Intuition The ONE core ideaEvery emerging memory (MRAM, ReRAM, PCM) stores a bit as a resistance — a physical, stubborn property of a material that stays put when the power dies. So before you can understand any of them, you only need to be fluent in three things: what resistance is, how current and voltage relate to it, and how heat comes out of a resistor. Master those and the whole topic is just "how do we flip a material between high-R and low-R?"
Before any symbol, let us name the players so no abbreviation is a mystery later.
Definition The memory acronyms — spelled out
SRAM = Static Random-Access Memory (fast, volatile, big) — see SRAM .DRAM = Dynamic Random-Access Memory (dense, volatile, needs refresh) — see DRAM .Flash = flash memory (non-volatile, slow to write, wears out) — see Flash Memory .MRAM = Magnetoresistive Random-Access Memory — bit stored as magnetic orientation.ReRAM (a.k.a. RRAM) = Resistive Random-Access Memory — bit stored as a conductive filament.PCM = Phase-Change Memory — bit stored as the atomic phase (crystalline vs amorphous) of a glass-like material.
The last three (MRAM, ReRAM, PCM) are the "emerging memories" of the parent topic; the first three are the old charge-based memories they aim to replace.
Old memories (SRAM, DRAM, Flash) store a bit as charge — a puddle of electrons that is either present or absent. Charge leaks, so those memories forget.
Emerging memories (MRAM, ReRAM, PCM) throw that away. They store a bit as how hard it is for electricity to flow through the cell . Two flavours:
Electricity flows easily → low resistance → call it one bit value.
Electricity flows poorly → high resistance → call it the other bit value.
Everything else in the parent note is machinery for setting and reading that resistance. So let us build every symbol from the ground up. Figure s01 below shows exactly this contrast — one cell that lets charges stream through, one that chokes them.
Figure s01 — Left (red): a low-resistance cell, many charge arrows pass through easily. Right (black): a high-resistance cell, only one arrow makes it. Same push, different flow — that difference is the stored bit.
I current is how many electric charges flow past a point each second . Picture: water flowing through a pipe — current is the number of litres per second passing a cross-section.
Unit: the ampere (A). Its symbol is the letter I intensité ).
Why the topic needs it: to flip a memory bit you push a current through the material; to read a bit you check how much current gets through.
Definition Sign convention (which way is "positive"?)By convention, positive current means positive charge flowing from the + terminal, through the cell, to the − terminal (electrons actually drift the other way, but we track the conventional direction). This matters for ReRAM/PCM writes, where reversing the current direction (opposite polarity) does the opposite job (SET vs RESET). Throughout this note we take the read/write current as flowing in that positive direction unless stated.
Sub-units you will meet:
V voltage is the electrical "push" that drives current . Picture: the height of a water tank — the taller the tank, the harder the water is shoved through the pipe below.
Unit: the volt (V). Symbol: V U V V r e a d across two points; we call the + terminal the higher-potential one. V r e a d > 0
The parent note writes things like V r e a d = 0.2 V se n se
This is the heart of the whole topic. Resistance is the thing the bit is stored in.
R resistance is how strongly a material fights the flow of current . Picture: a narrow, gravelly section of pipe — same push, less water gets through.
Unit: the ohm , symbol Ω 1 k Ω = 1 0 3 Ω 2 k Ω
Now the single most-used equation in the parent note:
Figure s02 — The red curve is I = V / R V = 1 R I
V = I R I = V / R { V , I , R }
To read a resistive cell you put it in series with a fixed resistor. "In series" means one after the other on the same wire, so the same current flows through both.
Definition Series & the load resistorR L two resistors in series share one current; the total resistance is their sum . Picture: two gravelly pipe sections joined end to end — the water fights through the first, then the second.
R L load resistor , a fixed reference resistance the designer adds. The cell R ce l l
Because the same current I voltage divider , drawn in Figure s03 :
Figure s03 — The read circuit: the source pushes V r e a d R ce l l R L V se n se I
Now the two states of a resistive cell get their standard names:
Definition LRS, HRS and the "memory window"
LRS = Low-Resistance State — the cell is a good conductor (small R L R S HRS = High-Resistance State — the cell blocks current (large R H R S Memory window = the ratio (or gap) between those two resistances , R H R S / R L R S how far apart the "0" and "1" readings are . A big window = the two V se n se
Worked example Sanity-check the divider extremesIf R ce l l → 0 V se n se = V r e a d ⋅ R L R L = V r e a d R ce l l → ∞ V se n se = V r e a d ⋅ ∞ R L = 0 R H R S / R L R S
MRAM's reads and ReRAM/PCM's writes all care about how much heat a current dumps into the material . That heat is what melts GST (the phase-change glass) in PCM.
P power is energy delivered per second . Unit: the watt (W) = joules per second.
Picture: how fast a kettle heats — high power = fast heating.
Definition Temperature riseΔ T C t h Δ T Δ how many kelvins hotter the material gets — a rise , not an absolute temperature.
C t h thermal capacitance : how much energy it takes to raise that little volume by one kelvin (unit: J/K). Big C t h E = C t h Δ T Δ T ≈ C t h E = C t h I 2 R t . Δ T ≈ 550
The parent note keeps taking ratios of resistances . A ratio answers "how different are these two numbers, relative to their size?" — which is exactly the question a sense amplifier asks.
R P R A P same way or opposite ways :
R P Parallel state (magnetizations aligned) — this is the low resistance.R A P Anti-Parallel state (magnetizations opposed) — this is the high resistance.
These are just the LRS and HRS of MRAM, given magnetic-specific names.
Definition Percent and relative change% 200% = 200/100 = 2 relative change compares a difference to a baseline:
old new − old . TMR (Tunnel Magnetoresistance ):
TMR = R P R A P − R P . WHY divide by R P To strip out the units (ohms cancel) and get a pure number you can compare between devices. Bigger TMR ⇒ the two states look more different ⇒ a bigger memory window ⇒ easier, more reliable reading.
Joule Heating P equals I squared R
Resistance Ratios TMR window
Emerging Memories MRAM ReRAM PCM
Self-test: cover the right side, answer, reveal.
Expand the acronyms MRAM, ReRAM, PCM. Magnetoresistive RAM, Resistive RAM, Phase-Change Memory.
What does the symbol I Electric current — charges flowing per second, measured in amperes (A).
What does the symbol V The electrical push (voltage), measured in volts (V).
What does R Resistance — how hard a material fights current — in ohms (Ω
State Ohm's law two ways and its key assumption. V = I R I = V / R R
In a series pair, what is shared between the two resistors? The same current I
Write the voltage divider for a cell R ce l l R L V se n se = V r e a d ⋅ R L + R ce l l R L
What is the "memory window"? The ratio/gap R H R S / R L R S
What are R P R A P MRAM's Parallel (low) and Anti-Parallel (high) resistance states.
In the Joule-heating section, what do E t E t
Why prefer P = I 2 R P = V I Because PCM writes are current-controlled, so we want power in terms of the current I
What is Δ T A change (rise) in temperature, in kelvins, not an absolute value.
What does C t h Thermal capacitance — energy needed to warm the tiny heated volume by 1 K (J/K).
Convert 33 μ A 33 × 1 0 − 6
What do LRS and HRS stand for? Low-Resistance State and High-Resistance State.
Express TMR = 200% 200% = 2.0 R A P 3 × R P
Next up: the parent topic itself → Emerging memories (MRAM, ReRAM, PCM) . Related deeper ideas: Memristor , Memory Hierarchy , In-Memory Computing .
