# Trends in Performance Limits of the HOT Infrared Photodetectors
## 5 2D Material Infrared Photodetectors
### 2D Material Photodetectors: Current Responsivity Versus Response Time
As shown, the graphene high mobility along with the extension of the charges lifetime trapped in QDs caused a photodetector responsivity up to seven orders of magnitude higher in relation to the standard bulk photodiodes, where g \(\approx\) 1. Higher responsivity of Si avalanche photodiode (APD), up to 100 A/W, is caused by avalanche process. However, due to the  
Figure 14: Photogating effect in 2D material photodetectors: (**a**) the operation of hybrid phototransistor, (**b**) closed channel under illumination, (**c**) photoconductive gain, and (**d**) I-Vg trace under illumination. long lifetime of trapped carriers, the response time of 2D material photodetectors is very slow (\(<\)10 Hz), what considerably limits real detector functions. It is interesting to underline the unusual electrical and optical properties of gold patched graphene nanostripe detectors presented by [PERSON] et al. [60]. The photodetector exhibits a spectral response in the ultrabroad range from visible to the IR with high responsivity ranging from 0.6 A/W (for wavelength 800 nm) to 11.65 A/W (for 20 \(\upmu\)m) and frequency exceeding 50 GHz. As is shown in Figure 15, its current responsivity (black circles) coincides well with curve (black line) theoretically predicted for ideal photodiode in NIR spectral range. 2D materials show potential for operation in wide spectral range from UV to THz, although majority of them cover visible and SWIR (see Figure 16). Similarly, for graphene photodetectors, both high responsivity and short response time cannot be reached simultaneously in many 2D material-based photodetectors. Figure 15: The graphene-based photodetectors spectral responsivity compared with commercial detectors. Black line presents spectral responsivity for ideal photodiode with 100% QE and g = 1. [PERSON] and green colors correspond to \(\leq\)1 ns, while the blue color \(\geq\)1 second response times. The graphene detectors are labelled with proper reference and brief description. The commercial photodiodes are marked in green (adapted after reference [58; 59]). The two major factors determine the development of the 2D material high sensitivity photodetectors. It is a short carrier lifetime and low absorption in a thin active region (\(\sim\)100-200 nm). In consequence, the broadband operation sets a trade-off between high responsivity and response time. The 2D materials-based detectors display a large variation in their responsivity and response time [62; 63; 64] about 9 orders of magnitudes as is shown in Figure 17. In order to improve the IR absorption, the multiple layers instead of the single layer are chosen. In photogating effect photodetectors, 2D materials are used as the fast transfer channel for carriers. However, as is mentioned above, their overall disadvantage is the very slow response time attributed to traps and high capacitance. The response time is typically longer than \(\sim\)\(1\times 10^{-2}\) ms, what indicates on considerably longer response time in comparison with commercial silicon, InGaAs, and HgCdTe photodiodes, while for HOT LWIR photodiodes is typically tens of nanoseconds. The up-left blank panel on Figure 17 shows the lack of photodetectors with both high responsivity and short response time. Figure 17 summarizes the responsivity and response time of different 2D material photodetectors. It is shown that black phosphorus is a good candidate for fast detection and falls into a region between graphene and TMDs. Figure 16: The layered 2D material photodetectors spectral responsivities at 300 K (after reference [43; 61]). Black line shows spectral responsivity for ideal photodiode with 100% QE and g = 1. The responsivities of commercially available photodetectors (InGaAs and HgCdTe photodiodes) are presented for comparison reasons. Figure 17: Current responsivity versus response time for HOT 2D material photodetectors in relation to the commercial silicon, InGaAs and HgCdTe detectors (experimental data taken from reference [63]).