a memory device employing optoelectronic elements which in combination form a dynamic ram , the elements comprising first and second photodiodes electrically connected for exchange of a reverse bias condition from one photodiode to the other when either one is exposed to light to cause a photocurrent to flow , thereby to define logical ` 0 ` and ` 1 ` states , and an optical switch such as a mos structure connected in parallel with one of the photodiodes so as to be rendered light transmissive in only one of the reverse bias conditions , thereby to detect the logical state of the cell when exposed to incident light .