A novel transistor with a graphene base embedded between two n-type silicon emitter and collector layers (graphene-base heterojunction transistor) is fabricated and characterized electrically. The base voltage controlled current of the device flows vertically from the emitter via graphene to the collector. Due to the extremely short transit time for electrons passing the ultimately thin graphene base, the device has a large potential for high-frequency RF applications. The transistor exhibits saturated output currents and a clear modulation of the collector current by means of the graphene base voltage. The vertical transfer current from the emitter via the graphene base to the collector is much lower than expected from device simulations. A comparison of the graphene-base transistor and a reference silicon n-p-n bipolar transistor is performed with respect to the main DC transistor characteristics. A common-emitter gain of larger than one has been achieved for the reference device while the graphene-base transistor so far exhibits a much lower gain.
A very high frequency plasma enhanced chemical vapor deposition technology (VHF PECVD) is widely used for deposition of amorphous and microcrystalline silicon layers 1 . The satisfying deposition rates and process efficiency combined with adequate material quality make this technology a good solution in the industry applications, especially in the large scale deposition systems. The modelling of the plasma inside a reactor helps developing the design of the deposition systems, as well as to enhance the deposition parameters. The advantage of the excitation frequency increase on the PECVD technology has been introduced 2 . The already reported plasma model analyses are applied only in the frequency range up to 100 MHz. However, the frequency range from 140 MHz to 200MHz is a new field of interest and requires enhanced simulation procedures. Therefore, two 1D plasma models of a CCP reactor for two different frequency ranges (13.56 MHz and 140 MHz) are introduced in this work. Due to the complexity of the silane plasma chemistry, pure argon was used to reveal a clear picture of the modelling issues. The discharge conditions inside the large scale reactors (length from 30 cm up to 3 m, the pressure up to 1 Torr) require to use a fluid model instead of the particle models. This work describes the difference in the fluid model definition caused by the frequency increase. In particular the modifications of the electron energy distribution function (EEDF) as well as the species transport coefficients are taken into consideration. The variations of the plasma parameters are also presented. 1. B. Leszczynska, et.al, "Adaptation of PECVD deposition system for very high excitation frequencies over 81.36 MHz", 28th European Photovoltaic Solar Energy Conference and Exhibition, 2013, pp. 2568 -2572. 2. B. Leszczynska, et.al, "High-rate deposition of silicon thin film layers using linear plasma sources operated at very high excitation frequencies (80-140 MHz)", Thin Film Solar Technology IV.
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