussed in detail, which can be used as a reference for the application of the HgCdTe APD FPA.
Conclusions and Prospects Firstly, the key parameters that characterize the performance of HgCdTe APD focal plane chip are analyzed. Secondly, based on the characteristics of HgCdTe linear avalanche focal plane devices, the applications of HgCdTe avalanche focal plane devices in active/passive imaging, fast imaging and 3D imaging are discussed. Finally, the future development of HgCdTe avalanche focal plane devices is prospected. With the development of HgCdTe material growth, fabrication of devices, readout circuit design and processing and testing technology, there will be HgCdTe APD focal plane products with better performance, larger area, smaller pixel center distance and higher frame rate, which meet the demands of high-performance detectors in various applications such as 3D imaging, active/passive dual-mode imaging and single-photon detection.
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