S. P. Singh, M. Mishra and G. Srivastava / IJECCT 2013, Vol. 3 (3) 20 Leakage Reduction in Nanometer SRAM cell using Power Gating V DD control technique Suryabhan Pratap Singh¹, Manish Mishra¹, Geetika Srivastava² 1 Department of Electronics, DDUGU, Gorakhpur (273009), U.P, INDIA 2 Amity school of Engineering& Technology, Amity University Lucknow (226010), U.P, INDIA sbhpsingh@gmail.com¹,geetika_gkp@rediffmail.com² Abstract: Increased demand of storage capacity in commercially available products has led to increased attention of researchers in the field of memory design. As today, memory block dominates more than 90% of entire chip area; improvement in their performance will lead to overall system performance improvement. This paper focuses on optimization of power dissipation with decreasing data retention voltage and size of virtual supply node transistors. An 8T-SRAM cell simulation results shows leakage power improvements compared with previous 6T SRAM cell. The analysis proves suitability of 8T-SRAM cell in low-power applications. This paper presents access and virtual supply transistor sizing analysis with respect to overall cell leakage power performance. The data retention gates reduces the leakage current of the SRAM cell in hold mode of operation and propose the leakage improvement as high as 22 % at 90nm at 0.7V with (W/L)cell/(W/L)access=1,5% AT 65nm at 0.4V,17% at 45nm and 6.5% at 32nm at 0.2 V compared with respective 6T SRAM cell and the leakage power can be reduce 20.8% at 90nm with sizing of sleep transistor is 2 at voltage 0.9V, 6.15% at 65nm with sizing of sleep transistor is 2 at voltage 0.6V, 20.16% at 45nm with sizing of sleep transistor is 2 at voltage 0.3V and 8.45% at 32nm with sizing of sleep transistor is 2 at voltage 0.2V with respective G-gated SRAM cell. Keywords SRAM, Low power Design P-gated G-gated, Leakage reduction I. INTRODUCTION CMOS digital integrated circuits are the enabling technology for modern information age [1]. The size of MOS devices is approaching physical limit and their submicrons leakage currents are increasing dramatically. Most of the time of its operation the memory cell remains in standby mode and hence standby leakage currents of SRAM cells typically contributes a major portion of chip leakage. Since several millions of memory cells are integrated in one SRAM chip, standby leakage currents of each cell is accumulated to consume larger amount of total chip power. As SRAM is most popular devices for digital storage so, reduction of the leakage power and improvement performance capabilities of SRAM is the main focus of chip designer today. SRAM is a type of semiconductor memory that uses bi- stable latching circuitry to store a bit as voltage. Each memory cell required six transistors. In this paper, 8T SRAM p-gated and g-gated cell with VDD control power reduction technique is analyzed and compared with 6T- SRAM cell in nanometer technology. The performance of cell is evaluated in different technologies with varying virtual nodes voltages and size of sleep transistors. The power consumption by cell is reduced by cutting off supply terminals in standby mode of cell operation. A. 6T-SRAM Cell Circuit Design Figure 1: 6T SRAM cell [2] Figure 1 shows basic SRAM cell configuration with 6 transistors, which stores bit in two cross-coupled inverters made of four transistors (Q1, Q2, Q3 and Q4). This storage cell has two stable states which represent stored 0 and stored 1. Two additional access transistors serve to control the access to a storage cell during read and write operations. Access to the cell is enabled by the word line (WL) which controls the two access transistors Q5 and Q6 connected to the bit lines BL and BL BAR. They are used to transfer data for both read and write operation. B. Sizing of Conventional 6t Sram Cell The SRAM cell should be sized as small as possible to achieve high density in memory design. However, issues related to robustness impose a sizing constraint to the 6TSRAM cell. Fig. 1 shows the conventional 6TSRAM cell configuration. The transistor ratio between Q1 and Q5 must be greater than 1.2 to keep a proper SNM during the read operation.[3,4,5] C. Leakage Current Component Leakage current is the main source of standby power dissipation in SRAM cell. In nano-scale MOS devices, the major components of leakage current are the sub-threshold leakage, the gate-tunneling leakage, and junction leakage
S. P. Singh, M. Mishra and G. Srivastava / IJECCT 2013, Vol. 3 (3) 21 (Figure 2). The sub-threshold leakage, which is defined as a weak inversion conduction current with Vgs < Vth, is significant component of off-state transistor leakage [6, 7]. Figure 3: 8T g-gated SRAM data retention circuit Figure2: leakages component in the transistor. The sub threshold current (I sub) is given by I sub=a sub w exp ( ) (1-exp ( V ds )) The Gate-tunneling current (I gate) is dominated by gate to channel current of ON NMOS transistors. I gate =A ox W N ( )² (iii) Junction leakage (Ijn) is small contributed to total leakage current. D. SRAM Operation An SRAM cell has three different states: standby or hold mode, reading and writing. For proper operation of SRAM read mode and write mode, it should have good "read stability" and "write ability" respectively. The cell operation in three different states can be defined as-(i) Standby mode: The word line is not asserted, access transistors Q 5 and Q 6 disconnect cell from the bit lines and data inside cell remains Unaffected. (ii) Read mode: The stored bit in the cell is transferred from Q and Q bar to bit lines with a positive going pulse applied on word line. (iii)write mode: For writing a 0, it is applied on BL and 1 is applied on BL bar, WL is asserted and the value stored in cell is latched to bit lines. Input drivers are designed much stronger than the weak transistors in the cell for proper operation of SRAM II. 8T SRAM CELL 8T SRAM cell is designed for reducing power dissipation by addition of two NMOS transistor in pull down of 6T SRAM cell in G-gated mode and similarly two PMOS transistor in pull up for P-gated mode (Figure 3 and figure 4). An 8T SRAM cell structure is analyzed for improved power dissipation in standby. Figure 4: 8Tp-gated SRAM cell data retention circuit In G-gated SRAM cell (Figure 3), the extra pair of NMOS replaces mode from ground node with evaluated potential by virtual ground node [8,9,10]. Q7 has main function of cutting off the cell from ground in standby mode which reduces leakage current in this mode, and Q8 is used to provide a fix voltage Vy at this node. In P-gated SRAM cell (Figure 4), the extra pair of PMOS replaces VDD node by virtual VDD node.q7 has main function of cutting of the cell in standby mode from VDD which reduces leakage current in this mode, Q8 is used to provide a fix voltage VX at this node. Drawback: virtual ground (supply) node may charge (discharge) to VDD (0) is stored bit may be destroyed. Solution: In the standby mode, strap the virtual ground or virtual supply to a fixed voltage node is Data retention capability. III. SIMULATION AND ANALYSIS Circuits have been simulated using BSIM 4 at 90nm, 65nm, 45nm and 32nm technology. To make the impartial testing environment all circuits has been simulated on the same input patterns. In this paper, two different low power techniques is presented and compared in detail for nanometer technologies on SRAM cell. The relative power dissipation at varying node voltage and sizing of different transistors has been evaluated and compared with conventional one. IV. RESULT Figure 5 and 6 shows that power dissipation of 6T-SRAM cell decreases with increase of size of access transistor (Q5 and Q6) i.e. (W/L) access Power dissipation for all technology (figure 1). Figure 7 show that power dissipation of G-gated SRAM cell decreases with increase data retention voltage (Vy) then after it (power dissipation) increases at 90nm technology. figure 8,9 and figure 10
S. P. Singh, M. Mishra and G. Srivastava / IJECCT 2013, Vol. 3 (3) 22 shows that power dissipation of G-gated SRAM cell increases with increase data retention voltage (Vy) with respect to different size of sleep transistor (Q7 and Q8) at 65nm,45nm and 32nm technology (figure 3). figure11, 12, 13 and figure14 shows that power dissipation of P-gated SRAM cell decreases with increase data retention voltage (Vx) then after it (power dissipation) increase with respect to different size of sleep transistor (Q7 and Q8) for all technology. And figure15, 16, 17 and figure18 show that percentage power dissipation of P gated and G gated SRAM cell increases with increase data retention voltage (V) at different size of sleep transistor for all technology. Figure8: Power dissipation vs. voltage (Vy) with different sizing of transistor at 65nm technology in G-gated SRAM cell. Figure5: Power dissipation Vs. W/L of access transistor at 90nm technology in 6T SRAM cell. Figure 9: Power dissipation vs. voltage (Vy) with different sizing of transistor at 45nm technology in G-gated SRAM cell. Figure 6:Power dissipation Vs. W/L of access transistor at 65nm, 45nm, and 32nm technology 6T SRAM cell. Figure10: Power dissipation vs. voltage (Vy) with different sizing of transistor at 32nm technology in G-gated SRAM cell. Figure7: Power dissipation vs. voltage (Vy) with different sizing of transistor at 90nm technology in G-gated SRAM cell. Figure11: Power dissipation vs. voltage (Vx) with different sizing of transistor at 90nm technology in P-gated SRAM cell.
S. P. Singh, M. Mishra and G. Srivastava / IJECCT 2013, Vol. 3 (3) 23 Figure12: Power dissipation vs. voltage (Vx) with different sizing of transistor at 65nm technology in P-gated SRAM cell. Figure 16: Power dissipation in % vs. voltage of P and G-gated SRAM cell at 65nm technology. Figure13: Power dissipation vs. voltage (Vx) with different sizing of transistor at 45nm technology in P-gated SRAM cell. Figure 17: Power dissipation in % vs. voltage of P and G-gated SRAM cell at 45nm technology. Figure14: Power dissipation vs. voltage (Vx) with different sizing of transistor at 32nm technology in P-gated SRAM cell. Figure: 18: Power dissipation in % vs. voltage of P and G-gated SRAM cell at 32nm technology. Figure15: Power dissipation in % vs. voltage of P and G-gated SRAM cell at 90nm technology. V. CONCLUSION The most efficient technique to reduce the power dissipation is the reduction of supply voltage (data retention voltage Vx and Vy), the power dissipation reduction in SRAM cell is not only due to power supply voltage reduction, but also to the operating sizing of sleep transistor In this paper, proposed circuit is presented for reducing power consumption through scaling the supply voltage as compared to conventional circuit at different technologies. We have shown that the leakage power can be reduce 20.8% at 90nm with sizing of sleep transistor is 2 at voltage 0.9V, 6.15% at 65nm with sizing of sleep transistor is 2 at voltage 0.6V, 20.16% at 45nm with sizing of sleep transistor is 2 at voltage 0.3V and 8.45% at 32nm with sizing of sleep transistor is 2 at voltage 0.2V in 8T P- gated SRAM cell than 8T G-gated SRAM cell. 8T P-
S. P. Singh, M. Mishra and G. Srivastava / IJECCT 2013, Vol. 3 (3) 24 GATED SRAM is better than conventional 6T SRAM cell and 8T G-gated SRAM cell at different technology. REFFERENCES [1] Sung-Mo Kang,Yusuf Leblebici, CMOS digital integrated circuits analysis and design, ISBN -13:978-0-07-053077-5/0-07-053077-7, third edition, Tata McGraw Hill education,2003,30th reprint 2012,pp preface pp (xi). [2] Adel S. Sendra and Kenneth C. Smith microelectronic circuits, ISBN 0 19 511690 9, fourth edition, oxford, pp 1117-1118. [3] Kevin Zhang, Uddalak Bhattacharya, Zhan ping Chen, Fatih Hamzaoglu,Daniel Murray, Narendra Vallepalli, Yih Wang, B. Zheng, and Mark Bohr, SRAM Design on 65-nm CMOS Technology With Dynamic Sleep Transistor for Leakage Reduction IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 40, NO. 4, APRIL 2005, pp. 895-896 [4] Bhavya Daya, Shu Jiang, Piotr Nowak, Jaffer Sharief Synchronous 16x8 SRAM Design pp.1-2 [5] A.Chandrakasan, W.J. Bowhill, F. Fox, Design of High- Performance Microprocessor Circuits, IEEE Press, 2000. [6] Behnam Amelifard, Farzan Fallah, and Massoud Pedram Low Leakage SRAM Design in Deep Submicron Technologies Jan 25, 2008 Presentation at SNU, pp.29 [7] Sung-Mo Kang,Yusuf Leblebici, CMOS digital integrated circuits analysis and design, ISBN -13:978-0-07-053077-5/0-07- 053077-7, third edition, Tata McGraw Hill education,2003,30th reprint 2012,pp 449. [8] Geetika Srivastava &R.K.Chauhan Deasign of a new 10T SRAM cell for leakage reduction & stability enhancement IEEE,VOLUME 3,Number 39 (2010) pp.225-230. [9] Geetika Srivastava &R.K.Chauhan Effect of technology scale down on power reduction stretegies ISBN-978-1-4577-0694-3 PN-717, IEEE Explorer conference proceding May 2012. [10] Geetika Srivastava &R.K.Chauhan Effect of process parameter on 6T SRAM cell design for low power reduction International Journal of Microcircuits & Electronics, ISBN 0974-2204,VOLUME 1, Number1 (2010) pp.35-42.