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To evaluate detector diodes, tangential signal sensitivity was chosen as the test parameter. This is defined as “that power below a 1-mW reference (0 dBm) required to produce an output pulse sufficient to raise the noise fluctuation by an amount equal to the average noise level, or 4 dB above the minimum detectable signal.”

Results of this evaluation are shown in Fig. 24, which clearly shows the comparison between X-band Schottky and point-contact diodes. When operating without dc bias, the Schottky diodes are approximately 30 dB poorer in this test than the point-contact units. This would be expected, because the Schottky diodes are non-conducting at low bias levels. With dc bias, the Schottky diode is comparable to the point-contact unit.

The use of dc bias changes the video impedance and noise characteristics of the diodes. Point-contact diodes without bias have video impedances of approximately 4 kΩ to 100 kΩ. Schottky diodes are in the 10’s of megohms. With 50 μA dc bias, point-contact diodes have video impedances in the range of 1.5 to 4 kΩ, whereas the Schottkys are in the range of 5 to 6 kΩ. The video impedance, coupled with the capacitance of the holder-amplifier combination, has a critical effect on the plus fidelity due to the charging time-constant of the circuit. Bias on point-contact diodes will result in better pulse fidelity than with Schottkys because of the lower video impedance.

The noise generated within the diode by the dc bias is lower in the Schottky than in the point-contact device. However, Schottky diodes with very low capacitances have shown high noise. This is attributed to the high RS associated with low capacitance.

Temperature runs of both diodes show the Schottkys to be more stable with temperature variation, while the point-contact tangential signal sensitivity varied 3-5 dB at high temperatures.

Families of curves, showing voltage output vs. power input and load resistance for both point-contact and Schottky diodes, are shown in Fig. 25. The higher peak inverse voltages of the Schottkys, as compared with the point-contacts, are evidenced by their higher open-circuited voltage. The higher starting voltage of the Schottky diode, compared to the point-contact, is also evident.

The future of the Schottky diodes

What does the future hold in store for the Schottky diode?

At present, these diodes are limited to an upper frequency of about 40 Gc. Research and development now in process will push this limit higher. The Schottky diode has the potential of being the circuit designer’s “dream” diode—the active junction with no package.