Comparison Between Forward Bias Currents in P/I/N and P/C(B/P)/N Hydrogenated Amorphous Silicon Diodes
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COMPARISON BETWEEN FORWARD BIAS CURRENTS IN P/I/N AND P/C(B/P)/N HYDROGENATED AMORPHOUS SILICON DIODES G. WINBORNE, LE XU* AND M. SILVER Department of Physics And Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 * Permanent address: Nankai University, Tianjin, P.R. China ABSTRACT The drift mobility in compensated a-Si:H as measured by standard T.O.F. is much less than in intrinsic material; presumably due to charge defects. In order to determine other differences in behavior between compensated and intrinsic material, we have studied the transient forward bias current of each in double injecting devices. We have found that: a) a much higher voltage is required in compensated material to obtain the same current as in the intrinsic; b) intrinsic and compensated samples show similar transient current responses but the compensated devices are much slower; c) the steady state current degrades in the compensated current but not in the intrinsic; and d) mobilities obtained by the voltage pulse technique in compensated material are at least three times larger than by the standard T.O.F. technique.
INTRODUCTION It was suggested by Silver et al. 1 that the long range potential fluctuations due to charge defects limit the mobility in intrinsic a-Si:H rather than the short range random potential fluctuations to dihedral angle disorder. One prediction arrived at from this suggestion is that the extended state mobility would be lower in compensated material than in intrinsic due to the plethora of charged donors and acceptors. Spear et al. 2 found that at 300*K the drift mobility for 5.0 PPM P/B compensation yielded a mobility of approximately .09 cm 2 V- 1s- 1 rather than the value of 1.0 cm 2 V- ls- I usually found in intrinsic ma, -rial. This result suggested that further comparision of the transient transport properties of the two materials should be made to better understand the relationship of charged defects with the nobility. We have carried out a study of the forward bias transient current in these materials. While they show qualitatively similar behavior, there are significant quantitative differences which are not presently fully understood. The transient current response in both materials has a repetition rate dependence but the compensated material requires a much larger voltage and rise time to achieve the same current level as the intrinsic material. Further, the current in compensated material degrades with continuous pulsing in a time on the order of minutes. This effect is not observed in the intrinsic material. Finally, we have determined that mobilities measured under forward bias by the voltage pulse technique are larger than those obtained by the standard, laser pulse, reverse bias
Mat. Res. Soc. Symp. Proc. Vol. 118. c 1988 Materials Research Society
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v-V St4 -V=O Pulse Generator Figure 1. Circuit schematic. The beginning of one applied voltage pulse is shown.
method. This suggests that forward bias even in compensate
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