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

带间隧道场效应晶体管(TFET)最近引起了人们的极大兴趣，被认为是迈向超低功耗电子系统的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，TFET不依赖于通过势垒上的热发射对电荷载流子注入的调制，而是使用场效应控制的带到带隧道，以便在接通和关断状态之间切换器件。因此，TFET潜在地允许在显著较低的电源电压下运行，并且表现出显著较少的泄漏电流，从而大幅降低动态和待机功耗。然而，目前的技术还没有达到那个阶段，最先进的TFET表现出不如传统MOSFET的性能。其原因是频带间隧道概率仍然不够高。TFET的两个最有效的性能增强器是：i)采用异质结构，在发生带对带隧道传输的源极通道界面处具有较小的带隙，并在器件中的其他任何位置使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，从而增加了带到带的隧道几率。石墨烯代表最终的超薄沟道层，由于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，并因此适当地设计带隙以优化TFET性能。在目前的方案中，我们研究了几种不同的基于单层和双层石墨烯的TFET。有针对性的器件设计包括：i)T形纳米带，在带到带隧道界面处具有T的短端，以减小该界面的带隙，从而提高器件性能；ii)基于双层石墨烯的TFET被用于以适当的方式调节源沟道和漏极中的带隙，以及iii)包括双层和单层石墨烯的异质结构TFET。在实验工作的同时，还进行了基于量子力学计算的器件模拟。为了实现沿电流传输方向所需的n-i-p掺杂结构，我们已经开发了包括具有单独可寻址栅极的掩埋三栅极结构的衬底。石墨烯沉积后，通过直接剥离或转移工艺，石墨烯将被图案化或用额外的顶栅加固，以实现工作的石墨烯TFET。实验装置将用随温度变化的输运测量进行全面的表征，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.