Semiconductor Memory Types

Semiconductor Memory

• RAM

—   Misnamed as all semiconductor memory is random access

—   Read/Write

—   Volatile

—   Temporary storage

—   Static or dynamic

Dynamic RAM

• Bits stored as charge in capacitors


• Charges leak


• Need refreshing even when powered


• Simpler construction


• Smaller per bit


• Less expensive


• Need refresh circuits


• Slower


• Main memory

Dynamic RAM Structure

DRAM Operation

•   Address line active when bit read or written

—   Transistor switch closed (current flows)

• Write

—   Voltage to bit line

–     High for 1 low for 0

—   Then signal address line

–     Transfers charge to capacitor

• Read

—   Address line selected

–     transistor turns on

—   Charge from capacitor fed via bit line to sense amplifier

–     Compares with reference value to determine 0 or 1

—   Capacitor charge must be restored

Static RAM

• Bits stored as on/off switches


• No charges to leak


• No refreshing needed when powered


• More complex construction


• Larger per bit


• More expensive


• Does not need refresh circuits


• Faster


• Cache

Static RAM Structure

SRAM v DRAM

• Both volatile

—   Power needed to preserve data

• Dynamic cell

—   Simpler to build, smaller

—   More dense

—   Less expensive

—   Needs refresh

—   Larger memory units

• Static

—   Faster

—   Cache