Experimental study and two-dimensional modeling of avalanche breakdown voltage in polycrystalline silicon p-n junctions

A two-dimensional (2D) model of the avalanche breakdown mechanism is examined to achieve a lateral polycrystalline silicon (polysilicon) p(+)-n diode with high forward current and high breakdown voltage (B-V). Samples with different film thicknesses (t(f)) were deposited by low-pressure chemical vapor deposition process. The p(+) zone and n zone are doped by ionic implantation with boron and phosphorus, respectively. The measured current-voltage (I-V) characteristics show that B-V varies between 6.4, 7.5, and 8.25 V when t(f) varies between 250, 350, and 450 nm, respectively. These data also show that when t(f) decreases, the forward current is high, the leakage current becomes higher under reverse bias, and B-V decreases. We reveal that the breakdown phenomenon of our samples is dominated by the impact ionization effect. A 2D simulation of avalanche breakdown voltage versus the critical parameters of polysilicon diodes is implemented. The algorithm is based on the solution of Poisson's equation and calculating the ionization integral along various electric field lines computed from the potential distribution. By taking into account the localization of trap states in the grain boundaries, the effects on the breakdown voltage of the doping concentration N-D, the intergranular trap state density N-T, the grain sizes L-g, the disposition of the grain boundaries, and the film thickness t(f) are investigated. The simulation results show that the impact ionization mechanism is more accelerated in polysilicon than in single-crystalline silicon, and the B-V(L-g), B-V(N-D), B-V(N-T), and B-V(t(f)) curves are characterized by a succession of descending stair shapes due to the trapping of free carries by trap states contained in grain boundaries that are parallel to the metallurgic junction. By comparing simulation results with experimental data, we select the electron-hole ionization coefficients characterizing our samples: alpha(infinity)(n)=1.0x10(6) cm(-1), E-n(crit)=5.87x10(6) V cm(-1), alpha(infinity)(p)=1.582x10(6) cm(-1), and E-P(crit)=2.036x10(6) V cm(-1). The fit shows that the extracted N-T varies from 1.07x10(13), 4x10(12), and 2x10(12) cm(-2) when the film thickness varies from 250, 350, and 450 nm, respectively. These results validate the developed model and confirm that structural and electronic properties of the polycrystalline deposition films are improved during the layer growth. (c) 2007 American Institute of Physics.