Photodiode having voltage tunable spectral response

Abstract

A photodetector ( ) includes a substrate ( ) having a surface; a first layer ( ) of semiconductor material that is disposed above the surface, the first layer containing a first dopant at a first concentration for having a first type of electrical conductivity; and a second layer ( ) of semiconductor material overlying the first layer. The second layer contains a second dopant at a second concentration for having a second type of electrical conductivity and forms a first p-n junction ( ) with the first layer. The second layer is compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap in a direction away from the p-n junction. The compositional grading can be done in a substantially linear fashion, or in a substantially non-linear fashion, e.g., in a stepped manner. Preferably the first dopant concentration is at least an order of magnitude greater than the second concentration, and more preferably is at least two orders of magnitude greater. When the first p-n junction is reverse biased, a depletion region ( ) exists substantially only within the second layer, and varying the magnitude of the reverse bias shifts the wavelength at which a maximum spectral sensitivity or responsiveness is obtained. At least one electrical contact is provided for coupling the second layer to a source ( ) of variable bias voltage for reverse biasing the p-n junction. As the magnitude of the bias voltage is changed a wavelength of electromagnetic radiation to which the photodetector is responsive is thus changed. An alternating current signal can be superimposed on the reverse DC bias voltage and a synchronous detection technique used to detect photons corresponding to a certain bandgap energy.