Some reactive effects in forward biased junctions

The small-signal equivalent parallel capacity of forward-biased semiconductor junctions is strongly dependent on the current. At very low currents (less than 10 µa for a junction area of 1 mm²) the capacity appears to be chiefly due to space charge effects. For currents up to approximately 100 µa, the capacity complies with Shockley´s predicted low-level theory. For larger currents, however, there is a definite deviation from the low-level diffusion predominance and capacity reaches a maximum after which it decreases through zero and then goes to large inductive values. The latter phenomena is explained, qualitatively, by considering an inductance in series with the diffusion capacity. The capacity increases linearly with current but the inductance (due to conductivity modulation) increases faster. The result is that a change from an equivalent RC circuit to an equivalent RL circuit is made at high enough currents (5 ma is a typical value for the 1 mm²junction area). Measurements were made on abrupt silicon junction diodes with junction areas of about 7 × 10^-4, 10^-2, 10^-1cm²and on the emitter junction (about 5 × 10^-5cm²) of a diffused base silicon transistor.