A method of testing the impedance of an IMPATT diode operating as a pulsed oscillator at a predetermined pulse repetition frequency is shown to comprise the steps of alternately applying through different, but known, paths, a test signal to the bias line to such diode and observing the change in the test signal as reflected from the IMPATT diode.

The operating temperature of the active region of an RF semiconductor device such as a microwave hybrid circuit having a gallium arsenide field-effect transistor therein is determined by measuring signal gain of the device at variable times following application of bias voltage to the device. Bias voltage is applied in response to a constant duty cycle pulse train from which are derived a sample-and-hold command pulse and a synchronized bias control pulse.

The disclosed method qualifies diodes for microwave power combiners having a central cavity, and having N diode oscillator circuits spaced around the cavity for furnishing energy thereto. It includes the steps of activating only one of the N oscillator circuits, and then measuring the frequency at which the diode in that circuit supplies maximum power to the cavity. This power is measured by means of a probe having an electric field coupling of N.sub.11 with the cavity, where N.sub.11 equals N.sub.12 /.sqroot.N, and where N.sub.12 is the corresponding electric field coupling that is used to remove power from the cavity when all N of the oscillator circuits are simultaneously activated. These steps are repeated on each of the individual diodes to be tested. The diodes which qualify for simultaneous use in the combiner are only those which have measured frequencies of maximum power lying within a predetermined frequency band.

An attenuator useful in measuring low level leakage currents is disclosed. The attenuator includes a plurality of current dividers coupled in cascade. Each current divider includes an input and two outputs between which the current entering the input is divided. The current exiting the last divider is significantly attenuated from that entering the attenuator. The attenuator output is coupled to the device under test and to one input of a differential amplifier. A known current is input to the differential amplifier and part is directed to the attenuator input and the other part to a current measuring device. The difference between the known current input to the differential amplifier and that measured is the current input to the attenuator. In the steady state, the current input to the differential amplifier from the current attenuator is about zero. Accordingly, the leakage current is equal to the known current entering the differential amplifier less the measured current divided by m, the attenuation provided by the attenuator.

A method and an apparatus for testing semiconductor devices, especially overvoltage protectors, involves the application to the device to be tested of a standard surge waveform which is truncated by terminating it early. The amount of heating of the device by the truncated surge waveform is ascertained by monitoring or measuring a heat sensitive parameter of the device during and/or shortly after the application of the waveform. Devices which undergo less heating are capable of withstanding higher electrical loads in service than those which become more heated. The early termination is effected by a controllable short circuit operating on the standard waveform. The differential coefficient with respect to time of a heat sensitive parameter may be monitored continually during the truncated waveform and used to operate the short circuit even earlier if the differential coefficient exceeds a threshold value indicating an undue temperature rise in the device.