Reverse-Bias-Dependence of Spectral Photoresponse of Si and GaAs Shallow p-n Junctions Near the Band Edge

Abstract

The spectral photoresponse of Si and GaAs photodiodes near the band edge has been measured with reverse bias up to nearly 100 V at room temperature. The measurement shows that the reverse voltage V raises the photoresponse ΔI, especially in the long-wavelength region, and hence also shifts the spectrum toward longer wavelength. At a fixed wavelength, ΔI is proportional to Vn, where, n decreases as the photon energy increases. For the step-type Si junction, n ranges from ⅕ to ½ as the photon energy changes from 1.6 (α−1=10 μ) to 1.15 eV (α−1=1000 μ); this voltage dependence may mainly result from the widening of the junction depletion-layer by the reverse bias. For GaAs photodiodes, the exponent n changes from ⅙ to 1.3 as the photon energy varies from 1.44 (α−1=1.6 μ) to 1.36 eV (α−1=1000 μ). The strong voltage-dependence of the photoresponse of a GaAs junction, which behaves like a p-i-n junction electrically, in the long wavelength region may result from the photoexcitation of electrons or holes to shallow levels in the pseudointrinsic region followed by some strongly field-dependent ionization process, such as tunneling, etc.