Alternatives for Doping in Nanoscale Field-Effect Transistors

In the present article, alternatives to impurity doping in nanoscale field-effect transistors (FETs) are investigated. The discussion is based on conventional and tunnel FETs. The impact of dopant deactivation due to dielectric mismatch or quantization, random dopant effects, and the degeneracy level on the performance is discussed. As alternatives metal-semiconductor-contacts, gate-controlled doping and an interface engineering approach are studied. One of the main requirements for proper device functionality is the existence of a band gap in the contacts. Thus, metal-semiconductor contacts are less suited since they lead to ambipolar operation with increased leakage and to a deteriorated on-state performance. With gate-controlled doping, electrodes areused to create doped regions leaving behind a pristine band gap. Moreover, it enables reconfigurable devices with nFET, pFET and tunnel FET operation. Furthermore, with multiple nanoscale gates, electrostatic doping allows manipulating the potential within the device on the nanoscale. Experimental demonstrations of such devices with triple-gates and multiple gate structures are presented. Finally, the interface engineering approach allows combining a metallic contact electrode with an almost unmodified band gap in the source/drain contacts by adjusting an ultrathin insulator in-between metal and semiconductor yielding quasi-doped contacts whose polarity depends on the work function of contact metal.