Contents I. Introduction
Millimeter wave (mm-wave) systems have sparked an increased interest in communications and security applications. These systems often employ a III-V material such as InP or Gallium Arsenide (GaAs) for the generation and processing of the sub-THz signal. The various properties of InP in the optical and electrical domain makes it an ideal semiconductor material for the manufacturing of cutting-edge active devices such as lasers, amplifiers, and photodiodes. On the other hand, the isotropic etching profile of InP presents design challenges for the implementation of waveguiding structures such as microstrip [1]–[3], grounded-CPW and substrate integrated waveguide (SIW). The main obstacle lies in the fact that via-holes cannot be etched efficiently using conventional wet-etching [4]–[7]. Furthermore, the substrate of microstrip or grounded-CPW needs to be 30 – 40 µm thick, thus thinning and handling of InP substrates becomes challenging. Therefore, InP based devices often incorporate transitions to effectively couple the mm-wave signal into a conventional rectangular waveguide. These transitions are created by adding a radiating element at the end of active device based on InP which radiates into a cavity formed by introducing a slot in the wide/short wall of the rectangular waveguide [8]–[10]. Such cavities require vertical placement of the InP chip within the waveguide. Hence, they are more susceptible to mechanical defects resulting from placement difficulties due to a smaller contact area with the slot [11]. Furthermore, the width of the substrate cannot exceed the height of a rectangular waveguide for vertical cavity structure, which creates a design limitation on the width of the InP device and radiating element. As an alternative way to overcome this problem, here we present an in-plane waveguide slot created at one of the open ends of the rectangular waveguide. The InP chip sits horizontally in the waveguide, thereby avoiding any mechanical instability. By optimizing the cavity dimensions, it is possible to tune the operating frequency of the transition. The proposed CPW-WG transition can operate over a bandwidth of 16 GHz (288 - 304 GHz), and is optimized to interface with a InP based resonant tunneling diode (RTD) while being suitable for fabrication using split-block machining.
