Radiation Resistance of Test npn IC Transistors with Dielectric Insulation, Manufactured on Silicon, Isovalently Doped with Germanium (SiGe)

Abstract

There is a contradictory assessment of the possibility of germanium (Ge) use to increase the radiation resistance of silicon (Si) homogeneously doped with an isovalent impurity. A number of publications show that only a limited effect of germanium doping on the radiation stability of the pn-structure, irradiated by high-energy electronsis observed. Simultaneously there is a noticeable improvement in the radiation resistance of npnp-structures made on SiGe under γ-irradiation. In order to remove the contradiction, this work compares the β radiation degradation of test bipolar transistor npn Integrated Curcuit (IC) structures, manufactured using the same technology, "silicon with dielectric insulation", on isovalent germanium-doped SiGe silicon with different Ge content, N_Ge=1,2·10¹⁹…1,2·10²⁰ cm^-3. The static gain coefficient β was measured before and after α-irradiation. Irradiation of unencased npn structures with α-particles with an energy of 4.5 MeV carried out in a specially designed and manufactured laboratory installation using radioisotope sources; npn structures with two base thicknesses: 0.25 and 0.35 μm were studied experimentally. The dependence approximating the experimental data, β(Φα), an equation, describing the change in the gain factor of the transistor structure upon α-irradiation, obtained using the OriginPRO program. Obtained results for structures with a base thickness of 0.25 μm show a strong nonlinear dependence of β(Φα) equations on N_Ge. The degradation of the control transistors gain, manufactured according to the standard technology (N_Ge = 0) is described by the S-curve. Irradiation of npn structures formed on SiGe wafers with different levels of doping with an isovalent impurity leads to a complete change of the nature of the dependence. For  Φ_α ≤ 10¹¹ см^-2 the nature of the change in β is practically the same for structures made on wafers with N_Ge= 0 and N_Ge= 2.5·10¹⁹ сm^-3, as well as for N_Ge=1,2·10¹⁹ сm^-3 and N_Ge=1.2·10²⁰сm^-3. When increasing Φ_α≥ 10¹¹ сm^-2 there is an accelerated degradation of the gain factor of npn structures made on wafers with N_Ge= 2.5·10¹⁹ cm^-3. This level of doping of silicon with germanium is not acceptable from the point of view of Si radiation resistance. At Φ_α ≤ 10¹⁴ см^-2 radiation stability of npn structures made on SiGe wafers with N_Ge=1.2·10¹⁹ см^-3 approximately two times lower than the same of control structures with N_Ge= 0. For transistors with a base thickness of 0.35 μm, no effect of changing the nature of the npn structures β(Φα) degradation. Observed dependence, which confirms the possibility of slowing down the radiation degradation of the amplification factor value of the npn structures made on SiGe. Increase in radiation resistance by 2-3 times for test transistors, made on SiGe wafers, doped with N_Ge= 7,5·10¹⁹ см^-3, observed  in a wide range of doses of α-irradiation,  10¹¹≤ Φ_α≤ 10¹⁴ см^-2.