Transparent Zinc Oxide Gate Metal–Oxide–Semiconductor Field-Effect Transistor for High-Responsivity Photodetector (original) (raw)
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We report that photoresponse of ZnO in UV can be enhanced substantially by simultaneously applying a gate bias in an Electric Double Layer Field Effect Transistor configuration fabricated on ZnO as a channel. The effect arises from synergy between UV illumination and applied gate bias which leads to a substantial enhancement in the device current. We propose that large carrier density created by the illumination and the gate leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to enhanced field effect mobility. This is verified by gate bias controlled Photo Luminescence experiment. Adding new functionalities or enhancing functionalities of known materials through synergy is a fruitful approach to research in materials science. In this paper we demonstrate that a synergy between an applied gate bias and illumination can lead to a substantial enhancement of photoresponse of ZnO when it is used as a channel in an electric double layer-field effect transistor (EDL-FET) configuration.The response is more than simple addition of two effects when they act individually. A photocon-ductor is a 2-terminal device without any need of a gate as in a FET [1]. In this case the illumination leads to creation of carriers. A gate on a semiconductor channel in a Field Effect Transistor (FET) also creates carriers by field effect. Both these mechanisms can independently lead to carrier creations in the semiconducting channel of the FET. We demonstrate that, these two independent mechanisms when applied together can lead to a substantial synergy that enhances the response many fold compared to that when the two effects are applied separately. The observed effect is more than simple addition of the two effects. The phenomena is shown in an n-ZnO FET where the gate is made from an electric double layer (EDL) formed by a solid state electrolyte. The synergy effect makes a generic tool that has application in substantially enhancing response in a photo-detector when it is gated. An EDL sandwiched between an electrolyte and a semiconductor surface forms nano-gap capacitor, which gives rise to large capacitance compared to that obtained in a conventional metal oxide dielectric gate, leading to a large surface carrier density (≥10 14 /cm 2). EDL gate has been used in the past to control electronic properties of wide band gap II-VI semiconductors like ZnO [2, 3]. However, use of EDL-FET in a ZnO photodetecor to enhance its photoresponse has not been done before. Particularly , the synergy of gate and illumination to enhance its response has not been demonstrated. The EDL-FET formed on ZnO, like all FETs, has a threshold gate voltage V th , beyond which the channel current between drain and source (I ds) increases. The device current under dark with no gate I 0 (dark), is enhanced to a much larger value I ph under UV illumination alone due to photoconductivity in the ZnO. We show that with an applied gate bias V g >V th under UV illumination, the device current achieved I ds (U V) can be much larger than I ph , showing that presence of gate and illumination together enhances the device optical response many times over the simple photoconductive response. We establish below that this synergy of light and gate is brought about by enhancement of the field effect mobility (µ F E) in the channel region under illumination and the bias. The effect has been seen in EDL-FETs fabricated on three types of samples: bulk single crystal, an epitaxial thin film of ZnO on Sapphire (0001) substrate with MgZnO buffer layer and also a thicker polycrystalline ZnO film on glass substrate. The qualitative nature of the effect reported in this paper is generic and does not depend on specific physical state of the ZnO used.
Light-Driven WSe 2 -ZnO Junction Field-Effect Transistors for High-Performance Photodetection
Advanced Science, 2019
Assembling nanomaterials into hybrid structures provides a promising and flexible route to reach ultrahigh responsivity by introducing a trap-assisted gain (G) mechanism. However, the high-gain photodetectors benefitting from long carrier lifetime often possess slow response time (t) due to the inherent G–t tradeoff. Here, a light-driven junction field-effect transistor (LJFET), consisting of an n-type ZnO belt as the channel material and a p-type WSe2 nanosheet as a photoactive gate material, to break the G–t tradeoff through decoupling the gain from carrier lifetime is reported. The photoactive gate material WSe2 under illumination enables a conductive path for externally applied voltage, which modulates the depletion region within the ZnO channel efficiently. The gain and response time are separately determined by the field effect modulation and the switching speed of LJFET. As a result, a high responsivity of 4.83 × 103 A W−1 with a gain of ≈104 and a rapid response time of ≈10 µs are obtained simultaneously. The LJFET architecture offers a new approach to realize high-gain and fast-response photodetectors without the G–t tradeoff.