The graded-gap Alx Ga1 − x As–GaAs heterojunction

The electrical properties of n-n (AlGa)As–GaAs heterojunctions and the photoresponse of n-p and p-n (AlGa)As–GaAs heterojunctions, all grown by liquid-epitaxy techniques, are explained in terms of a graded-gap model of the heterojunction interface. This model, in which the grading occurs over a few hundred angstroms now makes possible a consistent explanation of the operation of double-heterostructure lasers in contrast to the abrupt model of the heterojunction which predicts a series resistance due to reverse biasing of the n-n heterojunction. Most n-n heterojunctions measured were Ohmic with the percentage of non-Ohmic (rectification ratio ∼ 5 : 1) junctions decreasing with increasing growth temperature. The grading and its statistical nature are explained in terms of the initial nonequilibrium growth condition in liquid-phase epitaxy when the (AlGa)As melt is placed in contact with a GaAs solid rather than the (AlGa)As solid alloy determined from the equilibrium phase diagram. The measured photoresponse of the n-p junction extends from the (AlGa)As band energy to the GaAs band energy as predicted by the graded-gap model and is not consistent with previous abrupt models in which there is a spike in the conduction band.