| Download this article in .PDF format |
This file type includes high resolution graphics and schematics when applicapable.
Burnout is a major cause of operation failure of microwave diodes. Burnout (for this discussion) is defined as catastrophic failure such as an open or short circuit or a 3-dB degradation in electrical performance. Burnout may be due to excessive energy (continuous or transient) dissipated in the diode.
Continuous power seldom exceeds ratings in most applications since microwave mixer diodes operate with a local-oscillator power input of a few mW, well within their rating. As a detector covering a wide dynamic range, the diode’s capability may be exceeded at the high-power inputs. In practice, this problem can be eliminated by adding enough attenuation to protect the diode at the maximum input-power level while accepting reduced signal output at low-input power.
It is more difficult to protect against transient energy. Two major causes of excessive transient energy are TR tube leakage and high-energy signal pulses.
Using the same MQM packaging configuration, burnout comparison of Schottky and point-contact S- and X-band diodes is shown in Fig. 26. The tests were performed by subjecting groups of diodes, one at a time, to a single pulse from a Torrey line. The charging voltage was increased with successive pulses until the diodes were destroyed.
The S-band Schottky diodes were better at higher voltage levels than the point-contact diodes, as shown at α. The X-band Schottky diodes were better than the point-contact diodes at the lower voltage levels, as shown at b. X-band devices withstand less voltage than the S-band diodes because of the smaller junction of the higher frequency diodes.
Using a repetitive-pulse test, results were somewhat different as shown in Fig. 27. The tests were performed by subjecting a group of diodes one at a time to a series of pulses and then repeating at increasing voltages until all diodes were destroyed. A diode that will withstand one pulse may be destroyed by repetitive pulses of the same energy content.
The Schottky diodes appear better at lower charging voltages at S-band, as shown in Fig. 27a. At X-band, the point contact diodes appear better, as shown at b. These curves point out the significance of the application and the test procedure in evaluating diode burnout resistance.
Another type of pulse burnout of interest is that encountered with high-powered pulses in the order of 50 ns to a few ms duration. Such pulse widths might be experiences by a diode in a video receiver. The peak pulse power is the significant factor in causing diode burnout. In testing for this, Schottky diodes are superior at both the 1.0- and 10.0-ms pulse widths, as shown in Fig. 28.
Cw power capability
The ability to withstand cw power is of interest in some applications of microwave diodes, such as in power monitors on an intermittent basis or in harmonic generators on a continuous basis.
Cw power-capability tests were performed by applying power to the group of diodes at a sequence of increasing power levels for short periods of time until the diodes were destroyed or the maximum power of the test circuit was reached. X-band point-contact and Schottky diodes were mounted in a holder one at a time. Each diode was matched to the power source at each power level, in order to get maximum power dissipation in the diode. Two different diode loads were used.
With the 10-kΩ load, point-contact diodes started to fail at 640 mW while Schottky diodes showed no failures up to 800 mW. With the 10-Ω load, Schottky didoes showed changes at 160 mW while the point-contact diodes showed no change up to 800 mW.
These results can be understood in terms of the diode characteristics. With the 10-Ω load, the forward current will be greater in the Schottky diode because of its lower series resistance; and consequently, the power dissipated in the device will be higher at the same input power level. With the 10-kΩ load, less forward current will exist and less power will be dissipated in the diode.
However, the voltage built up across the diode in the reverse direction will be much higher with the 10-kΩ load; and the point contact diode with its much lower PIV characteristics will start to conduct much sooner in the reverse direction with subsequent diode damage. The better PIV characteristics of the Schottky diode will also be an advantage in some applications, such as in video detector systems, where a possibility of pulse leakage from nearby transmitters exists.
Schottky didoes withstand burnout better than point-contact diodes. The amount of superiority demonstrated varies from one type of test to another and is a reflection of the problems of test instrumentation. The experimental data support the theoretical superiority of a junction device, with its greater uniformity, over a point-contact device which depends on a pressure contact between tungsten and silicon for rectifying junction.