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

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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.