← IGCSE Computer Science Paper 1

Transistors as control gates and floating gates

A flash memory cell: a transistor with a control gate and a floating gate that traps charge for one bit value and none for the other, so the bit stays stored when power is off.

Part of: IGCSE Computer Science 0478 — 3.3 Data Storage — Module 15

Understanding Transistors as control gates and floating gates in IGCSE Computer Science Paper 1

What it does: Flash memory reads the presence or absence of charge in the floating gate as the two bit values, 0 and 1. Pits and lands belong to optical storage. Flash cells use trapped charge. Trapped charge stands for one value and no charge stands for the other. Both are valid and each stores one bit. Flash has no moving parts. Each transistor cell holds a single bit. The control gate is the connection the computer uses to control the transistor when it reads, writes or erases a bit.

Common mistakes: A flash memory transistor has two gates: a control gate and a floating gate. There is no 'power' gate. Line 1 should be gates = control, floating. The floating gate is insulated, so trapped charge stays there even when the power is off. It does not lose the charge. The line should be floating gate keeps charge.

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This module's key terms are also in a printable crossword (Modules 304–313) and word search (Modules 304–324).

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Learning
Duo
Which part of a flash memory cell traps electrical charge to store a bit?
The control gateThe floating gate
Matches
The control gate traps the charge that stores the bit
↓
It is the floating gate that traps the charge. The control gate only controls the transistor by applying voltage.
Applying
Meanings
Term
floating gate transistor
Meaning
A flash memory cell: a transistor with a control gate and a floating gate that traps charge for one bit value and none for the other, so the bit stays stored when power is off.
Practice
IF floatingGate = charged THEN
Testing
Gaps
The gate traps the charge
Check Answer
Spots
1gates = control, power
2IF charge is trapped THEN

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Quick Questions

1. Which part of a flash memory cell traps electrical charge to store a bit?
A. The floating gate ✓
B. The control gate
The floating gate is insulated, so it can hold trapped charge. The control gate is used to control whether charge is added or removed.
2. What does the control gate do in a flash memory cell?
A. Permanently stores the data as a magnetic pattern
B. Controls whether charge is moved into or out of the floating gate ✓
The control gate manages the charge. The data is held by charge in the floating gate, not by magnetism.
3. How does a flash cell store one of its two bit values?
A. Charge is trapped in the floating gate ✓
B. A tiny pit is burned into the surface
Pits and lands belong to optical storage. Flash cells use trapped charge.
4. A cell has no charge in its floating gate. What does this stand for?
A. The cell is broken and cannot store data
B. The other bit value to the one stored by trapped charge ✓
Trapped charge stands for one value and no charge stands for the other. Both are valid and each stores one bit.
5. What happens to the charge in the floating gate when the power is turned off?
A. It stays trapped, so the data is kept ✓
B. It drains away, so the data is lost
Charge staying in the floating gate is why flash memory is non-volatile.

Match the Pairs

control gate → The part of a flash transistor that receives the voltage used to write, read or erase the bit stored in the cell
floating gate → The part of a flash transistor that traps electrical charge so that the cell can hold a bit value
Trapped charge in the floating gate stands for one bit value, and no charge stands for the other → charge trapped = one value; no charge = other value
The charge stays in the floating gate when power is off, so flash memory is non-volatile → power off -> charge stays -> data kept
The control gate traps the charge that stores the bit → It is the floating gate that traps the charge. The control gate only controls the transistor by applying voltage.
When power is off, the floating gate loses its charge and the data is erased → Flash memory is non-volatile. The trapped charge stays in the floating gate when power is off, so the data is kept.

Key Term

floating gate transistor
A flash memory cell: a transistor with a control gate and a floating gate that traps charge for one bit value and none for the other, so the bit stays stored when power is off.

Fill in the Blanks

1. The floating gate traps the charge
Hint: Type the name of the gate that holds trapped electrical charge in a flash cell.
2. The control gate is used to read or write
Hint: Type the name of the gate that is used to read or write the cell.
3. Trapped charge = one bit value No charge = the other bit value
Hint: Type the short word that describes the cell when its floating gate holds nothing.
4. Charge stays in the cell when power is off
Hint: Type the power state in which flash memory still keeps its stored charge.
5. A cell is a transistor with two gates
Hint: Type the electronic component that makes up one flash memory cell.

Spot the Error

Question 1
A gates = control, power
B IF charge is trapped THEN
C bit = 0
D ELSE bit = 1
Line A — A flash memory transistor has two gates: a control gate and a floating gate. There is no 'power' gate. Line 1 should be gates = control, floating.
Question 2
A power = OFF
B floating gate loses charge
C bit stays stored
Line B — The floating gate is insulated, so trapped charge stays there even when the power is off. It does not lose the charge. The line should be floating gate keeps charge.
Question 3
A cell stores 1 bit
B transistor has 3 gates
C control + floating gate
Line B — A flash memory transistor has exactly two gates: the control gate and the floating gate. Line 2 should say 2 gates.
Question 4
A to store a 0:
B control gate sends charge
C floating gate drains it
Line C — To store a 0 the floating gate must trap the charge. If it drained the charge, nothing would be stored. Line 3 should be floating gate traps it.
Question 5
A // trapped charge = 0
B // no charge = 1
C IF floating gate is full THEN
D bit = 1
Line C — The comments say that no charge means 1. The condition for bit = 1 must therefore be an empty floating gate, not a full one. Line 3 should be IF floating gate is empty THEN.

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