CHAPTER 7 HOW LIGHT BECOMES DATA^115
Unlike ordinary memory, in which a single transistor is used to store either a 0 or a 1 and must
be constantly refreshed by a supply of electricity, flash memory uses two transistors on different
electrical lines that intersect each other to save a single 0 or 1 bit. One of these transistors is
called thecontrol gate and the other is called a floating gate.To write a 1 to the intersection
of the two transistors, an electri-
cal charge passes through the
floating gate, through a layer
of metal oxide that separates
the transistors, and on to the
control gate.2
Energy from the current also causes electrons to
boil off the floating gate and through the metal
oxide, which drains them of so much energy that
they cannot return.3
Abit sensor near each of these intersections
compares the strength of electrical charges on
both gates and, if the charge on the control gate
is at least half that of the floating gate (which it
would be after getting that transfusion of elec-
trons), the bit sensor proclaims that the value
stored there is a 1. If the sensor doesn’t detect a
significant charge, the bit is a 0.4
When it comes time to erase or rewrite the bits
stored in flash memory, a strong electrical current
flows through the circuitry to predetermined sec-
tions of the chip called blocks. The energy in
the blocks disperses the electrons so they are
again evenly spread among the gates.5
1
How Flash Memory Works with the Power Off
Number of Photos Based on Flash Card Capacities (Approximate)
Sensor Size File Size 16MB 32MB 64MB 128MB 256MB 512MB 1GB 2GB
2 Megapixel 900KB 17 35 71 142 284 568 1,137 2,275
3 Megapixel 1.2MB 13 26 53 106 213 426 853 1,706
4 Megapixel 2.0MB 8 16 32 64 128 256 512 1,024
5 Megapixel 2.5MB 6 12 25 51 102 204 409 819
6 Megapixel 3.2MB 5 10 20 40 80 160 320 640
8 Megapixel 3.5MB 3 9 18 36 73 146 292 565Source: Multiple Internet sites.