Page 211 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

192 w Chapter Sixteen

Source Control gate Drain
Source Control gate Drain terminal terminal terminal
terminal terminal terminal I
Silicon -
I I I
-

-
Silicon -
dioxide

(su bstrate)

M05 Transistor EPROM Transistor
Figure 16-20. Standard MOS transistor versus EPROM transistor

the normal operation of the control gate. To program the transistor, a relatively
high voltage in the order of 12V is applied between the control gate and drain
terminals. This causes the transistor to be turned hard on, and energetic electrons
force their way through the oxide into the floating gate in a process known as
hot (high energy) electron injection. When the programming signal is removed, a
negative charge remains on the floating gate. This charge is very stable and will
not dissipate for more than a decade under normal operating conditions. The
stored charge on the floating gate inhibits the normal operation of the control
gate, and thus distinguishes those cells that have been programmed from those
which have not.
EPROM cells are efficient in terms of silicon real estate, being half the size
of DRAM cells and an order of magnitude smaller than fusible links. However,
their main claim to fame is that they can be erased and reprogrammed. An
EPROM cell is erased by discharging the electrons on the floating gate. The
energy required to discharge the electrons is provided by a source of ultraviolet
(UV) radiation. An EPROM device is delivered in a ceramic package with a
small quartz window in the top; this window is usually covered with a piece of
opaque sticky tape. For the device to be erased, the tape is first removed from
the circuit board, the quartz window is uncovered, and the package is placed in
an enclosed container with an intense ultraviolet source.
The main problems with EPROM devices are their expensive ceramic
packages with quartz windows and the time it takes to erase them, which is in
the order of 20 minutes. A foreseeable problem with future generations of these