Technology CAD simulation is helping designers of CMOS image sensors meet the challenges presented by increasing device complexity and decreasing pixel size.

Technology computer aided design (TCAD) of semiconductor devices exhibits the advantages of reduced development costs and development time. In this worka TCAD methodology has been developed for high-speed photodetectors. The calibration procedure for fixing the free parameters in the physical models employed in the simulation has been illustrated for a commercially available InGaAs/InP p–i–n photodetector. This approach has been illustrated using a specific example where the task was to optimize the absorption layer thickness of a novel photodetector structure.

In this paper, the spectral responsivity of a photodetector is determined by extracting optical parameters (refractive index and extinction coefficients) from the reflectivity measurement data and by treating the surface recombination velocity as a free parameter. A transfer matrix method based model has been implemented to compute the optical fields within the device. The electrical simulation was performed using ISE TCAD simulation package.

A Fokker–Planck equation for carrier transport in semiconductors is derived from the Boltzmann transport equation by expanding in Legendre polynomials and assuming the phonon energy exchanged at momentum randomizing collisions is small compared with the mean carrier energy. The method is used to compute impact ionization probability distributions in space and time and the results agree well with those generated by an equivalent Monte Carlo model over a wide range of electric fields from 300 kV/cm to 1 MV/cm.

The probability-distribution function (PDF) for impact-ionisation path length is a crucial quantity for understanding and modelling the low-noise behaviour of avalanche photodiodes with short multiplication regions. In these devices the high electric fields needed to produce avalanche narrow the PDE reducing the randomness in ionisation position and hence the noise in the multiplication. A simple method is presented for calculating PDFs using the 'lucky-drift' model. The results are compared with those predicted by corresponding Monte Carlo calculations employing a parabolic energy band deformation-potential optical-phonon scattering and hard-threshold impact ionisation. The overall behaviour expected for the PDF is reproduced by the simple model. However the unphysical, catastrophic assumption made for energy-relaxing collisions in lucky drift results in an unphysical delta function in the PDF which carries a significant weight at high electric fields.

The probability distribution function (PDF) for impact ionization path length is a crucial quantity for understanding and modeling the low noise behavior of avalanche photodiodes with short multiplication regions. In such devices the ionization coefficient is no longer in equilibrium with the local electric field but depends on the carrier’s history. The high electric fields needed to produce avalanche gain narrow the PDF, thereby reducing the randomness in ionization position and hence the noise in the multiplication. In this article we present a method for calculating PDFs using a Fokker–Planck model. The results are compared with those obtained from an equivalent Monte Carlo simulation employing a parabolic energy band, deformation potential optical phonon scattering, and a hard energy threshold for impact ionization.

The impact ionization probability functions, P(x), are effectively histograms of the positions carriers ionize along the field direction, x, which determine the avalanche multiplication properties of a semiconductor device. In this work, we use a Monte Carlo model to investigate the form of these as multidimensional functions; in not just x, but also of the elapsed time, t, and the distance traveled perpendicular to the electric field direction, y. Despite most previous temporal calculations of the avalanche process assuming that all carriers travel at the drift velocity, vd, it is shown that electrons which ionize at the shortest distances travel several times faster than vd. There is also a significant spread in possible velocities with which ionizing carriers travel along the x direction due to diffusion. Diffusive spreading of the ionization probability in both x and y is also described.