On the use of Po210and Am241 collimated alpha sources for the characterization of the onset of carrier multiplication in power devices

The design of robust power devices and the definition of their safe operating area require the quantitative characterization of charge multiplication in reverse biased junctions. This is mandatory especially for those mechanisms like single event burnout, where the failure is triggered by the massive impact ionization arising in the reverse-biased device from the charge generated by the impinging ionizing radiation, transported and eventually multiplied. Traditional DC characterization techniques exhibit usually limited sensitivity, such that they are not capable to detect the reverse current and the related charge multiplication before the occurrence of the junction breakdown. In the past, alternative methods aimed to improve the sensitivity at lower electric fields have been proposed that exploit either optical, or particle beams. However, these solutions cannot be simply applied to real commercial devices and in addition they just deliver averaged values of the multiplication factor. In this paper, single ionization events generated at the close vicinity of the reverse-biased junction of a commercial power PiN diode are acquired to measure the distribution of the multiplication factor at different reverse bias conditions. Here, the fast reverse current pulse produced by the initial ionization charge burst is collected by a dedicated spectrometry chain and processed to obtain the probability distribution of the current pulses and the related multiplication factor. This analysis accounts for the stochastic nature of the initial charge generation, as well as of the impact ionization process. The measurements results are compared with the multiplication values and distributions as obtained by TCAD, analytical models and by Monte Carlo simulation. The performance of two different exempt quantity alpha sources is investigated, namely Polonium (Po
²¹⁰
) and Americium (Am
²⁴¹
) that are used in conjunction with dedicated collimators. The technique is demonstrated based on a commercial 1.2 kV-70 A power PiN diode in the reverse bias range from 700 V to 1250 V. Detailed information is provided about the proposed hardware solutions, which can easily be implemented under usual laboratory conditions.