Black phosphorus flexible thin film transistors and GHz circuit applications (2017)

Two-dimensional (2D) semiconductors have attracted significant interest in the last decade for applications in high speed and low power consumption nano electronics. Their intrinsic thickness scalability down to atomic layer and superior mechanical stiffness enabled novel applications in flexible nanosystems[1]. Black phosphorus (BP) captured immediate attention in electronics community owing to its superior electrical performance including high carrier mobility >1000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, highly tunable energy band gap ranging from 0.3 eV (bulk) to 2 eV (monolayer), and its unique anisotropy in electrical, thermal and mechanical properties[2, 3]. In this work, we demonstrate few layer BP based flexible nano electronics for robust RF applications with intrinsic cut-off frequency of 20 GHz[4]. Ambipolar inverter based on BP transistors was demonstrated with high inverter gain of 66. Flexible dual-finger configurated top-gated (TG) BP RF transistors (Fig. 1(a)) were fabricated on top of highly bendable polyimide substrate. The DC transfer and output characteristics for a typical transistor with channel length L=0.5 μm were shown in Fig. 1(b) and (c), with carrier mobility ~233 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs obtained. The intrinsic RF response was obtained after standard open-short de-embedding. As shown in Fig. 1(d) intrinsic cutoff frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> =20 GHz was obtained, which was the highest value obtained for the flexible BP transistors. Superior mechanical robustness of the reported flexible BP RF transistors was verified via bending test under tensile strain up to 1.5%, with less than 25% of degradation observed for extrinsic cutoff frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> (Fig. 1(e))[4]. Compare with graphene, MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and inorganic metal oxide thin films, few layer BP based RF transistors successfully extended the application scenario such as wireless transceiver and smart gadgets where the operation frequency lies in tens of GHz (Fig. 1(f))[4]. Based on the ambipolar transport characteristics of few layer BP transistors, ambipolar inverter was successfully realized as the schematic shown in Fig. 2(a). Two ambipolar BP bottom gated transistor were configurated into a complementary C-MOS structure, where the charge neutral point of the two series transistors split by the applied power supply voltage. The inverter functionality with sharp inversion of the output voltage signal with respect to input signal as shown in Fig. 2(b). The peak inverter gain was reaching ~66 under V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dd</sub> =5V (Fig. 2(c)). This value is much higher than the previously reported BP invertors[5] and makes this material promising for high speed logic applications.