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From: patel@uicsgva.csg.uiuc.edu
Newsgroups: comp.arch
Subject: Re: Questions about DRAM's
Message-ID: <1600020@uicsgva.csg.uiuc.edu>
Date: 10 Apr 90 00:29:16 GMT
References: <28686@cup.portal.com>
Lines: 45
Nf-ID: #R:cup.portal.com:28686:uicsgva.csg.uiuc.edu:1600020:000:1942
Nf-From: uicsgva.csg.uiuc.edu!patel    Apr  9 16:07:00 1990


  I am not a VLSI or Memory Expert but some of this information you
can find in literature on memory.  Most likely, this note is not being
read by RAM designers, so I thought I should clear up the picture on
RAM.

DRAMs:
Present day DRAMs use a 3-D capacitor, called Trench Capacitor.
A Trench Capacitor has a small footprint on chip but effectively
gives you a larger surface area.  By contrast a 2-D (planar)
capacitor has an effective area equal to its footprint.
From Physics/EE 101 everyone should recall that the capacitance is
proposrtional to the area of the parallel plates.

       2-D Capacitor           Trench Capacitor
      ---~~~~~---                ---     ---
      ---~~~~~---                --\\   //--
                                    \\_//
                                     \_/

So the trecnch capacitor has a capacitance larger than an
equal-footprint planar capacitor.  You need a large capacitor 
1. To hold the data for reasonable time before you need a refresh.
Charge retention depends heavily on your process, how good is your
manufacturing process to hold the leakage currents to a minimum etc.

2. To tell the difference between charge (1) and no-charge(0)!
In fact this is a very serious problem.  Unlike ordinary logic
levels, (0 for < 1 volt and 1 for > 2.5 volts) you have to sense
very small voltage swings between a logic 0 and 1.  To sense the
presence of an extremely small charge against various odds
(noise, leakage, long wires, temperature variations etc.) is
no small feat.  Design of sense amps is considered a
black art.

SRAMs:
Static RAM cell does not need a capacitor to work.
Static RAMs canont be scaled down arbitrarily for the same reason
as DRAM can't.  The same problem exists with small transitors
as with small capacitors, namely, driving long lines and
reliably differentiating between 1 and 0 at the sense amp.

-J. Patel
Univ. of Illinois, Urbana-Champaign
patel@csg.uiuc.edu