UBC Theses and Dissertations
An experimental and theoretical investigation of the characterstics of dark illuminated junction diodes of germanium and silicon. Pinson, William Edwin
Measurements have been made on a variety of germanium and silicon junction diodes in order to determine departures from the ideal behavior both under illuminated and dark conditions. The diodes included alloyed, grown and diffused junction types and the experimental study was largely confined to forward voltage. Carriers injected by a forward voltage into the bulk regions of a p-n junction in Ge or Si reduce the resistances of these regions, A model of a photodiode consisting of an ideal diode (Shockley, 1949) in series with this carrier modulated resistance produces good agreement with the experimentally observed dark chs.; in one Ge alloy diode the agreement was exact up to at least 0,7V forward voltage. For large forward currents (density of injected carriers comparable to injected carrier density) the Shockley expression relating I and V is no longer valid nor consequently is the conductivity modulation theory. This theory, making use of the experimental dark chs, and the experimental dynamic capacity measurements, is able to deduce many of the parameters of the diode material e.g. Ƭ𝞺, Pո,Ψo. The chs. of the diode dark and illuminated have been found to intersect at large forward currents. Theoretical investigation of the condition for crossover in various models of the diode is made. The physical meaning of this condition is that at some forward voltage the resistance of the diode is sufficiently reduced on illumination to offset the effect of the opposed internal photo-e.m.f.. For an ideal diode the relation between the short circuit photo-current and open-circuit photo-e.m.f. (photochs.) should simply be the same as the dark forward chs., except for a reversal of sign of the current. The fact that the experimental photochs falls below the ideal photochs is attributed to internal resistance in the diode and an expression is developed from this assumption; the theoretical expression is largely consistent with experiment. Several other models are examined for the non-ideal behavior of junction diodes which attempt to take into account the possible effects of the resistance of the bulk material and of the electrode-semiconductor contact; the implications of non-linear recombination are also investgated. It is found that none of these alternative models is as successful as the conductivity-modulation scheme in explaining the dark, the illuminated and the photocharacteristics.
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