An Ultrafast Diffusion-Driven Detector

Abstract

Numerous fast semiconductor photodetectors have been developed in the last decade. These devices have used short carrier lifetimes [1,2] or large electric fields and small geometries [3,4] to produce fast impulse responses. In contrast to these methods of making fast photodetectors, we describe a photovoltaic device whose operating speed is due to the diffusion of carriers. This device involves interfering light waves to form a grating of photoexcited carriers with a cos K x dependence. Such a distribution will decay with a time-constant of (D _a K ²)⁻¹ where D _a is the material’s ambipolar diffusion constant. Because K can be made very large, the response time can be made very short. For example, if the grating is formed in silicon, with optical beams of 1 μm wavelength having antiparal lel k-vectors, then the diffusion-driven detector response time will be 0.27 psec.