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Since most microwave diodes will be used in military equipment with rugged service environments, it is necessary to know the ability of microwave devices to withstand severe environmental stress. Schottky didoes will withstand high mechanical and thermal stresses as demonstrated by tests performed under the standard conditions of MIL-STD-750 for semiconductors. Schottky didoes will withstand mechanical stresses of 20 G vibration, 1500 G shock, and 20,000 G acceleration. Schottky diodes will withstand temperatures cycling from -65 to 150-deg. C. No deterioration was evident after 1000-h storage at 150-deg C.

These tests prove that Schottky diodes are suitable for severe military and space environments. Point-contact devices, likewise, withstand the same degree of mechanical and thermal stress as the Schottky diode.

Schottky diodes have two fabrication advantages over point-contact devices that result in greater uniformity and, therefore, in greater reliability in large-scale production. Schottky diodes are made by forming a large number of junctions on a single wafer of semiconductor material. This process, which is one used for the majority of semiconductors, is capable of close control. These junctions can be measured for dc characteristics and capacitance while still on the wafer or in chip form, which results in tight process control. Point-contact devices, however, are made by a pressure contact between the semiconductor and the metal contact. This fabrication method results in significant variation in dc characteristics from device to device. Furthermore, these dc characteristics can only be measured in the finished assembly.

The Schottky diode should be much more uniform in high-volume production than the point-contact diode. Variations in package materials and processes can be separated from semiconductor variations so that impedance variations from device to device should be much less with Schottky diodes.

Using Schottkys in circuits

Beamlead technology has been applied to produce a unique microwave mixer circuit configuration, in which two silicon Schottky-barrier diodes are fabricated monolithically and beamleaded in series with a center beam between them. The resultant diode pair is shown in Fig. 29.

The complete mixer is shown in Fig. 30. Here the diodes are bonded to the arms of a quarter-wavelength branch line hybrid, and the i-f signals are combined at the center top of the diode pair and fed out through a low-pass filter. This mixer has successfully performed at X-band with a noise figure of 7 dB.

As an illustration of other applications of this technique, a demonstration model of a cw integrated Dopplr radar has been fabricated as shown in Fig. 31. It consists of a 9.5-Gc avalanche-diode oscillator, which serves as transmitter and local oscillator (the system operates at zero i-f frequency), a directional coupler, separate transmit and receive antennas, a Schottky-diode balanced mixer, low-pass filter, audio amplifier and loud speaker. A moving target produces a Doppler-shifted return signal, which is down converted to an audio frequency, amplified and fed to the speaker.

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