石墨烯射频器件研究进展

石墨烯因具有优良的电学特性,在半导体行业中受到广泛关注,特别因其具有超薄的结构和极高的载流子迁移率,为解决短沟道效应提供了可能,并且在高速电子领域具有应用前景.近年来,使用石墨烯作为沟道材料制备射频晶体管及射频电路是发挥石墨烯材料优势的一个重要研究方向.制造高性能的射频器件,首先要制备出高性能的石墨烯材料.在金属衬底上沉积均匀的单层石墨烯材料或者在绝缘衬底上外延生长单层、双层石墨烯材料都是获得高质量石墨烯材料的常用方法.器件结构及工艺流程的设计也是提升晶体管射频性能的重要因素,多指栅结构、T型栅结构、埋栅结构以及自对准工艺的发展能够有效改善石墨烯射频晶体管的截止频率及最大振荡频率.石墨烯晶体管独特的电学特性使得其除了可以构造与其他半导体材料电路相似的射频电路结构,还可以构造出功能完整并且结构更加简单的新型射频电路结构.

Research progress of graphene radio frequency devices

Graphene, the first realized two-dimensional material, has received much attention in electronic applications in recent years. With ultra-high carrier mobility and one atom thick structure, graphene becomes a promising semiconductor candidate for solving the problem of short channel effect in nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET), and exploring its applications in radio frequency devices. How to develop the advantages of graphene transistor in radio frequency is an attractive research area. The first step is to obtain high quality graphene material. In this article we summarize the graphene growth methods commonly used in electronic field, including chemical vapor deposition on metal substrates and epitaxial method on wide bandgap semiconductor and insulator substrates. Another key factor to improve graphene transistor performance is to carefully design the device structure and process flow. Multi-finger gate and T-shaped gate are widely used in MOSFET. These two structures can significantly reduce gate resistance, and result in a better radio frequency performance. “Inverted process” is introduced for graphene FET fabrication, which is compatible with silicon-based back-end-of-line technology. It can reduce the damages to graphene during fabrication. Another improved self-aligned gate deposition process can lead to a good gate coupling and less parasitic parameters. These newly developed process play a prominent part in increasing the cut-off frequency and maximum oscillation frequency of graphene radio frequency devices. In addition, single crystal graphene is helpful in eliminating carriers scattering and improving the radio frequency properties of graphene transistor. So far, the highest cut-off frequency of graphene transistor reaches over 300 GHz by a few groups, but the maximum oscillation frequency remains low. Record-high maximum oscillation frequency is 200 GHz when gate length is 60 nm. Further improvement of maximum oscillation frequency needs to be tried out. Several graphene radio frequency circuits are also discussed in the paper. Some of the circuits have similar structures to silicon-based circuits, and others are designed based on the unique property of graphene transistor, like ambipolar transport properties. The new concept circuits have simpler structures than conventional circuits. With the rapid development of graphene growth and related integrating technology, the potential to use graphene in radio frequency field will be further increased.