Giant injection of two-dimensional electron gas

Abstract

Summary form only given. Many modern semiconductor devices, such as field effect transistors, Charge Coupled Devices, RF switches, Quantum Photodetectors, lasers, and light emitting diodes, use two-dimensional electron (2DEG) and/or hole gas (2DHG) with the 2D concentration varying as a function of coordinate. For example, in a field effect transistor, the electron concentration under the gate and in the ungated regions can be quite different. Numerous computer models exist for analyzing such systems. However, the analytical model has been lacking, even though it can be extremely useful for the device design and parameter extraction. The results predicted by such model developed in this paper are somewhat surprising, even though they are physically clear and transparent. In a nutshell, we show that the 2DEG injected from the region with a higher concentration into the region with a lower concentration decays very slowly (compared to an exponential function) with the characteristic length of such decay being much larger than that for a three dimensional case (where it is simply a Debye length). The characteristic scale of the concentration decay from the 2DEG region with a high carrier concentration into the 2DEG low concentration region is the Bohr radius but the transition region could be s long as 10 Bohr radii. Another important result is that a passivation with high dielectric permittivity layer might dramatically increase the carrier concentration in the low doped region. Hence, such passivation can be used for large reductions in parasitic resistances of normally off FETs relying on space charge injection into the ungated regions. We refer to this effect as <;<;giant injection>;>; and predict that this effect will find application in designing new types of passivation to reduce parasitics in short channel 2DEG devices. A similar effect could be achieved using floating metal layer separated from the 2DEG by a thin dielectric.