Flexible Light‐to‐Frequency Conversion Circuits Built with Si‐Based Frequency‐to‐Digital Converters via Complementary Photosensitive Ring Oscillators with p‐Type SWNT and n‐Type a‐IGZO Thin Film Transistors

Abstract

In this study, as system‐level photodetectors, light‐to‐frequency conversion circuits (LFCs) are realized by i) photosensitive ring oscillators (ROs) composed of amorphous indium‐gallium‐zinc‐oxide/single‐walled carbon nanotube (a‐IGZO/SWNT) thin film transistors (TFTs) and ii) phase‐locked‐loop Si circuits built with frequency‐to‐digital converters (PFDC). The 3‐stage ROs and logic gates based on a‐IGZO/SWNT TFTs successfully demonstrate its performance on flexible substrates. Herein, along with the advantage of scalability, a‐IGZO films are used as photosensitive n‐type TFTs and SWNTs are employed as photo‐insensitive p‐type TFTs for better photosensitivity in circuit level. Through the controlling a post‐annealing condition of a‐IGZO film, responsivities and detectivities of a‐IGZO TFTs are obtained as 36 AW−1 and 0.3 × 1012 Jones for red, 93 AW−1 and 3.1 × 1012 Jones for green, and 194 AW–1 and 11.7 × 1012 Jones for blue. Furthermore, as an advanced demonstration for practical application of LFCs, a unique circuit (i.e., PFDC) is designed to analyze the generated oscillation frequency (fosc) from the LFC device and convert it to a digital code. As a result, the designed PFDC can exactly count the generated fosc from the flexible a‐IGZO/SWNT ROs under light illumination with an outstanding sensitivity and assign input frequencies to respective digital code.

Progress in light-to-frequency conversion circuits based on low dimensional semiconductors

“…In this section, as an advanced demonstration for practical application of LFCs, a digitization process of generated oscillation frequency from light information is introduced, which is practically essential point to be applied in the product level. For securing the afore-mentioned purpose, in 2021, Jeong et al proposed and designed complementarymetal-oxide-semiconductor (CMOS)-based phase-locked-loop circuits built with frequency-to-digital converters (PFDC) to analyze the frequency of clock signal from LFCs and convert the frequency information to multi-bit binary signals [170]. Especially, they implemented LFCs using channel materials of n-type a-IGZO and p-type SWNT TFTs.…”

“…The VO was known as typically available defects in a-IGZO films, which may render a-IGZO TFTs with large energy bandgap photo-responsive under visible light illumination [200,201]. Thus, they controlled VO defect levels in an engineering mode by adjusting annealing conditions in air ambient at 300 °C for 1 h [170]. As shown in Fig.…”

“…In Ref. [170], it was reported that a-IGZO TFTs with larger VO (i.e., larger g VBT ) under low annealing temperature at 300 °C would generate more photocurrent than those of a-IGZO with smaller VO associated with high annealing temperature (450 °C). Consequently, responsivities and detectivities of a-IGZO TFTs were achieved as 36 A/W and 0.3 × 10 12 Jones for red (660 nm), 93 A/W and 3.1 × 10 12 Jones for green (530 nm), and 194 A/W and 11.7 × 10 12 Jones for blue (450 nm).…”

“…The role of CFD and FFD is counting edges of the input clock signal from photosensitive a-IGZO/SWNT ROs within a certain period of PLL output clock, and PLL provides the required clock signals according to the output of CFD by using the divider in PLL. The advantages on the designed architecture are conceived in the following aspects: (i) reduction of the power consumption caused by unnecessarily high frequency of sampling clock signal and (ii) the linearity of photosensitive a-IGZO/SWNT ROs, which are promoted to remove additional sequence controller, required to perform the calibration of the PFDC system [170]. They explained the detailed operation principle of PFDC as follows.…”

“…6(c) and 6(f))[81,[93][94][95]170]. The photo-inverter is composed of (i) the enhancement-mode typed drivers with depletion-mode loads (DLEDs), (ii) complementary type transistors, and their light-detection for photo-inverters is achieved through variation of transition point or gain of inverters, which result from photo-response of transistors.…”