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Direct modulation lasers are attractive for short-to-mid range fiber communication system. Transistor lasers (TLs), which are hetero-bipolar transistor with an active layer in the base, have a potential to overcome modulation bandwidth limitation of conventional laser diodes (LDs). One of the reasons of the modulation bandwidth limitation of the conventional LDs is delay of carrier diffusion to quantum wells (QWs). With proper design of TLs with some current gain, this delay can be reduced and TLs can have wider modulation bandwidth under the emitter current modulation configuration. In the first part of the presentation, these theoretical analyses will be discussed. By theoretical analysis of TLs with rate equations taking account of carrier diffusion lifetime, 50 GHz modulation bandwidth can be expected with a current gain of ~1.7. Also, by applying voltage bias between the base and collector under the common-base configuration, which causes Frantz-Keldysh effect at the interface layer (GaInAsP in our case), internal loss can be controlled if the bandgap of layers at the interface is appropriately wider than the energy corresponding to the lasing wavelength. This is a kind of “built-in” electro absorption modulator. Therefore, voltage modulation, which corresponds to “loss” modulation, can be configured and has wider modulation bandwidth. The reason why loss modulation has wider modulation bandwidth is that the slope of decay curve of small signal modulation over a relaxation oscillation frequency is slow compared with that of conventional current modulation. In the second part, buried-hetero (BH) regrowth techniques of AlGaInAs QWs will be discussed. AlGaInAs TLs with lasing wavelength of 1.3 µm is very important for fiber communication system. By adopting proper annealing conditions in an organo-metallic vapor-phase-epitaxy (OMVPE) reactor, non-radiative recombination at the AlGaInAs/InP regrowth interface can be significantly reduced even after exposing in the air. Annealing gas, temperature, and the time are important parameters to find optimum annealing conditions. We found using PH3, instead of AsH3, as the annealing gas is effective to remove oxidation at the interface under 650°C, 45-min. annealing. Using this annealing condition, 1.3-µm AlGaInAs BH-LD was realized with a low threshold current density of 140A/cm2/well. Finally, the fabrication process and characteristics of 1.3-µm AlGaInAs TLs will be discussed. Several regrowth steps by MOVPE were required including above mentioned BH regrowth. CW operation up to the temperature of 40°C was demonstrated with both 2 and 3-terminal configurations. For 2-terminal configuration (this means the collector was floating), the threshold current of 33 mA was achieved with the cavity length of 1000 µm and stripe width of 1.7 µm. For 3-terminal configuration, the device shows optical characteristics (I-L) and transistor (collector current) characteristics, simultaneously, under common-emitter and common-base configuration. For the common-emitter configuration, threshold “base” current was reduced by applying voltage between the emitter and collector, On the other side, for the common-base configuration, we observed threshold “emitter” current increase by applying voltage between the base and collector due to Frantz-Keldysh effect. From transistor characteristics, we found Early effect also had an important role for TL characteristics.
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