4 .0 SCHOTTKY BARRIER DIODE DEVELOPMENT The material discussed in this section is closely associated with the material described in Section 3. 0. Section 3.0 stresses the work done in modifying the circuits to a higher impedance level and the results of the total circuit and diode modification efforts in terms of higher efficiency obtained for the lower levels of microwave power input. It was acknowledged in Section 3. 0 that the diode modifications were an important part of the effort. In this section the work on the diode will be stressed. Historically, much of the improvement in rectenna element performance has been attributable to improvements in diodes. (Figure 1-16.) The major improvements in diodes have resulted from reducing the series resistance of the diode. This reduction was accomplished in part by using GaAs as the semiconductor material. GaAs has a much higher mobility which results in much lower series resistance. Another major improvement was making the thickness of the epitaxial layer as thin as possible consistent with thickness required for the reverse breakdown voltage, V^r, specified. Most diodes are constructed with excess thickness of the epitaxial layer to obtain greater yield. A quality control problem is basically involved in making them thinner. Another important improvement in diode construction, the plated heat sink design, reduced the contribution of the resistance of the substrate to the total series resistance by greatly reducing the substrate thickness. But even after these improvements it was known that the losses in the diodes were considerably greater than circuit losses; therefore, further improvements would most likely come from improvements in diodes. Confirmation of the source of losses of a more quantitative nature has resulted from the present work as described in Section 2.2.3. Diode losses have now been measured quite accurately, and the circuit losses sufficiently so by experiment and computer simulation. These measurements indicate that the diode losses are significantly greater than circuit losses. Circuit losses other than those represented by the diode itself tend to be 3% or less of the power input, while diode losses range from a minimum of 6% to much higher values. Rectenna element efficiency with the diodes available from the RXCV program were found tp_be close to 90% at the power levels corresponding to such optimum efficiency. ' These levels were in the three to ten watt region. However, at lower power levels of 50 milliwatts the RXCV rectenna elements would perform at only 58%. (Figure 3 -1 , ) Since the circuit losses tend to remain relatively constant, such low efficiency must be associated with the diode or the manner in which it is used. Prior to this technology development program it had been established that most of the loss in the diode at such low power levels was associated with the voltage drop across the Schottky barrier itself and that the extent of this loss relative to the microwave power input was roughly given by the ratio of the drop across the Schottky barrier to the DC output voltages as discussed in Section 2. 2. 4. To improve the efficiency the DC load resistance could be increased or the drop across the barrier reduced.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==