Abstract:

The development and application of a three-dimensional finite-difference time-domain (FDTD) model for a travelingwave heterojunction phototransistor (TW-HPT) are presented. The model is enhanced using effective permittivity schemes at the dielectric interfaces and special techniques for the treatment of very thin material sheets. The 3D fullwave electgromagnetic model allows the numerical calculation of the output photocurrent, electrical characteristic impedance, light absorption, microwave losses as well as microwave and optical dispersion. Run in a fast, parallel processing, machine the simulation herein allowed (for the first time, to the best of the authors' knowledge) the simultaneous investigation of the optical and microwave characteristics of the traveling-wave structure. This is in contrast to the approach followed by other researchers in the past, as well as by popular simulation packages, based on which results only for the microwave property can be obtained. Snapshots of the field propagation inside the device provide valuable insight into its passive behavior and clearly demonstrate the device's velocity mismatch between the optical signal and the photogenerated electrical pulse. Numerical results for the effective refractive indices of the optical and electrical wave quantify the difference in the velocities of the two waves.

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