Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: programming EPROMS
Message-ID: <1991May27.152421.4122@zoo.toronto.edu>
Date: Mon, 27 May 1991 15:24:21 GMT
References: <1991May26.231046.28772@comp.vuw.ac.nz>
Organization: U of Toronto Zoology

In article <1991May26.231046.28772@comp.vuw.ac.nz> cyborg@kauri.vuw.ac.nz (Alex Ivopol) writes:
>... Could someone please explain to me why the
>EPROM needs such high voltage (12.5, 21 or even 25 volts) just to erase
>certain bits. How exactly is this voltage used in the EPROM to erase bits ?

A programmed bit in an EPROM has electrons sitting on a "floating gate", an
electrode that is electrically isolated from the rest of the world by a
very good insulator (ultrapure glass, essentially).  The insulator has to
be good, because if it lets the electrons leak out, the bit flips back to
the unprogrammed state.  You erase EPROMs by exposing them to intense
short-wave ultraviolet because it kicks up enough ionization in the
insulator to let the electrons leak out.  Of course, to program the thing,
you have to somehow get the electrons *in*.  Speaking sloppily, this is
done by punching them through the insulator by brute force.  Hence the need
for a high programming voltage.
-- 
"We're thinking about upgrading from    | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 to SunOS 3.5."              |  henry@zoo.toronto.edu  utzoo!henry