Transient over-voltage protectors (TOVP's) are commonly used in telephone lines to ward off electric transients induced by lightning. The TOVP is basically a fourlayer device, similar to a thyristor, but without gate, and with a number of shorting dots between the cathode and the nearest base. The shorting dots, among other things, control the current distribution during fast turn on transients: the more uniform the current, the higher the surge capability of the device. The position of the shorting dots has been optimized in the past by failure analysis methods, and combinations of 2D simulations and analytical methods. 2D simulations, however, give an incomplete picture of turn-on processes, because of their three-dimensional (3D) nature. On the other hand, full 3D simulations are impractical because they would require long computation times in high-end workstations and even in supercomputers. With our quasi-3D Spice-based simulator, transient current density distributions of four layer devices with shorting dots can be quickly simulated. This method consists of dividing the device into four-layered square prisms, and a 1D PNP-NPN transistor pair model associated to each of them. The resulting equivalent circuit is simulated with Spice in a personal computer. In the present paper, a cuasi-3D simulation-based optimization method for the shorting-dot positions of a bipolar TOVP, consisting of two four-layer devices connected in anti-parallel, is presented for the first time. It was found that the boundary between the two sections of the device disturbs the transient current density distribution when 10 gs/300 gs current spikes are applied to the device, and that the distribution could be improved by shifting 50 ,um the position of the shorting dots. It was also estimated, by quasi-3D simulation, and confirmed with 2D Atlas electro-thermal simulations, that at anode currents of 100 A/cm2, temperature rise is less than 5 K.
Since strong avalanche is a central phenomenon in four-layer (PNPN) devices such as thyristors and TOVP's, the usual compact model consisting of two transistors (PNP)-(NPN) must include a numerically stable strong avalanche model. To this end, Miller's empirical expression for avalanche multiplication M(V CB ) in bipolar transistors was modified to eliminate its singularity at the collector-base breakdown voltage (V CBO ), by substituting a hyperbolic function for Miller's equation when the collector-base voltage (V CB ) is higher than 0.99999 V CBO . The combined function is such that it converges asymptotically to a large but finite value for V CB >V CBO . Furthermore, the function and its first derivative were made continuous at 0.99999 V CBO , thus ensuring stability under very strong avalanche. The model, as tested with four layer PSpice (PNP)-(NPN) simulations, did not show any instability or nonphysical results. The resulting limited multiplication factors introduced only a 0.2% voltage overshoot error at breakdown. It was also tested satisfactorily within a circuit based quasi-3D device simulator consisting of several thousand (PNP)-(NPN) transistor pairs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.