Patent ID: 8283662

Claim:
A memory device comprising: a first memory cell; and a second memory cell, the first memory cell comprising a first transistor and a first memory element, the first transistor comprising a first channel, a first gate electrode, and a first source and drain electrodes, wherein the first gate electrode is part of a first word line or is electrically connected to the first word line; and wherein one of the first source and drain electrodes is part of a first bit line or is electrically connected to the first bit line and the other is electrically connected to the first memory element, the second memory cell comprising a second transistor and a second memory element, the second transistor comprising a second channel, a second gate electrode, and a second source and drain electrodes, wherein the second gate electrode is part of a second word line or is electrically connected to the second word line; wherein the second channel is formed of an oxide semiconductor film; and wherein one of the second source and drain electrodes is part of a second bit line or is electrically connected to the second bit line and the other is electrically connected to the second memory element, wherein a threshold voltage (V 21 ) of the second transistor before ultraviolet ray irradiation is higher than a threshold voltage (V 1 ) of the first transistor; wherein a threshold voltage (V 22 ) of the second transistor at the time of ultraviolet ray irradiation is lower than the voltage V 21 ; wherein a threshold voltage (V 23 ) of the second transistor after ultraviolet ray irradiation is lower than the voltage V 21 and higher than the voltage V 22 and the voltage V 1 ; wherein data stored in the second memory element is read when a voltage (V G ) is applied to the second gate electrode of the second transistor which is being irradiated with ultraviolet rays so that the second transistor is turned on; and wherein V 1 ≦V G (the voltage V G is equal to or higher than the voltage V 1 ), and V 22 ≦V G <V 23 <V 21 (the voltage V G is equal to or higher than the voltage V 22 ) are satisfied.