Reconfigurable Field-Effect-Transistors

在过去的几十年中，通过在相同的芯片面积上增加尺寸减小的晶体管的数量，已经实现了高度集成电路（IC）的性能提高和因此增加的功能。然而，晶体管的可扩展性很快就会结束。因此，进一步增加IC功能的一种有吸引力的替代方法是向器件本身添加功能。这可以主要通过所谓的可重构接触来实现，所述可重构接触允许动态地改变器件以作为a型、p型以及带到带隧道场效应晶体管（TFET）工作。具有可重构接触的场效应晶体管（FET）由至少两个栅电极组成，即，一个是实际的栅极，另一个栅电极（极性栅极）允许重构晶体管。基于这种多栅极器件架构，已经展示了许多基于纳米管、纳米线和二维材料的可重构器件。特别地，在作为常规晶体管和TFET的操作之间的切换是非常有吸引力的，因为这将使得电路能够操作为最高性能（常规FET操作）或最低功耗（TFET操作）。所有当前可重构FET的主要缺点是极性栅极不改变接触金属的功函数的事实，因此，为了获得电子和空穴的类似注入，在中间间隙处的费米能级排列是必要的。因此，载流子总是通过大量的肖特基势垒注入，这会严重恶化器件特性。在本项目中，实现了可重构的接触与电子和空穴的欧姆接触类似的注入概率。此外，以实现单极器件操作的方式来制造接触，这对于适当的TFET功能是重要的。为此，将采用由硅沟道层、SiNx和石墨烯组成的栅极夹层。石墨烯将用作金属源电极。虽然在文献中已经研究了石墨烯-硅二极管/接触结构，但这里重要的添加是石墨烯和硅之间的SiNx层。该层是决定性的，以便在硅中获得非常低的间隙状态密度，这对于单极器件操作是必要的，并且更重要的是，对于具有低关断状态泄漏的TFET操作是必要的。在狄拉克点周围的石墨烯中的低态密度进一步降低了硅间隙态的密度，并且此外，使得能够通过石墨烯/SiNx叠层对硅进行充分的选通，使得导带/价带可以利用适当的栅极电压移位。因此，当前的方法提供了可以作为n型、p型和隧道场效应晶体管操作的真正可重新配置的FET。

During the last decades, a performance increase and thus increased functionality of highly integrated circuits (ICs) has been achieved by increasing the number of transistors of decreasing size onto the same chip area. The scalability of transistors, however, will soon come to an end. Therefore, an attractive alternative approach to further increase the functionality of ICs is to add functionality to the devices themselves. This can primarily be achieved with so-called reconfigurable contacts that allow changing the devices to work as a-type, p-type as well as band-to-band tunnel field-effect transistor (TFET) dynamically.Field effect transistors (FETs) with reconfigurable contacts consist of at least two gate electrodes, namely one that is the actual gate and the other gate electrode (the polarity gate) that allows to reconfigure the transistor. Based on such multiple-gate device architectures, a number of reconfigurable devices based on nanotubes, nanowires and two-dimensional materials have been demonstrated. In particular, switching between the operation as a conventional transistor and a TFET is very attractive since this would enable circuits that can operate either optimized to highest performance (conventional FET operation) or to lowest power consumption (TFET operation). The major drawback of all current reconfigurable FETs is the fact that the polarity gate does not change the work function of the contact metal and as a result, a Fermi level line-up at mid-gap is necessary in order to obtain similar injection of electrons and holes. Hence, carriers are always injected through a substantial Schottky-barrier which yields strongly deteriorated device characteristics. Within the present project, reconfigurable contacts with an injection probability similar to ohmic contacts for electrons and holes are realized. Furthermore, the contacts are made in a way that enables unipolar device operation which is important for proper TFET functionality. To this end, a gated sandwich consisting of a silicon channel layer, ultrathin SiNx and graphene will be employed. The graphene will be used as a metallic source electrode. While graphene-silicon diode/contact structures have been studied in literature the important addition here is the ultrathin SiNx layer in-between the graphene and the silicon. This layer is decisive in order to obtain a very low density of gap-states in the silicon which is necessary for unipolar device operation and, more importantly, for TFET operation with low off-state leakage. The low density of states in graphene around the Dirac point further decreases the density of silicon gap-states and, in addition, enables sufficient gating of the silicon through the graphene/ultrathin SiNx stack so that the conduction/valence bands can be shifted with an appropriate gate voltage. As a result, the current approach provides truly reconfigurable FETs that can operate as n-type, p-type and tunnel field-effect transistors.