Modeling the C-V Characteristics of Heterodimensional Schottky Contacts R. Ragi, J. Manzoli, M. A. Romero University of São Paulo - Brazil muriloa@sel.eesc.sc.usp.br B. Nabet Drexel University - USA nabet@cbis.ece.drexel.edu Abstract This paper addresses the capacitance-voltage (C-V) characteristics of heterodimensional Schottky diodes, in which the Schottky metal is placed in direct contact to a two-dimensional electron gas and the confined electron behavior directly dictates the device performance. We develop a novel quasi-2D model for the C-V characteristics of the device, by starting from a self-consistent solution of the Schrödinger and Poisson equations in the growth direction. The model is validated by contrasting the theoretical results with experimental data from an AlGaAs/GaAs device fabricated in our laboratory. 1. Introduction The properties of electrons in an inversion layer have attracted interest since the 1930’s, when Lilienfeld conceived the field-effect transistor. Further attention has been motivated by the enhanced transport properties of the two-dimensional electron gas (2-DEG) formed at modulation doped heterointerfaces, where the inversion layer is quantized in the growth direction. Already in the early 90’s High Electron-Mobility Transistors (HEMTs) based on this principle displayed power amplification well above 100 GHz with outstanding noise performance. This paper is concerned with devices based on a further extension of the modulation doping concept by using heterodimensional interfaces, i.e., interfaces between materials of dissimilar dimensions. In our specific case, this interface is a Schottky barrier laterally connecting a three-dimensional (3D) metal and a two- dimensional (2D) electron gas. In fact, heterodimensional diodes, transistors and photodetectors present several attractive features such as low capacitance due to the small effective cross-section, excellent noise and transport characteristics due to the 2D electron gas and a high breakdown voltage, making them very promising for high-frequency applications [1- 2]. In order to illustrate the motivation for studying heterodimensional devices we briefly revisit the question of computing the thermionic emission current in such devices. In fact, straighforward extension of Bethe’s theory, considering both the proper two-dimensional density of states as well as energy quantization in the growth direction for a 2-DEG with only one significantly populated subband, yields [3]: (1) 1 kT qV exp kT E exp kT q exp T A J 0 B 2 3 * 2D n       −               −         − = φ where A * 2D is the two-dimensional equivalent of the Richardson constant [3], Ε o is position of the first allowed energy level in the 2-DEG and the other terms have their usual meanings. Taking the ratio between the expression above and Bethe’s formula for thermionic emission, results: (2) kT E exp kT πm 2 h W I I r 0 * 0 2D 3D         = = where W is the effective length of the Schottky contact for a conventional device, typically around 1 µm and the other terms have their usual meanings. Fig 1 computes the ratio given by eq. 2 for a typical AlGaAs/GaAs structure (see section 4) as a function of both the doping density and thickness of the AlGaAs layer, values of Ε o were calculated using the model described in detail in section 3. The large values achieved represent a strong supression of thermionic emission current, essentially due to the exponential term in E o . This term acts as an effective Schottky barrier enhacement due to energy level quantization. In fact, despite the simplifications involved in Bethe’s formalism, there are experimental data avalaible [3-4] to support the above predictions by demonstrating one order of magnitude improvement in dark current. By itself, this feature certainly makes heterodimensional structures very attractive as low noise photodetectors as well as low leakage gate contacts. Despite this vast amount of device-related work, the number of investigations on the modeling of the capacitance-voltage characteristics heterodimensional structures is still limited. In the next sections, we develop a novel quasi-2D model for the C-V characteristics of the device, by starting from a self- consistent solution of the Schrödinger and Poisson 623 ESSDERC 2002