Junction geometries to improve power output of incoherent light from GaAs diodes

Incoherent radiation is usually observed emanating from the p or n region of a wafer containing a p-n junction. In the region in which the radiative recombination occurs, the P/I ratio is 1.5 watts per ampere at 77° K for unity quantum efficiency. The ratio of the maximum power which may exit either the p or n surface to that generated within the crystal is approximately 1 : 57. This relationship results in an upper limit of 26.3 milliwatts per ampere. The small ratio is a consequence of the small critical angle in gallium arsenide. When the p-surface is employed as the exit surface, for junction areas approaching 1 cm², the highest value obtained at 77°K is 14 milliwatts per ampere. For junction areas of 10^-2cm², values of 20 milliwatts per ampere have been observed. The maximum average current which may be passed by such a junction and still maintain a positive slope for dP/dI is 10 to 20 amperes. Thus, the total power obtainable from 1 cm²is less than 0.280 watt. This limitation arises primarily because of the excessive joule dissipation present in such large area junctions. This investigation has resulted in geometrical configurations suitable for average power output in excess of 10 watts and configurations for pulse power output in excess of 1 kilowatt. The total semiconductor area for each of the above systems is less than 20 cm². The measurement technique and method of calibration is briefly discussed. The data obtained from these structures shows that P/I values as high as 170 milliwatts per ampere have been obtained. It is shown that one of the structures regularly achieves 40 to 60 milliwatts per ampere. Moreover the structures exhibit much less power dissipation which permits a higher power output than present junction structures. The radiation characteristics of the present junction structure are compared to the new structures for dc, average and peak power. For pulsed operation, the average and peak power are given as a function of the period. The maximum peak power is at least 100 times greater than the average power at low duty cycle operation. Under this condition the quantum efficiency is several times greater than the dc value. This is also true for duty cycles of 1 to 10 percent for which the junction heating effect is appreciable. Junction heating becomes appreciable at a duty cycle of 10 percent for junctions exhibiting a resistance value of 0.04 ohms and a current value of 2 amperes. Absorption of the radiation at the contacts also contributes to junction heating, however, to a lesser degree.