The computer simulation program has also indicated that there is the possibility of a combination of effective diode capacitance, input microwave power level, and setting of the effective inductance in the rectifier tank circuit that can cause the generation of much power in the higher order harmonics and decrease the efficiency significantly. Thus the computer simulation program provides a useful means of providing a large variety of operating conditions that would be difficult to reproduce experimentally. 5. Development of Improved Diodes A significant number of different diode designs were produced under this contract as an aid in investigating their impact upon improved rectenna element performance. The most significant development was the use of a GaAs-W barrier in place of the well-established GaAs-Pt Schottky barrier. As anticipated, this reduced the forward voltage drop by about 0. 2 volts or about 25%. This reduction is of great significance in efficiency improvement when operating elements at a very low incident microwave power level. The development of the GaAs-W barrier is a significant achievement because of the difficulty of the bonding properties of their physical interface. A solution to this problem was found under separate contractual support and enabled us to adapt the same fabrication techniques to the diodes that were developed under this contract. The effort to develop diodes for improved performance of rectenna elements when they are subjected to low values of incident microwave power has established the importance of the resistance of the back contact as a limiting parameter. Improved diodes for low power operation are made with lower junction capacitance and therefore with reduced junction area. Because the distance between the junction area and the back contact is so small in the plated heat sink diode, the current does not have a chance to fan out before making contact with the back contact. Since the series resistance in the epitaxial layer is very small for these diodes which have a high doping density as well as a thin epitaxial layer the back contact resistance becomes a dominating element. Since the technique of applying the back con- tack for these diodes is the best available, there is no way of reducing the specific resistance (per unit area) of the back contact without active development effort. However, it is possible to increase the distance between the back contact and the epitaxial layer in order to let the current fan out. Diodes were made with this feature and it was found that the series resistance as measured by DC voltage was indeed reduced. However, it was found that these diodes did not provide improved performance and it was then found that the microwave resistance was higher than the DC resistance, indicating that there was a skin effect involved.
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