Intuition The one core ideaAll ROM-family memory answers a single question: how do we make a switch that remembers its position with the power switched off, and who is allowed to flip it? Every symbol below — voltage, charge, threshold, oxide, exponential leakage — exists only to describe one tiny switch that stays put.
Before you can read the parent note on ROM, PROM, EPROM, EEPROM , you need to understand each word and symbol it throws at you. This page builds them from nothing, in an order where each idea leans on the one before it.
A bit is a single yes/no answer: a 0 1 one of two physical states of some tiny device — high or low, connected or broken, charged or empty.
The whole memory game is: find a physical thing with two stable states , and agree that one state means 0 1
Think of a light switch on a wall. Up = on, down = off. It has exactly two positions and it stays where you left it — you can leave the room and come back and it hasn't moved. That "staying put on its own" is the property we chase throughout this topic.
Why the topic needs this: a memory chip is millions of these two-state switches. Every difference between ROM, PROM, EPROM and EEPROM is just a different way of building the switch and a different way of flipping it .
Definition Volatile / Non-volatileVolatile memory forgets everything the moment power is removed. Non-volatile memory keeps its stored bits with no power at all .
The wall switch above is non-volatile : no electricity needed to remember its position. Compare that to a person you ask to "keep holding this button" — the instant they get tired (power off), the button pops back. That tired-button behaviour is what RAM does.
Non-volatile is the whole reason the ROM family exists. When you press the power button, the CPU wakes up knowing nothing . Something must have already remembered "here is how to boot." That something must survive being powered off — see Boot Firmware and BIOS and Memory Hierarchy for where this sits in a real machine.
Deeper background: Volatile vs Non-Volatile Memory .
Definition Voltage, symbolV Voltage is the amount of electrical push between two points, measured in volts . The symbol is the single letter V V
Picture water in a tank raised high above the ground. The height of the water is like voltage: the higher it sits, the harder it shoves water out of a pipe at the bottom. Voltage is "electrical height." A small V V
Why the topic needs it: writing to a floating-gate cell requires a big push (high V reading uses a small , gentle V
Definition Charge, symbolq Charge is the "stuff" that voltage pushes around — carried by electrons , each carrying one tiny fixed unit of negative charge q
Electrons are like marbles. Voltage rolls them downhill. If you can roll a batch of marbles into a sealed box with no exit , they just sit there forever — and their mere presence changes how the surrounding device behaves. That sealed box is the coming "floating gate," and the trapped marbles are your stored bit.
Why the topic needs it: EPROM and EEPROM store a bit as "electrons present" vs "electrons absent" on an isolated gate. No electrons = one logic value, trapped electrons = the other.
A MOSFET is a tiny electronic switch with three terminals: source , drain , and gate . Current can flow from source to drain — but only if the voltage on the gate is high enough. The gate is a controlling "faucet handle" that never touches the water; it works purely through its electric field.
Look at the figure. Source and drain are two docks separated by a gap (the "channel"). The gate hovers above the gap. When you raise the gate voltage past a certain point, its field pulls a conducting bridge into existence across the gap — current flows. Below that point, no bridge, no current. It's a switch operated by voltage, not by a mechanical finger.
Why the topic needs it: every memory cell in this family is (or contains) a transistor like this. "Reads as a 1 or a 0" literally means "conducts or doesn't conduct when we apply the read voltage."
Deeper: MOSFET and Threshold Voltage .
Definition Threshold voltage, symbolV t h threshold voltage V t h exact gate voltage at which the MOSFET switches from OFF to ON . Below V t h V t h
V t h tipping point of the faucet handle — the precise angle where water starts to flow. Every transistor has one. Here is the crucial trick the whole topic hangs on: if you can secretly change a cell's V t h
Read the figure carefully. We always read with the same gate voltage V r e a d
If the cell's V t h low (below V r e a d
If trapped electrons have pushed V t h high (above V r e a d V r e a d
Definition Floating gate + oxideA floating gate is a second gate added inside the transistor that is completely wrapped in an insulator called oxide (a glass-like layer, thickness written t o x trapped , because oxide blocks their escape.
Recall the sealed box of marbles from §4 — the floating gate is that box, and the oxide is its walls. It sits between the control gate (the handle you touch) and the channel. Any electrons you manage to sneak inside can no longer leave, so they keep shifting V t h That is non-volatility, built from geometry.
Symbols to know here:
t o x thickness of the oxide (how thick the walls of the box are).ϕ B phi , subscript B) = barrier height = how "tall" the insulating wall is that an electron must climb over or tunnel through.
Deeper: Floating-Gate Transistor .
The parent note's scary formula uses exp ℏ
Tunnelling is a quantum effect where an electron can pass through a thin wall it doesn't have the energy to climb over — like a ball occasionally appearing on the far side of a hill it never crested. The thinner the wall, the more often it happens.
Definition The exponential functionexp ( x ) exp ( x ) e x changes extremely fast . Make x exp ( x ) huge factor. It is the natural language for "leakage that dies off dramatically."
Intuition Why an exponential and not a straight line?We use exp decays multiplicatively with distance: each extra atomic layer of oxide cuts the escape chance by the same fraction , not the same amount . Repeatedly multiplying by a fraction is exactly what exp ( − something × t o x )
The symbols in the parent's leakage formula, in plain words:
Deeper: Fowler–Nordheim Tunneling .
A fuse is a thin metal thread that conducts when intact but permanently melts open if you push too much current through it. Intact = one bit value; melted = the other.
Same idea as the fuse in a household plug: shove enough current and the wire vaporises, breaking the circuit for good. In PROM you deliberately blow selected fuses to write your data — once. There is no un-melting, which is exactly why PROM is one-time programmable.
Why the topic needs it: PROM stores bits with no floating gate at all — it's the simplest, crudest two-state switch (connected / broken). Contrast this with the reversible charge trick of §4–8.
Definition Write cycle & enduranceA write cycle is one erase-and-rewrite of a cell. Endurance is the maximum number of write cycles a cell survives before the oxide wears out. Written as powers of ten: 1 0 5 1 5 = 100 , 000
1 0 n 1 0 n 1 n 1 0 4 = 10 , 000 1 0 5 = 100 , 000 1 0 6 = 1 , 000 , 000
Why the topic needs it: every write shoves electrons through the oxide (§8), and each pass slightly damages those insulating walls. So even "non-volatile" memory eventually dies — the parent note's endurance table and wear-leveling example are built entirely on this idea.
Voltage V = electrical push
Charge q = trapped electrons
Endurance and write cycles
Read it top-down: the plain ideas (bit, voltage, charge) feed the device ideas (MOSFET, floating gate, V t h
Test yourself — each line hides its answer.
A "bit" stored in hardware is physically… one of two stable states of a tiny device (connected/broken, charged/empty).
Non-volatile memory means… it keeps its stored bits with no power at all.
Voltage V electrical "height" or push — like water raised in a tank.
Charge q electrons; trapped electrons are the marbles-in-a-sealed-box that store a bit.
A MOSFET conducts only when… its gate voltage rises above the threshold V t h
Threshold voltage V t h the exact gate voltage where the transistor flips from OFF to ON.
Trapped electrons on a floating gate change the bit by… raising V t h
The floating gate stays charged for years because… oxide insulates it; leakage tunnels out only exponentially slowly.
We use exp ( − … ) tunnelling probability decays multiplicatively (by a fixed fraction) per extra oxide layer.
A fuse stores a bit as… intact (conducting) vs permanently melted-open (broken) — one-time only.
1 0 5 100,000 rewrites before the oxide wears out.
