As the scaling down of Si devices in the range less than few nm has been expedited up to a physical limit of Si, low dimensional materials have been regarded as one of next generation semiconductors. Among a variety of applications, studies on photodetectors have been actively investigated with their novel optical properties as well as astonishing electrical properties. However, most of research has focused on single device-type photodetector (i.e., photo-diode or photo-transistor). Contrary to common photodetector, light-to-frequency circuits (LFCs) are based on frequency reading with photosensitive ring oscillators, which has better noise immunity and reduced system complexity, thus, can be utilized to novel application even in internet of things (IoT) and bio & medical fields. In this review, low dimensional materials based circuit level photodetectors, which are core elements as the form of either inverters or ring oscillators for demonstration of LFCs, are introduced. Along with the introduction of low dimensional materials and their optical properties for optoelectronics, a fundamental concept for LFCs is specifically described. Thereafter, research progress on low dimensional material based photosensitive inverters is addressed according to the types of devices. Furthermore, as one of practical method for the improvement of photodetector performance, molecular doping technology is presented. Lastly, complete system of LFCs and its digitization for demonstration of production level, and potential application in the respective four aspects, (i) medical SpO₂ detection, (ii) biological fluidic system, (iii) auto-lighting in agriculture, and (iv) optical feedback and sensing systems, are presented as systematic way to address the envisioned practical applications for the future displays including virtual reality and augmented reality, and others. As a remark, LFCs based on low dimensional semiconductors are expected to be one of core components in trillion’s sensor area.

Confronted by the inherent physical limitations in scaling down Si technology, transition metal dichalcogenides (TMDCs) as alternatives are being tremendously researched and paid attention to. However, mature counter doping technology for TMDCs is still elusive, and thus, a controllable and reversible charge enhancer is adopted for acceptor (or donor)-like doping via octadecyltrichlorosilane (ODTS) (or poly-L-lysine (PLL)) treatment. Furthermore, multiple counter doping for TMDC field-effect transistors (FETs), combined with a threshold voltage (V_th) freezing scheme, renders the V_th modulation controllable, with negligible degradation and decent sustainability of FETs even after each treatment of a representative charge enhancer. In parallel, the counter doping mechanism is systematically investigated via photoluminescence spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy (AFM), surface energy characterization, and measurement of optoelectronic properties under illumination with light of various wavelengths. More impressively, complementary inverters, composed of type-converted molybdenum ditelluride (MoTe₂) FETs and hetero-TMDC FETs in enhancement mode, are demonstrated via respective ODTS/PLL treatments. Herein, driving backplane application for micro-light-emitting diode (µ-LED) displays and physical validation of a corresponding counter doping scheme even for flexible polyethylene terephthalate (PET) substrates could be leveraged to relieve daunting challenges in the application of nanoscale Si-based three-dimensional (3D) stacked systems, with potential adoption of ultralow power and monolithic optical interconnection technology.