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Technology Stocks : WDC/Sandisk Corporation -- Ignore unavailable to you. Want to Upgrade?

To: gdichaz who wrote (8118)	11/14/1999 9:06:00 PM
From: Artslaw	Read Replies (2) \| Respond to of 60323

How about a start at the beginning? What is a multi-level cell and what function does it perform? A multi-level cell is a cell that contains multiple levels. :) Flash and DRAM historically either store charge or they do not, representing a logical "true" or "false". A multi-bit cell contains several levels, normally factors of 2. So SanDisk's DD technology has four levels, which allows it to store two bits. Let's say it can go up to two volts (I have no idea what the "high" for SanDisk is). That might allow it to store the the logical values in the following voltage ranges: (completely made up--just an example) 1.7-2.0 -> 11 (true true, or 3) 1.3-1.6 -> 10 (true false, or 2) 0.9-1.2 -> 01 (false true, or 1) 0.0-0.8 -> 00 (false false, or 0) However, the bits aren't measured by the voltage stored, but rather the current that passes through the cell when they are read. Thus, you have to be able to differentiate between the current from a "11" and the current from "10". This is done with "sense amp" circuitry, not entirely unlike A to D converters. As you shrink the voltage range that represents the logical values, it become much harder to program those amounts (and make sure you aren't storing some other state). Over time, as you damage your gate oxide from the programming process, it becomes harder still, requiring special circuitry to verify the written value, etc, all of which takes silicon space. Steve