Study of Plasma Effects in HEMT-like Structures for THz Applications by Equivalent Circuit Approach

The growing interest to terahertz (THz) region of electromagnetic spectrum is pulled by a variety of its possible applications for free-space communications, sensing and imaging in radio astronomy, biomedicine, and in security screening for hidden explosives and concealed weapons. Terahertz imaging may also be useful for industrial processes, such as package inspection and quality control. Despite strong demand in compact solid-state devices capable to operate as emitters, receivers, photomixers of the THz radiation their development is still a challenging problem. Plasma waves with linear dispersion law can exist in the gated two-dimensional electron gas (2DEG) (Chaplik 1972) in systems similar to field-effect transistor (FET) and high-electron mobility transistor (HEMT). At high enough electron mobility and gate length in submicrometer range 2DEG channel can serve as a resonant cavity for plasma waves with the resonant frequencies in the THz range. Experimentally observed infrared absorption (Allen,1977) and weak infrared emission (Tsui,1980) were related to plasma waves in silicon inversion layers. Excitation of plasma oscillations in the channel of FET-like structures has been proposed as a promising approach for the realization of emission, detection, mixing and frequency multiplication of THz radiation (Dyakonov,1993, Dyakonov,1996). Nonresonant (Weikle,1996) and weak resonant (Lu,1998) detection have been observed in HEMTs experimentally. Resonant peaks of the impedance of the capacitively contacted 2DEG at frequencies corresponding to plasma resonances have been revealed (Burke,2000). Terahertz detection and emission in HEMTs fabricated from different materials have been demonstrated (Knap,2002; Otsuji,2004; Teppe,2005; El Fatimy,2006; Shur,2003). Theoretical models for plasma waves excited in the HEMT 2DEG channel are usually based on the similarity of the equations describing the behavior of electron fluid and shallow water (Dyakonov,1993; Shur, 2003; Satou,2003; Satou,2004; Veksler,2006). On the other hand, electromagnetic wave propagation in the gated 2DEG channel is similar to that in a transmission line (TL) (Yeager,1986; Burke,2000) which makes it possible to represent the gated portion of 2DEG channel by a TL model. In this chapter, we will implement distributed circuit or TL model approach to the study of plasma waves excited in the 2DEG channel of the structures similar to HEMT. Once a system is represented by an electric equivalent circuit, its performance can be simulated using a circuit simulator like SPICE (Simulation Program with Integrated Circuit Emphasis). Such an approach is less time consuming comparing to full scale computer modeling. It enables an easy variation of the system parameters during the simulation procedure provides a quick way to facilitate and improve one’s understanding of the HEMT operation 5