ABSTRACT
This section will describe some of the practical design tradeoffs for state-of-the-art multipliers operating near THz frequencies. The figures and data in this section and in Section 14.6 were obtained from Mehdi [15]. The nonlinear modeling described in the earlier section is just the staring point in realizing a useful source. Parasitic resistances, losses, and shunt capacitance associated with semiconductor substrates, heat sinking, and power handling along with fabrication and assembly are all critical near THz frequencies. Research and development effort over the past decade has overcome many of these problems, resulting in practical multiplier chains producing useful power above 1 THz. Before discussing recent advances, it is useful to describe earlier multiplier configurations. Figure 14.8 shows a whisker-contacted diode structure. The figure shows a pointed metal whisker contacting a small Schottky barrier diode that is part of a “honeycomb” array. This configuration has several advantages. The semiconductor structure is relatively easy to fabricate and very small anode contacts are possible. The metal whisker contact has a well-understood impedance with low-parasitic capacitance. Perhaps the most important advantage was the years of experience invested in the design. However, there are disadvantages too. The semiconductor chip thickness does not easily scale down with decreasing waveguide dimensions at higher frequencies. The parasitic resistance depends on the spreading resistance from the circular anode. Assembly of the whisker can be an art form rather than a science, and although excellent performance is possible, reproducible results are sometimes difficult to obtain. Finally, the use of multiple junctions for additional power or for balanced configurations is not possible. Over the last decade, a planar diode technology has been developed to overcome these problems and to extend multipliers to above THz frequencies.
