The distribution of losses is such that the junction capacitance and therefore the area of the junction should be reduced. The reduction of the capacitance across the diode did not improve the low power performance of the diode as much as anticipated. This was because of a complication caused by the emergence of the back contact resistance as a significant portion of the series resistance when the active area associated with a low capacitance junction is small. The resistance in the depletion layer is very low in a diode designed for low reverse breakdown voltage, even in a small junction area associated with a small capacitance. The back contact resistance increases greatly, however, because the distance between the junction and the back contact is so small in a plated heat sink diode that the current does not have a change to spread out. This suggests making the base material out of a heavily doped material to keep the spreading resistance low and of sufficient thickness to allow spreading to a large area to cut down on the back contact resistance. Unfortunately diodes made in this fashion do not perform as predicted because current at the microwave frequency flows close to the surface, resulting in significant skin resistance between the active junction and the back contact. There are ways to reduce this skin resistance by evaporating a metal deposit over most of the surface flow path and thereby reducing its length. However, this is a recommended activity for the future since such diodes were not made under this study. 4. 1 The Diode Design and Construction Matrix The approach to the design and construction of the diodes took the following objectives into consideration: 1. To use the RXCV diode and rectenna element as a base and performance reference. To this end the RXCV diode has been included as one of the end items in the matrix. 2. To permit a systematic evaluation of the impact of various parameters and to note any inconsistencies in results or departures from expected behavior. 3. To use two types of barriers, GaAs-Pt and Ga-As-W, to investigate differences in their efficiencies. And also to investigate GaAs-W barrier construction techniques. 4. To investigate the performance of diodes having a low value of zero-bias capacitance and to confirm the design prediction that a lower value of capacitances would irr prove the efficiency of the diode and rectenna element at low power levels. c. To compare the performance of diodes having greatly different values of reverse breakdown voltage, particularly at low power levels.
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