Design and simulation of integration of vertical cavity surface emitting lasers and heterojunction bipolar transistor

Abstract

Vertical cavity surface emitting lasers (VCSELs) are widely used in the field of short-range optical communication and optical interconnection because of their advantages such as low threshold current, large modulation bandwidth, easy two-dimensional integration, easy coupling with optical fibers and low cost. The VCSELs and heterojunction bipolar transistor (HBT) are longitudinal current devices, so they can be well integrated on the same wafer, and the output light power can be modulated by the HBT base current. Integration of VCSELs and HBT are designed in this paper. The VCSELs and PNP InGaP/GaAs HBT form a direct series structure. The reflectivity of DBR is 99.72% at a resonant wavelength of 850 nm and 99.57% after adding HBT separately. Therefore, the addition of HBT has little influence on the reflectivity of DBR at the resonant wavelength. The electro-optical characteristics of the integrated structure are simulated by using PICS3D software. An interim collector is added into the model in order to ensure that the simulation can be carried out smoothly. Firstly, HBT is conducted and the current flows from the emitter to the interim collector. Then, the voltage across the interim collector and the N-type electrode is increased to make VCSELs conducted and the current of the transition collector drop to zero. The interim collector can be removed from the actual structure because the current is zero. The simulation results show that the current gain coefficient is 400, and the maximum modulation rate of the base current to the output light power rises up to 280 mW/mA. The maximum temperature in the active region increases with the base current increasing, and the output light power first increases and then tends to be saturated. The ac optical gain characteristics of the integrated structure is simulated by PICS3D, and the simulation result shows that cutoff frequency exceeds 1 GHz. The addition of HBT limits the modulation rate of the integrated structure, and further optimization of HBT structure parameters and geometric dimension are needed to improve the modulation rate. The integrated structure and simulation method established in this paper can also be used to integrate LED, LD, DFB or other luminescent devices with HBT.