In this paper, we report the fabrication and characterization of 1.3-mum AlGaInAs/AlGaInAs laser diodes (LDs) with an n-type modulation-doped strain-compensated multiple-quantum-well (MD-SC-MQW) active region and a linearly graded index separate confinement heterostructure. The barrier in the MD-SC-MQW active region contains the 28 Angstrom Si-doped modulation-doped region and two 29 Angstrom surrounding undoped regions that serve to prevent the overflow of Si doping atoms into the wells. We investigate the threshold current density, infinite current density, differential quantum efficiency, internal quantum efficiency, internal optical loss, threshold gain (for the cavity length of 300 mum), and transparency current density as a function of doping concentration in the n-type AlGaInAs barrier for the 1.3-mum MD-SC-MQW LDs. The theoretical and experimental results show that the optimum doping concentration of doped barriers is 5 x 10(18) cm(-3). With this optimum condition, the 3.5-mum ridge-striped LDs without facet coating will exhibit a lower threshold current and a higher differential quantum efficiency of 18 mA and 52.3% under the CW operation as compared to those of 22 mA and 43% for the undoped active region, respectively. In addition, a high characteristic temperature of 70 K, a low slope efficiency drop of - 1.3 dB between 20 and 70 degreesC, and a wavelength swing of 0.4 nm/degreesC for the LDs operated at 60 mA and 8 mW can be obtained in the LDs with doped barriers.