The contact-end-resistance method is applied to TLM structures to characterize in-depth the graphene-metal contact and its dependence on the back-gate bias. Parameters describing the graphene-metal stack resistance are extracted through the widely used transmission line model. The results show inconsistencies which highlight application limits of the model underlying the extraction method. These limits are attributed to the additional resistance associated to the p-p + junction located at the contact edge, that is not part of the conventional transmission line model. Useful guidelines for a correct application of the extraction technique are provided, identifying the bias range in which this additional resistance is negligible. Finally, the contact-end-resistance method and the transmission line model are exploited to characterize graphenemetal contacts featuring different metals.
The accurate extraction and the reliable, repeatable reduction of graphene–metal contact resistance (RC) are still open issues in graphene technology. Here, the importance of following clear protocols when extracting RC using the transfer length method (TLM) is demonstrated. The example of back‐gated graphene TLM structures with nickel contacts, a complementary metal oxide semiconductor compatible metal, is used here. The accurate extraction of RC is significantly affected by generally observable Dirac voltage shifts with increasing channel lengths in ambient conditions. RC is generally a function of the carrier density in graphene. Hence, the position of the Fermi level and the gate voltage impact the extraction of RC. Measurements in high vacuum, on the other hand, result in dependable extraction of RC as a function of gate voltage owing to minimal spread in Dirac voltages. The accurate measurement and extraction of important parameters like contact‐end resistance, transfer length, sheet resistance of graphene under the metal contact, and specific contact resistivity as a function of the back‐gate voltage is further assessed. The presented methodology has also been applied to devices with gold and copper contacts, with similar conclusions.
This paper reviews the experimental and theoretical state of the art in ballistic hot electron transistors that utilize two-dimensional base contacts made from graphene, i.e. graphene base transistors (GBTs). Early performance predictions that indicated potential for THz operation still hold true today, even with improved models that take non-idealities into account. Experimental results clearly demonstrate the basic functionality, with on/off current switching over several orders of magnitude, but further developments are required to exploit the full potential of the GBT device family. In particular, interfaces between graphene and semiconductors or dielectrics are far from perfect and thus limit experimental device integrity, reliability and performance
We examined the DC and RF performance of the graphene base transistor (GBT) in the ideal limit of unity common base current gain. To this purpose, we developed a model to calculate the current–voltage characteristics of GBTs with semiconductor or metal emitter taking into account space charge effects in the emitter–base and base–collector dielectrics that distort the potential profile and limit the upper value of fT. Model predictions are compared with available experiments. We show that, in spite of space charge high current effects, optimized GBT designs still hold the promise to achieve intrinsic cutoff frequency in the terahertz region, provided that an appropriate set of dielectric and emitter materials is chosen
We apply the contact–end resistance method to TLM structures in order to characterize the graphene–metal contact resistance. A critical analysis of the experimental results shows that the commonly used transmission line model fails to accurately describe the graphene–metal contact under specific biasing conditions. The experiments suggest the presence of an additional resistance contribution associated to the p–p+ junction induced in the graphene in the proximity of the contact. This voltage dependent resistance limits the range of applicability of the extraction technique. However, for carefully chosen bias conditions that reduce this additional resistance to small values, the technique provides reliable results, useful to investigate the graphene–metal contact properties and their technology dependence
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