Diffusion capacitance and laser diodes

The well-known diffusion capacitance is critical in determining the modulation response of p-n junctions and particularly of laser diodes. In this brief, we investigate the diffusion capacitance of a diode, as a function of the physical length of the diode and the carrier lifetimes in the narrow active region. We show that diode length and lifetime together, and not just the lifetime (which is well known), determine the bandwidth of the diode.