Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 CHAPTER 6 Semiconductor Memories

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 CLASSIFICATION OF SEMICONDUCTOR MEMORIES Semiconductor memories volatileNon-volatile loose their data once the power supply is turned off. SRAM, DRAM ROM, EPROM can retain their data even after power is removed.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Access parameters Read-access time Propagation delay from the time when the address is presented at the chip to the time data is available at the output. Cycle time Minimum time between initiation of a read operation and the initiation of another operation.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM a static memory. it is bistable. two inverters continuously maintain the value of the bit, as long as power is on. 6 transistors are required to store 1 bit (or 4 transistors + 2 registers).

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM  High-speed Low capacity Expensive Large chip area. Continuous power use to maintain storage

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM Single cell stores single bit. 4T+2R design (old) 6T design 4T+2R design (old) 6T design

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM Two SRAM cells dominate CMOS industry 6T Cell all CMOS transistors better noise immunity 4T Cell + 2R replaces pMOS with high resistance (~1GΩ) resistors slightly smaller than 6T cell requires an extra high-resistance process layer

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM 4T+2R Word line Asserted: connects to complementary bit lines.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 6T Cell Design Challenge

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM Cell Layout

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM A Resistor-Transistor pair divide voltage between V cc and GND T2 high resistance: –A close to V CC T2 low resistance –A close to Gnd.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM T2 high impedance: –A close to V CC –T 3 enabled –T 3 low impedance –B close to Gnd T2 low impedance –A close to Gnd. –T 3 disabled –T 3 high impedance –B close to V CC AB

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM Two stable states. Asserted word line sends complimentary values to the two bit lines. –This is the stored bit. –Bitline 0 contains bit –Bitline 1 contains inverse of bit

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM There is always a current through one of the transistor- resistor pairs. Use transistors instead of resistors to save energy. However, transistors can use up more space.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM (6 T) Cell consists of two lines of transistors, dividing the voltage between V CC and GND Cross-coupled. T2 in high impedance  T5 in low impedance T2 in low impedance  T5 in high impedance

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM (6 T) A B Assume T2 high impedance, T5 low impedance. –Point A ~ V CC –T3 in low impedance and T6 in high impedance –Point B ~ GND –T2 in high impedance, T5 low impedance. Stable State

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM (6 T) Assume T2 low impedance, T5 high impedance. –Point A ~ GND –T3 in high impedance and T6 in low impedance –Point B ~ GND –T2 in low impedance, T5 high impedance. Stable State A B

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 SRAM (6 T) 6T cell is in two stable states. If the word line is asserted, complementary values are placed on the two bit lines.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 DRAM Dynamic Random Access Memory –Dynamic: Periodically refresh information in a bit cell. –Else it is lost. –Small footprint: transistor + capacitor High density memory –Cheap. –Read complicated Slower than SRAM

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 DRAM a dynamic memory the bit is stored by internal capacitance and dissipates (leaks) over time it is also destroyed on a read operation and therefore required periodic refreshment. 1 transistor (with capacitance) is required to store 1 bit.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 DRAM First introduced (with a 3T cell) by Intel in –1kb capacity. Classic 1T cell introduced in –4kb capacity.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 DRAM DRAM cell –Capacitor –Transistor

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 DRAM To write a 0 Turn bit-line voltage to 0V. Turn word-line voltage to V CC. –Turns access transistor on. –Empties charge from capacitor. Turn word-line voltage back to 0V.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 DRAM To write a 1 Turn bit-line voltage to V CC. Turn word-line voltage to V CC. –Turns access transistor on. –Charges capacitor. Turn word-line voltage back to 0V.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Flash Memory

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Floating Gate Fundamentals Floating Gate between control gate and channel in MOSFET. Not directly connected to an outside line.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Floating Gate Fundamentals First used in Erasable Programmable Read Only Memory (EPROM) To function, Floating Gate EPROM cell needs to be able to maintain a charge on the floating gate. Charge completely isolated, hence can be stored for long times.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Floating Gate Fundamentals No charge in floating gate. –Assume control gate, source, drain at GND. –Increase voltage in control gate: Floating gate voltage also increases, but at a lower rate. Raises the threshold value of the transistor. When threshold voltage is high enough, creates a channel between source and drain. –Threshold value about twice as high.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Floating Gate Fundamentals Floating gate (negatively) charged: –Isolates control gate from forming a channel at normal threshold values. Discharging the floating gate: –Exposure to UV light for twenty minutes. –Chips have a crystal cover that allows UV light to hit the chip. –UV light charges electrons on the floating gate so that they can break through the isolation layer around the floating gate.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Floating Gate Fundamentals To charge the floating gate –Apply 12V (or higher) to control and drain –Maintain source and substrate at ground –For a few hundred microseconds. Creates a large drain current. Accelerates electrons to high velocity: hot electrons. Break through the silicon substrate SiO 2 barrier. Some get caught in the floating gate. Floating gate charge causes channel inversion. Electrons remain trapped on floating gate. Potential about -5V.

Norhayati Soin 06 KEEE 4426 WEEK 15/1 6/04/2006 Floating Gate Fundamentals Need to avoid trapping electrons in SiO 2 after several charge / discharge cycles. –Could raise threshold value of transistor. –Careful growth of SiO 2 layer.