Investigation of the electrical characteristics of GaAsP light-emitting diodes (LED's) under conditions of mechanical stress
The main line pursued is the mechanical stressing of light-emitting diodes (LED's) by pressing on the surface with spherically rounded sapphire probes ranging from 100 μm to 500 1m radius of curvature and with forces up to 50 grams weight on the largest probe, care being taken not to exceed the elastic limit of the material, which is estimated to be about 1.4 x 10^9 pascals (14,000 atmospheres) at the central bottom point of the probe. With the diode under forward bias a very wide range of current is covered from 10 mA to about 100 pA in the light-emitting region and from 100 pA to 100 nA in the generation/recombination region - and across the whole range a small decrease in current of about 1 per cent or less is observed. The classical theory of the distribution of stress by a spherical probe has been investigated by a modern computer technique, and by integration of the stress over the whole of the junction interface it is shown that the decrease in current can be ascribed to the increase in band gap of the semiconductor that is brought about by the axial component of pressure stress at right angles to the interface. With the larger currents in the light-emitting region a further decrease in current of the order of 1 per cent or less can be ascribed to heat conduction from the warm surface of the diode to the probe, thereby. lowering the temperature of the interface. Finally, the electrical characteristics of the diode under reverse bias have been investigated, with incident light playing an important role; in the absence of light the extremely small currents (10^-12 to 10^-10 amp) are consistent with thermal generation of carriers similar to the generation/ recombination mechanism for very low currents in the forward direction.
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Brunel University School of Engineering and Design PhD Theses