DRAM
What is the main reason 3D DRAM is hard to achieve simply by stacking many layers as in 3D NAND?
- ADRAM cannot rewrite data
- BDRAM cell transistors can only be made of silicon
- CEach cell needs a capacitor to store charge, and today's capacitors are very tall vertical structures
Show answer and explanation
Answer: C. Each cell needs a capacitor to store charge, and today's capacitors are very tall vertical structures
3D NAND stacks hundreds of word-line layers and etches vertical holes through them at once to form strings of cells. Each DRAM cell needs a capacitor to store charge, and planar DRAM secures capacitance by making the capacitor thin and deep, so it cannot simply be stacked. For 3D DRAM, researchers are studying structures that lay the transistor and capacitor on their sides in each layer, and capacitorless cells that hold charge on the parasitic capacitance of oxide-semiconductor transistors such as IGZO. Sideways cells take more area per layer, so stacking many layers at low cost is the key.
- A. DRAM can be freely read and written.
- B. Cells using oxide-semiconductor transistors such as IGZO are also being studied.
- C. Correct. The shape of the capacitor makes stacking difficult.