Uni-Traveling Carrier Photodiodes: Development and Prospects

The development of uni-traveling carrier photodiodes over the past 25 years is briefly reviewed and the key design in their structure are discussed. Monte Carlo simulation run for minority electrons locally excited in a p-type InGaAs structure reveals how they behave and leave the absorber. This shows the very diffusive nature of electrons, even in a moderately high quasi-field. Output linearity is carefully verified based on experimental THz-wave output vs. operation current. The first stage of saturation prior to severe field collapse is shown to be ruled by space charge density linearly changing with operation current. In the scaling down of a UTC-PD for faster operations, the space-charge effect is not significant, but junction self-heating is a factor imposing limitations.