| 摘要 |
PROBLEM TO BE SOLVED: To quicken the response to injection of both the small and large quantities of light during reading, in photodiodes for writing. SOLUTION: Capacitance and resistance of a photodiode are reduced by using a laminated substrate, where a P-type epitaxial layer 142 is formed on a P-type semiconductor substrate 141, the layer 142 being doped more lightly than the layer 141. Thus, the response of the photodiode to small quantities of light during reading is improved. Furthermore, to prevent reduction of its response due to flattened potentials even if large quantities of light are incident during writing, the thickness of the layer 142 is reduced to limit the width of a depletion layer and increase electric field strength within the depletion layer. For example, the electric field strength within the depletion layer is set to 0.3 V/μm or larger, in view of the response to injection of large quantities of light required for a photodiode for writing (e.g. 6 times the writing speed). Furthermore, the required width of the depletion layer is set to 5μm or larger for the response to injection of small quantities of light required of the photodiode for writing (e.g. 32 times the reading speed).
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