Planar THz Schottky Diode Based on a Quasi Vertical Diode Structure (1993)

Whisker contacted GaAs Schottky barrier diodes are presently the standard devices for heterodyne mixing applications at THz frequencies. Due to reliability and handling problems and with regards to the recently proposed integrated antenna structures, much effort has been undertaken to develop planar whiskerless Schottky diodes. Recent approaches to planar diodes, based on FET—like structures, could not overcome the principal problems leading to a high parasitic shunt capacitance and a high series resistance. In this paper we present a novel approach for the planar diode concept. It is directly deduced from the vertical structure of the whisker contacted diode, making use of the many advantages and mature technology of the whisker contacted Schottky diode concept. On top of a mesa, the anode is connected with an airbridge to a contact pad. By means of via hole etching, the backside ohmic contact is connected with a contact pad at the surface. We have developed a technology for fabricating these diodes and describe the structure and the main technological aspects. Simulations have shown that in comparison to the recent approaches this concept shows some very promising features. In particular the minimum series resistance is comparable to the series resistance of the whisker contacted diode, due to the vertical current flow through the mesa, which is similar to the whisker Schottky diode structures operable at THz frequencies. This structure also exhibits a low parasitic shunt capacitance, due to a combined via hole and air bridge technology. Fourth International Symposium on Space Terahertz Technology Page 393 Introduction For a variety of applications including radio astronomy, atmospheric studies and plasma diagnostics, heterodyne reception is the most promising technique in the Terahertz frequency range. There exist two different approaches to the realization of the nonlinear mixer element which is the key component of the receiver front end. The SIS device, which demonstrated record sensitivity at millimeter and long submillimeter wavelengths [1] and the GaAs Schottky barrier diode. Due to the required cryogenic cooling and a frequency limitation to about 700 GHz of SIS elements, GaAs Schottky barrier diodes are at the moment the most widely used mixer elements at submillimeter wavelengths and also in applications where cryogenic cooling is either too expensive or too heavy. Whisker contacted Schottky barrier diodes have demonstrated excellent performance up to 2.5 THz [2]. They benefit from a minimum parasitic capacitance and a simple and highly mature fabrication. However they are costly to assemble show reliability problems due to the whisker contact and are impractical for systems using more than one diode. With the need for cheap and reliable mixer elements, the developement of planar Schottky diodes is an important topic of investigation. The essential drawback of these devices with respect to THz applications is the high parasitic capacitance and the increased series resistance compared to whisker contacted diodes. In this paper we present a new approach to planar GaAs Schottky diodes which are designed for minimum parasitic capacitance and series resistance. The first section illustrates the design of the novel planar diode and gives an analysis of the parasitic resistance and the capacitance of FET-like planar diodes and the quasi-vertical diode. The second section describes the technological steps for the fabrication of the diode. Concepts of planar diodes In contrast to whisker contacted diodes an insulating substrate is used for planar diodes. The anode is connected to a contact pad by means of an air bridge. The common approach to the realization of planar Schottky diodes utilize FET-like structures [3]. Fig. I illustrates the structure. The ohmic contact pad is placed on the n+-layer as closely as possible to the Schottky contact [3, 4]. The anode of the diode is contacted by an air bridge to the second contact pad.