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

带对带隧道场效应晶体管（tfet）近年来引起了人们的极大兴趣，被认为是迈向超低功耗电子系统最有前途的途径之一。其原因是tfet的开关机制，与传统的mosfet相比，它不依赖于通过热辐射在势垒上调制载流子注入，而是采用场效应控制的带对带隧道，以便在开状态和关状态之间切换器件。因此，tfet可能允许在显著较低的电源电压下工作，并显示出大大减少的泄漏电流，从而大大降低动态和待机功耗。然而，目前的技术尚未达到这一阶段，最先进的tfet表现出不如传统mosfet的性能。造成这种情况的原因是，波段到波段的隧穿概率仍然不够高。tfet的两个最有效的性能增强器是i)在源通道接口处采用具有小带隙的异质结构，其中发生带对带隧道，并且在器件的其他地方采用较大的带隙来抑制泄漏电流。ii)超薄沟道层增加了栅极的电容耦合，从而增加了带对带隧穿的概率。石墨烯代表了最终的超薄通道层，因为带隙的大小取决于纳米带的宽度，横向改变纳米带的宽度允许产生空间相关的带隙，因此可以适当地设计带隙以优化TFET的性能。在目前的提案中，我们研究了几种不同的基于单层和双层石墨烯的tfet。目标器件设计包括i)在带到带隧道界面处使用T形纳米带，以减小该界面处的带隙，从而提高器件性能；ii)基于双层石墨烯的TFET，使用垂直电场以适当的方式调节源沟道和漏极的带隙；iii)由双层和单层石墨烯组成的异质结构TFET。实验工作伴随着基于量子力学计算的设备模拟。为了实现沿电流输运方向所需的n-i-p掺杂结构，我们开发了包含具有单独寻址门的埋式三栅极结构的衬底。通过直接剥离或转移工艺沉积石墨烯后，石墨烯将被图案化或添加额外的顶栅以实现工作的石墨烯tfet。用温度相关输运测量对实验装置进行全面表征，并与模拟结果进行比较。

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.