Technical Computer-aided Design Research Articles

Influence of the contacting scheme in simulations of radial silicon nanorod solar cells

Abstract Silicon nanorod solar cells were simulated using the Silvaco Technical Computer Aided Design (TCAD) software suite. For reasons of speed optimization the simulations were performed in cylinder coordinates taking advantage of the model's symmetry. Symmetric doping was assumed with a dopant density of 10 18 cm −3 in the p-type core and in the n-type shell, and the location of the pn-junction was chosen such that the space charge region was located adjacent to the shell surface. Two contact configurations were explored. In configuration A the cathode contact was wrapped around the semiconductor nanorod, while in configuration B the cathode was assumed just on top of the nanorod. In both cases the anode was located at the bottom of the rod. Cell efficiency was optimized with regard to rod radius and rod length. Optimization was performed in a three-step procedure consisting in radius optimization, length optimization and again radius optimization. A maximum in efficiency with respect to rod length L was visible in configuration A, leading to an optimum value of L = 48 μm. This maximum is explained by the combination of an increase of short-circuit current density J sc and a decrease of open-circuit voltage U oc with L . In configuration B, J sc also increases with L , but U oc stays rather constant and the maximum in efficiency only appears at very large values of L ≈ 12 mm. We restricted the rod length to L ≤ 100 μm for further optimization, in order to stay in an experimentally feasible range. During the optimization of rod radius R in configuration A the open circuit voltage increased continuously, while short circuit current density stayed rather constant. This leads to an increase in efficiency with R , which only stops at very large radii, where R starts to be comparable with L . In configuration B efficiency is almost independent of R , provided that the radius is large enough to comprise a well-formed space charge region, here only a shallow maximum can be estimated. With the demand of rod lengths being smaller than 100 μm, optimum parameters L = 48 μm, R = 32 μm and L = 96 μm, R = 2 μm were extracted for configuration A and B, respectively.

Impact of proton-radiation-induced spacer damage on the dc characteristics degradation in deep-submicron metal-oxide-semiconductor field effect transistors

The dc characteristics degradation of 0.18 μm metal-oxide-semiconductor field effect transistors (MOSFETs) after 10 MeV proton irradiation is comprehensively investigated in this paper. The measured results show that the off-state drain current is increased in N-channel MOSFETs, which is due to the turn on of the parasitic transistors induced by the shallow trench isolation regions. While in P-channel MOSFETs, the threshold voltage increase (absolute value), the transconductance degradation, and the saturation drain current decrease are observed. From the analysis, it is concluded that the basic damage mechanism is not ascribed to the gate oxide and the isolation region. The origin of the observed changes may be mainly due to the damage in the spacer oxides of the transistors. In order to verify the assumption, the leakage current passing through the spacer between the gate and the drain is measured before and after irradiation with floated source/substrate and grounded drain. We find that the leakage current is two to three times larger after irradiation. Finally, in order to confirm the extrapolation, 2-dimension (2D) simulation has been performed with Synopsys TCAD (technical computer-aided design) Sentaurus Device Simulation tools (ISE 10.0). The behavior of the simulated charges trapped in the spacers is qualitatively consistent with the experimental results.