Full-Swing, High-Gain Inverters Based on ZnSnO JFETs and MESFETs

et al. 2019

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circuitry is the TOAS zinc-tin-oxide (ZTO) since it can be deposited at room temperature (RT) enabling the low-cost fabrication of transparent, flexible circuits. [3,4] So far, several ZTO-based metal-insulator field-effect transistors (MISFETs) and related inverters have been reported in the literature, however, those devices required a deposition at elevated temperatures or post-growth annealing processes in order to perform well. [5][6][7] Reports on associated integrated circuits comprising ZTO MISFETs are restricted to thin film transistor (TFT) logic technology, based on depletion and enhancement transistors, as well as complementary metal oxide semiconductor logic technology, requiring compatible p-type and n-type FETs. Further, an additional fabrication step is necessary for the preparation of the gate insulator. Such MISFETs and related integrated circuits usually require high operating voltages due to the voltage drop across the insulator as well as a limited switching speed due to carrier scattering at the channel-insulator interface. [8] Previously reported ZTO-based ring oscillators, consisting of five or seven stages, exhibited oscillation frequencies between 0.85 and 800 kHz at operating voltages ranging from 5 to 60 V. [9][10][11] Single stage delay times, resulting in oscillation frequencies compatible with an ISM band, have so far not been reported for ZTObased ring oscillators.Recently, first metal-semiconductor field-effect transistors (MESFETs) and simple inverter circuits, comprising depletion-type MESFETs based on room temperature-deposited ZTO channel layers, have been reported. [12,13] Concerning device performance and fabrication efficiency, the absent gate insulator in case of MESFETs enables improved switching behavior as well as more fail-safe and faster processing. However, in case of inverters consisting of depletion-type FETs only, a shifting of the output signal is necessary to cover the voltage range required for switching a subsequent inverter. In the current work, we employ the Schottky diode FET logic (SDFL) approach that facilitates a sufficient level shift of the inverters output signal to enable a successful cascading of a series connection of inverters (see Figure 1). Since the SDFL layout consists of unipolar devices only, transistor channels can be deposited using a single photolithographic patterning process.Schottky diode FET logic (SDFL) ring oscillator circuits comprising metalsemiconductor field-effect transistors (MESFETs) based on amorphous zinc-tin-oxide (ZTO) n-channels are presented. The ZTO channel layers are deposited entirely at room temperature by long-throw magnetron sputtering. Best MESFETs exhibit on/off current ratios as high as 8.6 orders of magnitude, a sub-threshold swing as low as 250 mV dec −1 , and a maximum transconductance of 205 µS. Corresponding inverters show peakge gain magnitude (pgm) values of 83 with uncertainty levels as low as 0.5 V at an operating voltage of 5 V. Single stage delay times down to 277 ns are measured for three-stage ring...

Section: Resultsmentioning

confidence: 70%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Low‐Voltage Operation of Ring Oscillators Based on Room‐Temperature‐Deposited Amorphous Zinc‐Tin‐Oxide Channel MESFETs

Lahr

Vogt

et al. 2019

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“…Since previous investigations on ZTO-based FETs indicated the formation of a highly conductive ZTO layer close to the substrate, inhibiting sufficient depletion of the channel, the sputtering process was ignited in an oxygen-rich atmosphere of 25/30 sccm O 2 /Ar. [18,19] Subsequently, a conductive ZTO channel layer has been deposited on top of the intrinsic buffer layer. Source and drain contacts consist of ZnO : 4 wt% Ga 2 O 3 (GZO) with a nominal thickness of ≈70 nm grown by pulsed laser deposition at room temperature using a single ceramic target.…”

Section: Methodsmentioning

confidence: 99%

“…Increasing the thickness of the NiO layer by approximately a factor of three to 80 nm yielded an improved and rather constant leakage current for V G < 0 V, as has been reported for ZTO-based JFETs. [19] The field-effect mobility of each transistor has been estimated by calculating the transconductance g m from the transfer characteristics depicted in Figure 3a. Considering the channel resistance R S , the forward transconductance can be expressed by g m −1 = g max −1 + R S , where g max is the theoretical maximum of the transconductance, given by the channel conductivity.…”

Section: (4 Of 6)mentioning

confidence: 99%

All‐Oxide Transparent Thin‐Film Transistors Based on Amorphous Zinc Tin Oxide Fabricated at Room Temperature: Approaching the Thermodynamic Limit of the Subthreshold Swing

Lahr

Bär

et al. 2020

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TSOs are ZnO, SnO 2 , Ga 2 O 3 , and In 2 O 3 as well as several related compound materials and alloy systems, due to their broad application potential in the field of optoelectronic devices. [1-3] Especially transparent amorphous oxide semiconductors (TAOSs) have emerged into a thriving distinct area of research over the last few years, and since then, the field has grown rapidly toward, for instance, novel thin-film transistor (TFT) backplanes for next-generation flat-panel displays. [4,5] Alongside their high transparency, TAOSs exhibit a superior, at least tenfold higher free-carrier mobility compared to conventional amorphous silicon thin films and allow homogeneous large-scale deposition at sufficiently low temperatures, enabling the fabrication of transparent devices on flexible substrates. The by far most mature and already widely commercially exploited representative indium gallium zinc oxide (IGZO); however, contains scarce elements such as indium which innovative research is attempting to substitute by material systems consisting of abundant cations only. [6] One suitable candidate that meets these requirements and gained popularity particularly in the recent couple of years is amorphous zinc tin oxide (ZTO). ZTO is composed of earth-abundant, nontoxic elements only and exhibits, in addition to its optical transparency, a reasonably high free-carrier mobility with reported values up to 12.7 cm 2 V −1 s −1 in case of ZTO thin films fabricated at room temperature. [7] The first TFTs implementing amorphous ZTO as channel material have been reported by Chiang et al. back in 2005, followed by numerous studies on ZTO-based TFTs or rather metal-insulator-semiconductor field-effect transistors (MIS-FETs), fabricated using various deposition methods. [8-13] According to previous reports, however, the realization of MISFETs based on amorphous ZTO, including transparent devices, has up to date exclusively been limited to deposition at elevated temperatures or postdeposition annealing treatment in order to achieve sufficient functionality. [8,14-16] As alternatives to MISFET structures, metal-semiconductor fieldeffect transistors (MESFETs), implementing an n-ZTO/AgO x Schottky barrier diode as gate contact, have been reported by

Metal–Semiconductor Field‐Effect Transistors Based on the Amorphous Multi‐Anion Compound ZnON

Reinhardt

Grundmann

2020

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bias stress instability and performance changes of multi-cation AOS TFTs under visible light illumination, into the valence band resulting in an inherent stability of zinc oxynitride (ZnON) TFTs under bias stress and/or visible light illumination. [2][3][4] Besides attracting attention as active channel material in TFTs, ZnON was demonstrated to offer great potential for photosensitive devices with negligible persistent photoconduction due to its low band gap of about 1.0-1.3 eV and the position of the transition level of the oxygen vacancy within the valence band, respectively. [4][5][6] The issues of reproducibility and long-term stability that are associated with the nitrogen incorporation have been countered by argon plasma treatment, [7] annealing, [2,8,9] and/or encapsulation. [10,11] So far, amorphous ZnON (a-ZnON) was used as channel material within metal-insulator-semiconductor fieldeffect transistors (MISFETs), [2][3][4]8,9,[12][13][14] in this study, we investigate the electrical characteristics of metal-semiconductor field-effect transistors (MESFETs) based on amorphous n-type ZnON channel. MESFETs are especially suited for low-voltage and high-frequency applications due to the missing gate dielectric. Further, this reduces functionalization of the gate contact to the deposition of a metal layer for which a reactive magnetron sputtering process at room temperature is used in the current study. Recently, Schottky diodes and MESFETs based on amorphous zinc-tin oxide (a-ZTO) were demonstrated to be capable of realizing low-voltage operating logic circuits. [15] Combining this device concept with high-mobility a-ZnON seems to be a very promising approach for future sustainable integrated circuit based on AOS. Besides, Schottky diodes can be used for characterization of basic material properties by, e.g., thermal admittance spectroscopy or deep level transient spectroscopy. Results and DiscussionHall effect measurements at room temperature reveal a free electron concentration of n = 1.5 × 10 17 cm −3 and a Hall mobility of μ Hall ≈ 100 cm 2 V −1 s −1 for the as-received a-ZnON thin film. Figure 1a shows the Schottky diode jV characteristic at room temperature with a rectification ratio of 3.8 × 10 3 at ±2 V. Modeling of the diode forward current according to the Shockley equation j j e V IR k T V IR AR exp 1 s s B s p η ( ) = −       −       + − (1) Electrical properties of metal-semiconductor field-effect transistors (MES-FETs) based on the amorphous n-type multi-anion compound zinc oxynitride (ZnON) comprising reactively sputtered platinum as Schottky gate are presented. The Schottky barrier diodes reveal a rectification ratio of 4 × 10 3 at ±2 V and an ideality factor of 1.43. The investigated MESFETs show good switching characteristics with a switching voltage below 2 V, low subthreshold swing of 112 mV dec −1 and reasonable current on/off ratios up to 5 × 10 5 . Additionally, the stability of the devices under visible light illumination is proven.