Advanced memory concepts for DRAM and non-volatile memories | Digital Circuits - Electronics and Communication Engineering (ECE) PDF Download

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Introduction

As gadgets get smaller and we try to pack more memory into them, it’s getting tough because tiny transistors don’t work as well and making super-small patterns is hard. These problems push people to try new ideas, like building memory in 3D (stacking it up instead of flat). For DRAM (the main computer memory), special transistor designs—like fin-shaped or surrounding gate ones—can help. For flash memory and FeRAM (other types of memory), connecting cells in a chain is a cheap way to go. For newer MRAM (magnetic memory), crisscross layouts and stacking layers are big solutions.

Section Snippets

Advanced DRAM Technologies

When DRAM cells shrink below 100 nanometers (super tiny), it’s hard to keep the transistor strong enough to carry a good current. The channel (where current flows) is narrow, and adding more stuff to it slows down the electrons. Plus, old tricks to make transistors smaller don’t work here—you can’t thin the gate oxide (a key part) because it needs to block leaks to hold data, which limits how small things can get.

TIS/Fin/SGT DRAM

To fix these transistor problems, new designs like trench-isolated sidewall gates (TIS) or fin-shaped transistors (Fin) can boost performance—like how some fancy silicon-on-insulator (SOI) transistors work. Picture TIS as a transistor with a top gate and side gates (like a double-door setup), letting more current through when it’s “on” and blocking leaks when it’s “off.” These tricks help DRAM work better as it shrinks.

Floating Body Cell

The big headache with DRAM is keeping the capacitor (the part that holds the charge) big enough as cells get smaller. Making capacitors is tricky—they’re either dug into trenches or stacked up high. A new idea, called Floating Body Cell (FBC), skips the capacitor altogether. It stores the charge right in the transistor’s body instead. With just one transistor per cell, it’s simpler and takes up less space.

NAND Flash Memory

NAND flash is used for storing files—like photos on a camera or songs on a phone. It’s growing faster than NOR flash (used for phone program memory) because everyone wants memory cards, USB drives, and MP3 players. The NAND market is expected to boom soon thanks to all these gadgets.

FeRAM

FeRAM (Ferroelectric RAM) saves data using a special capacitor that holds a charge even when power’s off. A fancy 64 Mb FeRAM cell is super small (0.602 square micrometers) because it chains pairs of capacitors together and uses a simple etching trick to shrink it down.

MRAM

MRAM (Magnetic RAM) stores data by pointing tiny magnets in a layer either the same way or opposite to another layer. When they line up (parallel), resistance is low; when they don’t (anti-parallel), it’s high. New layouts—like crisscross designs—make cells smaller (down to 6F²) and stop sneaky currents from messing things up.

Multi-Layered MRAM

Lately, MRAM with magnesium oxide (MgO) barriers got a huge resistance boost (260%), and some cells now use two magnetic junctions together. Stacking MRAM in layers (multi-layered) could make it cheaper and better.

PRAM and Other Emerging Memories

PRAM (Phase-Change RAM) works by heating a special glass to switch it between a jumbled (amorphous) or neat (crystalline) state with electric pulses. It’s a top pick for cheap, nonvolatile memory (stuff that doesn’t forget when power’s off) like flash. Other new ideas, like RRAM (Resistive RAM) or organic memory, might take over when cells shrink even smaller than DRAM or flash can handle.

Conclusion

DRAM is still the king of memory with the biggest market, and it’s getting bigger and better for low-power or high-speed uses. But as shrinking gets tough, new transistor shapes—like TIS or fin designs—will keep it going. Even fancier vertical transistors (like surrounding gate ones) will help with tiny cells. The capacitor-less Floating Body Cell (FBC) could be the next big thing for super-dense embedded DRAM. All these ideas keep memory growing as gadgets shrink.