The spontaneous‐emission spectra of liquid‐phase‐epitaxial grown double‐heterostructure light‐emitting diodes with a Si‐doped GaAs active region and Ga1−xAlxAs barrier regions have been measured as a function of the Si concentration in the growth solution and the thickness of the active region. The results indicate that a graded band‐gap region is probably formed at the GaAs&sngbnd;Ga1−xAlxAs interface. The double‐heterostructure light‐emitting diodes are compared with homojunction diodes as potential excitation sources for upconverting phosphors.