Versatile wood cellulose, the most prototypical abundant polymer on earth, is considered a promising natural material for the fabrication of biodegradable electronics. The development of biodegradable electronics may help alleviate the adverse environmental impact caused by the fast‐growing electronic waste (e‐waste). The focus of this review is to discuss recent major advances in biodegradable electronics with versatile wood cellulose in terms of supporting substrates and functional components. First, the biological biodegradation and structural hierarchy of versatile wood cellulose is briefly introduced, followed by highlighting three types of cellulose substrates (opaque and hazy cellulose paper, transparent and clear cellulose film, and transparent and hazy cellulose film) for biodegradable electronics. Then, recent progress and research achievements in the use of versatile wood cellulose with multiscale dimensions in biodegradable electronics as a functional component (e.g., advanced light management layer, high capacitance dielectric, and ionic conductor) or even smart materials (e.g., mechanochromic layer, humidity sensing layer, adaptable adhesive layer, and piezoelectric component) are summarized in detail. Finally, an overview of challenges and perspectives for biodegradable electronics with versatile cellulose is provided.

2000759 (6 of 38) www.advmattechnol.de Figure 4. Flexible microwave transistors. a) Schematic illustration of fabrication process flow of flexible microwave TFTs based on Si nanomembrane. Left, two-step transfer-printing of Si nanomembrane on flexible substrate with assist of PDMS stamp. Right, direct flip-printing Si nanomembrane on flexible substrate. Reproduced with permission. [72] Copyright 2010, Wiley-VCH. b) Flexible microwave Si TFT using direct flip-printing approach in (a). Reproduced under the terms of the Creative Commons Attribution 4.0 International License. [75] Copyright 2016, Springer Nature. c) Flexible microwave Si TFT using two-step approach in (a). Reproduced with permission. [72] Copyright 2010, Wiley-VCH. d) Cross-section SEM image of flexible Si MOSFET made by thinning a 65 nm node SOI wafer. Reproduced with permission. [45] Copyright 2013, American Institute of Physics. e) Optical images of flexible GaAs HBT on cellulose nanofibril (CNF) substrate. Reproduced under the terms of the Creative Commons Attribution 4.0 International License. [90] Copyright 2015, Springer Nature. f) Schematic illustration and normalized on-state conductance (G/G 0), maximum transconductance (g m /g m0) of flexible InAs MOSFET with self-aligned gate. Reproduced with permission. [53] Copyright 2012, American Chemical Society. g) Cross-section schematic illustration and gain curves of flexible InGaAs/InAlAs HEMT as function of frequency under various bending conditions. Reproduced with permission. [91] Copyright 2013, American Institute of Physics. h) Output power density (P OUT), power gain (G P) and power-added efficiency (PAE) of the flexible GaN HEMT on thermal conductive tape with thermal conductivity of 1.6 W m −1 K −1. Reproduced with permission. [92] Copyright 2017, Wiley-VCH. i) Photography of AlGaN/GaN film grown on BN coated sapphire substrate and printed on flexible tape. Reproduced with permission. [19] Copyright 2017, Wiley-VCH. j) Cross-section schematic illustration of bottom gate flexible graphene FET (GFET). Reproduced with permission. [93] Copyright 2013, American Chemical Society. k) Cross-section schematic illustration of top gate flexible GFET with self-aligned gate configuration. Reproduced with permission. [94] Copyright 2014, American Chemical Society. l) High-frequency characteristics of flexible GFET with gate length of 50 nm. Reproduced with permission. [95] Copyright 2018, Wiley-VCH. m) Schematic illustration and gain curves of flexible microwave transistor based on MoS 2 as function of frequency. Reproduced with permission. [96] Copyright 2014, Springer Nature. n) High-frequency characteristics of flexible microwave transistor based on black phosphorus (BP) as function of frequency. Reproduced with permission.

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Recent demonstrations of grafted p-n junctions combining n-type GaN with p-type semiconductors have shown great potential in achieving lattice-mismatch epitaxy-like heterostructures. Ultrathin dielectrics deposited by atomic layer deposition (ALD) serve both as a double-sided surface passivation layer and a quantum tunneling layer. On the other hand, with excellent thermal, mechanical, and electrical properties, ZrO2 serves as a high-k gate dielectric material in multiple applications, which is also of potential interest to applications in grafted GaN-based heterostructures. In this sense, understanding the interfacial band parameters of ultrathin ALD-ZrO2 is of great importance. In this work, the band-bending of Ga-polar GaN with ultrathin ALD-ZrO2 was studied by x-ray photoelectron spectroscopy (XPS). This study demonstrated that ZrO2 can effectively suppress upward band-bending from 0.88 to 0.48 eV at five deposition cycles. The bandgap values of ALD-ZrO2 at different thicknesses were also carefully studied.

Cellulose nanofibril (CNF) substrates that are inexpensive, biodegradable, and quickly incinerable, are potential substrates for disposable RF applications. In this paper, high-performance flexible microwave lumped elements and filters fabricated on flexible CNF substrates are reported. A spiral inductor with a resonance frequency of 29.8 GHz and quality (Q) factor of 8.5 and a metal-insulator-metal (MIM) capacitor with a resonance frequency of 45 GHz and Q factor of 85.2 were achieved in a 200 μm thick CNF substrate. Meanwhile, the inductor and capacitor exhibit outstanding mechanical bendability, that is negligible performance changes were observed when they were bent to a radius as small as 15 mm. Based on the spiral inductor and MIM capacitor, a 5-GHz band-stop filter and a 4 GHz band-pass filter with excellent mechanical bendability were further demonstrated on the CNF substrate. These results indicate the potential of using CNF as substrates for broader microwave applications.INDEX TERMS Cellulose nanofibril, flexible electronics, microwave applications, radio-frequency inductorcapacitor and filters.