Abstract
Graphene has been proposed as the current controlling element of vertical transport in heterojunction transistors, as it could potentially achieve high operation frequencies due to its metallic character and 2D nature. Simulations of graphene acting as a thermionic barrier between the transport of two semiconductor layers have shown cut-off frequencies larger than 1 THz. Furthermore, the use of n-doped amorphous silicon, (n)-a-Si:H, as the semiconductor for this approach could enable flexible electronics with high cutoff frequencies. In this work, we fabricated a vertical structure on a rigid substrate where graphene is embedded between two differently doped (n)-a-Si:H layers deposited by very high frequency (140 MHz) plasma-enhanced chemical vapor deposition. The operation of this heterojunction structure is investigated by the two diode-like interfaces by means of temperature dependent current-voltage characterization, followed by the electrical characterization in a three-terminal configuration. We demonstrate that the vertical current between the (n)-a-Si:H layers is successfully controlled by the ultra-thin graphene base voltage. While current saturation is yet to be achieved, a transconductance of ~230 was obtained, demonstrating a moderate modulation of the collector-emitter current by the ultra-thin graphene base voltage. These results show promising progress towards the application of graphene base heterojunction transistors.
Highlights
Electronic devices with vertical transport and architecture have gained attention as a new path into greater performances
We present the electrical characterization of a graphene layer embedded between two (n)-amorphous silicon (a-Si):H layers deposited by VHF-plasma-enhanced chemical vapor deposition (PECVD) and the ability of graphene to modulate the vertical current in the structure up to 40%
We presented the electrical characterization of the interfacial diodes in (n)-a-Si:H/graphene/(n+ )-a-Si:H heterostructures in the vertical graphene-base heterojunction transistor (GBHT) configuration
Summary
Electronic devices with vertical transport and architecture have gained attention as a new path into greater performances. Graphene has been proposed by Mehr et al [1] in 2012 to be used as an ultra-thin base electrode to modulate the vertical transport barrier of heterojunction transistors. Simulations demonstrated attainable operation frequencies in the THz range. The proposed structure composed of dielectrics embedding graphene is based on the field emission of hot electrons as the charge transport mechanism. This was experimentally demonstrated by two different groups in 2013 [2,3]. To achieve the targeted operation frequencies, the insulators were simulated to be 2–3 nm thin with a low Schottky barrier (0.4 eV) at the metal contact [1]. In the experimental reports, 5 nm oxides with barriers of about 3 eV were used, Materials 2018, 11, 345; doi:10.3390/ma11030345 www.mdpi.com/journal/materials
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