We present a comparative study of silicon high-voltage diodes exhibiting the effect of delayed superfast impact-ionization breakdown. The effect manifests itself in a sustainable picosecond-range transient from the blocking to the conducting state and occurs when a steep voltage ramp is applied to the p+-n-n+ diode in the reverse direction. Nine groups of diodes with graded and abrupt pn-junctions have been specially fabricated for this study by different techniques from different Si substrates. Additionally, in two groups of these structures, the lifetime of nonequilibrium carriers was intentionally reduced by electron irradiation. All diodes have identical geometrical parameters and similar stationary breakdown voltages. Our experimental setup allows measuring both device voltage and current during the kilovolt switching with time resolution better than 50 ps. Although all devices are capable of forming a front with kilovolt amplitude and 100 ps risetime in the in-series load, the structures with graded pn-junctions have anomalously large residual voltage. The Deep Level Transient Spectroscopy study of all diode structures has been performed in order to evaluate the effect of deep centers on device performance. It was found that the presence of deep-level electron traps negatively correlates with parameters of superfast switching, whereas a large concentration of recombination centers created by electron irradiation has virtually no influence on switching characteristics.

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We present a comparative study of Si and GaAs high-voltage diodes operated in the delayed impact ionization breakdown mode. We use an experimental setup that allows measuring current and voltage on the diode simultaneously and independently during a 100-ps high-voltage switching transient. Si and GaAs structures with identical geometries and a stationary breakdown voltage of ∼1 kV were investigated. All devices trigger at close to 2 kV and are capable of forming a voltage ramp with a kilovolt amplitude and a 100-ps rise time. We found that Si p + nn + and p + pnn + structures differ in residual voltage: a relatively low residual voltage V res of 100-200 V was observed only for p + nn + structures, whereas for p + pnn + structures V res is about 1 kV. We report observing the lock-on effect in GaAs structures: after 100-ps avalanche switching GaAs diodes remain in a high conducting state as long as the applied voltage pulse lasts, whereas within the same time of 2 ns reference Si diodes fully recover the blocking capability.

The effect of delayed impact ionization breakdown initiated in a high-voltage Si or GaAs p+nn+ diode by a steep voltage ramp leads to 100 ps avalanche transient from the blocking to conducting state. Here, measurements of the voltage and current dependences in the Si diode exhibiting 100-ps kilovolt switching are presented together with simulations with focus on comparison. Device voltage and current are measured simultaneously and independently in a high-quality matched coaxial circuit. In simulations, we account for wave propagation and reflection processes in the coaxial driving/measuring circuit and for the inhomogeneity of the avalanche switching over the device cross section. This makes quantitative comparison with measurements possible. An agreement in switching time and transient characteristics can be achieved only under the assumption that a smaller part of the cross section is avalanching. The 100-ps switching time is formed not during the passage of superfast ionizing front in the “active” part of the device, as it is widely believed, but by the discharge time of the “passive part” over the conducting “active” part. The inner circuital current that flows within the device along the closed loop plays a dominant role in this process. Sources of initial carriers, the temperature dependence of the effect, and the limits of drift-diffusion transport model in describing the phenomenon of delayed breakdown are discussed.

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