ZnO Thin Film Transistors for RF Applications
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1201-H09-07
ZnO Thin Film Transistors for RF Applications Burhan Bayraktaroglu, Kevin Leedy and Robert Neidhard Air Force Research Laboratory, Sensors Directorate, AFRL/RYD WPAFB, OH 45433, U.S.A. ABSTRACT Nanocrystalline ZnO thin films grown by the pulsed laser deposition technique were used to fabricate high performance thin film transistors suitable for RF applications. It was shown that drain current on/off ratios of higher than 1x1012, sub-threshold voltage swing values lower than 100 mV/decade and hysteresis-free operation could be maintained with films grown across a wide temperature range (25°C to 400°C). Films grown at 200°C have the lowest surface roughness and result in devices with the highest current density operation. Devices with 1.2 Pm gate lengths and Au-based gate metals had record current gain and power gain cut off frequencies of fT = 2.9 GHz and fmax = 10 GHz, respectively.
INTRODUCTION Although thin film transistors (TFT) have been in use as long as the conventional transistors, they are still regarded as poorly performing devices that are only suitable for low speed applications. Very low electron mobilities (0.1-1 cm2/V.s) typically associated with conventional TFTs based on amorphous Si and organic semiconductors coupled with threshold instabilities due to poor grain boundary and interface charge control issues have prevented this technology from advancing to more demanding applications [1]. Recently, alternative TFT technologies based on heavy metal oxide compounds such as ZnO [2-3], GIZO [4], InGaO3 [5] and zinc tin oxide [6] have been investigated to overcome these limitations. As a binary compound, ZnO represents the simplest form of the metal oxide semiconductor. It can be fabricated as a high performance thin film easily by various deposition techniques including RF sputtering, pulsed laser deposition (PLD), atomic layer deposition, and solution based approaches. Most of these approaches produce very similar films that result in excellent thin film transistor characteristics. For example, using PLD, devices with 2Pm gate lengths produced record figure-of-merit numbers such as on/off ratio of 1x1012, current density of >400 mA/mm and field effect mobility of 110 cm2/V.s [7]. ZnO is one of the most versatile semiconductors available today and offers solutions in more classes of applications than any other semiconductor technology. The advantages of this semiconductor stem from the combination of two features that are unique to ZnO. First, it is a wide bandgap semiconductor (Eg = 3.37 eV) which makes it transparent to IR and visible light [8]. Wide bandgap enables low leakage current and high temperature operations. Also, it can support high electrical fields for high voltage device applications. Second, it maintains most of its intrinsic single crystal optical and electronic properties in a nanocrystalline thin film (i.e. not single crystal) form. This high level of tolerance to defects makes it suitable for a wide range of thin film applications including conformal and flexible electr
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