Study on betavoltaic battery with suspended 1.5 µm-Si thin film back-junction back-contact PN junction

This paper investigated the structural and area influence on the fabrication yield and the characteristics of the diode and betavoltaic battery. Different structures and areas of the back-junction back-contact (BJBC) PN junction with suspended 1.5 µm-Si thin-film were prepared by microfabrication based on silicon-on-insulator (SOI) substrates. The samples were fixed and wire-bonded on a custom-designed-and-processed sample holder and PCB board on the SEM stage so that their betavoltaic battery performance was tested using high-energy electron beams to simulate the tritium source. Before splintered to chips, all devices on the 4-inch wafer maintained their structures well and exhibited good rectification characteristics. However, the larger area the suspended thin film was, the more severely it was damaged after splintering. The collection efficiency of radiation-generated carriers were enhanced by reducing collection electrode spacing and increasing the number of electrodes, thereby improving the battery performance, which proves the advantage of design flexibility in doping regions and electrode shapes in BJBC structures. The large device area made the reverse saturation current of the PN junction decreasing an order of magnitude and so resulted in the betavoltaic battery performance degradation. As a result, the optimal betavoltaic battery performance was achieved by 1.2 × 1.2 mm² area BJBC PN junction with the interdigitated structure of 10 µm-wide P⁺/N⁺ regions and 25 µm spacing, yielding I_sc = 0.729 µA, V_oc = 0.194 V, P_m = 84.1 nW, η = 1.09 %.