How does a memory card remember with its power off?

A photograph remains on a memory card after power is removed because trapped charge preserves the electrical states that encode its bits. Flash cells read those states through their effect on transistor conduction. A floating-gate transistor is one design; many newer cells trap charge within regions of an insulating layer instead.

The storage region is surrounded by insulation, so electrons placed there do not have an ordinary conducting path to escape. Their electric field can still act through the insulator. In a floating-gate example, added negative charge makes it harder for the control gate to attract enough electrons into a conducting channel. More control voltage is then needed to switch that channel on.

This required control voltage is the threshold voltage. Reading tests the cell’s conduction under chosen voltages and assigns a bit from the result. The drawing supplies thresholds of 1 volt without the added electrons and 4 volts with them. At a 2-volt test, the first state conducts and the second does not. The labels 1 and 0 are a coding convention.

Programming and erasing use controlled electrical conditions that move charge across a very thin insulating barrier. Reading uses a different operation to sense the existing state. With power off there is no active read signal, but the stored charge can remain. Retention therefore does not require a continuously running circuit.

Insulation preserves charge without guaranteeing permanence: charge can escape or redistribute, and repeated programming and erasing can alter cell behavior. Real flash manages errors and may encode several bits using multiple threshold ranges. The binary diagram explains retained electrical states without specifying endurance or retention time; reading senses a memory whose storage does not require a running circuit.