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Nitrogen incorporation into GaAs has received much attention in the last decade, because of its application to long-wavelength lasers. However, nitrogen incorporation into GaP (100) has not received much attention to date despite the promising application of this material system to yellow-amber-red light-emitting diodes. In order to investigate the not yet well-studied (Al,In,Ga)NP material system, we use gas-source molecular beam (MBE), in which nitrogen radicals are used as nitrogen precursor, to grow these mixed group-V alloy semiconductors with excellent crystallinity. This dissertation is divided into two major parts. In the first part we describe the growth and characterization of the (Al,In,Ga)NP material system. Optical and structural properties of GaNP bulk layers, AlGaNP bulk layers, and InGaNP quantum wells are studied. The dependence of the GaNP band gap vs. nitrogen concentration and temperature dependent PL are analyzed. For AlGaNP layers, using a thermodynamic approach we explain the difference between nitrogen incorporation into GaP and AlP. The dependence of the emission wavelength vs. nitrogen and indium compositions is studied for InGaNP QWs. The electron effective mass is determined for InGaNP materials with different indium concentration. The conduction and valence band offsets are calculated for the InGaNP/GaP heterojunction. In the second part, we describe LED chip fabrication and contacts optimization. development of n-type and p-type contacts is discussed. A description of LED chip processing optimization is given for a p-i-n diode structure. The band offsets are compared for (Al,In,Ga)NP-based LED structures and conventional AlInGaP-based LED structures; they are 2-3 times higher in LEDs based on the (Al,In,Ga)NP material system. Growth and fabrication results for bulk GaNP-based amber LEDs are discussed. Color stability (electroluminescence peak wavelength shift vs. current) is compared for GaNPbased LEDs and AlInGaP-based LEDs; the wavelength shift of (Al, In,Ga)NP-based LED chips is ̃ 6 times less than that of AlInGaP-based LED chips, in the drive current range of 10 - 60 mA. The influence of In concentration in InGaNP QWs on EL properties of LED chips is reported. Single and multiple InGaNP QW-based LEDs are studied