Experimental and theoretical investigations of mono- and bilayer graphene nanoribbon band-to-band tunneling field-effect transistors

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Band-to-band tunnel field-effect transistors (TFETs) have recently attracted a great deal of interest and are considered as one of the most promising routes towards ultra-low power electronic systems. The reason for this is the switching mechanism of TFETs that in contrast to conventional MOSFETs does not rely on the modulation of charge carrier injection by therm emission over a potential barrier but rather employ field-effect controlled band-to-band tunneling in order to switch the device between on and off-state. As a result, TFETs potentially allow being operated at significantly lower supply voltages and exhibit substantially less leakage currents resulting in a strong reduction of dynamic and stand-by power consumption. However, current technology is not yet at that stage and state-of-the-art TFETs exhibit a performance inferior to conventional MOSFETs. The reason for this is that the band-to-band tunneling probability is still not high enough. Two of the most effective performance boosters for TFETs are i) employing a heterostructure with a small band gap at the source channel interface where band-to-band tunneling occurs and a larger band gap anywhere else in the device to suppress leakage currents. ii) An ultrathin channel layer increasing the capacitive coupling if the gate and hence the band-to-band tunneling probability. Graphene represents the ultimate ultrathin channel layer and because the size of the band gap depends on the width of the nanoribbon, lateral varying the width of the nanoribbon allows generating spatially-dependent band gaps and as such to engineer the band gap appropriately to optimize TFET performance. In the current proposal we investigate several different TFETs based on mono- as well as bilayer graphene. Targeted device designs include i) a T-shaped nanoribbon with the stub of the T at the band-to-band tunnel interface to decrease the band gap at this interface and thus increase the device performance, ii) TFETs based on bilayer graphene were vertical electric fields are used to adjust the band gaps in the source channel and drain in an approapriate way and iii) a heterostructure TFET comprising bi- and monolayer graphene. The experimental work is accompanied by device simulations based on quantum mechanical calculations. In order to realize the required n-i-p-doped structure along the direction of current transport we have developed substrates comprising buried tri-gate structures with individually addressable gates. After graphene deposition either by direct exfoliation or bya transfer process graphene will be patterned or fortified with additional top gates to realize working graphene TFETs. Experimental device will be thoroughly characterized with temperature dependent transport measurements and compared with simualtion results.