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Background. The unique spectral position of terahertz range determines the difficulties of developing compact solid-state sources of terahertz radiation. In most cases, the operating frequencies of existing devices are displaced in the terahertz part of the spectrum. It is known that impact ionization, especially in its initial stage, is a rather fast process that can be used to improve the devices frequency properties. The temporal and spatial restrictions of impact ionization in InGaN and InAlN compounds need to be evaluated to determine the prospects of its use for the terahertz range devices design. Purpose of Work. The aim of this work is to determine the values of time and space delays in the development of electron-initiated impact ionization in the InGaN and InAlN semiconductor compounds at the initial stage of impact ionization. Techniques and Methodology. The modeling of electronic transport was performed using the Ensemble Monte Carlo technique. It takes into account the all actual mechanisms of scattering іincluding the alloy potential scattering and impact ionization. The constant electric field approximation has been considered. The homogeneous materials and the materials with composition depending on coordinate were considered. The spatial distributions of impact ionization acts for a charge carriers ensemble were analyzed to determine a characteristic mean distance a carrier travels before ionizing ("dead space") and a delay time of impact ionization appearing. Results. The delay times of impact ionization in InGaN and InAlN compounds if electric field strengths greater than 100 kV/cm are less than 2 ps. Delay times can be an order of magnitude lower when Ga and Al content less than 60%, respectively in InGaN and InAlN. The mean distance a carriers acquire enough energy to impact ionize for this case are about 100-200 nm. This distance decrease with increasing electric field strength and may be less than 50 nm in the case of the InAlN. The dead space length can be changed by using a graded band layer in which the band gap decreases towards the anode. But the smallest values of the "dead space" correspond to homogeneous materials. Conclusions: Thus, impact ionization at the initial stage can be used in ultrahigh-frequency devices in the terahertz range, in particular, as a mechanism of energy relaxation. |
