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Light emitting transistors (LETs) and transistor lasers (TLs) are newly-emerging optoelectronic devices capable of emitting spontaneous or stimulated light while performing transistor actions. This dissertation describes the design, growth, and performances of long wavelength LETs and TLs based on InAlGaAs/InP material system. First, the doping behaviors of zinc (Zn) and carbon (C) in InAlGaAs layers for p-type doping were investigated. Using both dopants, the N-InP/p-In0.52(AlxGa1-x)0.48As/N-In0.52Al0.48As LETs with InGaAs quantum wells (QWs) in the base demonstrate both light emission and current gains (β). The device performances of Zn- and C-doped LETs have been compared, which is explained by a charge control analysis involving the quantum capture and recombination process in the QWs. A TL based on a C-doped double heterostructure (DH-TL) with single QW was designed and fabricated. The device lases at 77 K with a threshold current density (Jth) of 2.25 kA/cm2, emission wavelength (λ) at ~1.55 µm, and β of 0.02. The strong intervalence band absorption (IVBA) is considered as the main intrinsic optical loss that prohibits the device from lasing at room temperature. Based on a threshold condition analysis taking into account the strong IVBA, it is found that room-temperature lasing of a DH-TL is achieved only when the base thickness and doping level are within a specific narrow range and improved performance is expected in a separate confinement heterostructure (SCH) TL.