Effect of carrier lifetime on the forward characteristics of high-power devices

The influence of carrier lifetime on the characteristics of high-power devices has been examined from the standpoint of forward voltage drop at a given current, using existing theories of the p
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ln
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diode and allowing for carrier-carrier scattering effects. It is found that in the absence of recombination current in the heavily doped end regions, there exists an optimum base lifetime giving a minimum forward voltage. This minimum occurs because for increasing lifetime, the increase in junction voltage due to carrier buildup at the junction edge eventually overtakes the reduction in base voltage due to conductivity modulation. On the other hand, when the recombination currents in the end regions predominate over that in the base, their presence tends to inhibit carrier buildup, with the result that for sufficiently large values of base lifetime, the forward voltage falls to a limiting value. In certain cases, this value is less than the minimum voltage found in the absence of recombination in the end regions. In all the cases examined, the conclusion is that little is to be gained by further increase in lifetime beyond a certain value which depends on both the properties of the base and those of the end regions.