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Design Techniques for Gate-Leakage Reduction in CMOS Circuits

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Design Techniques for Gate-Leakage Reduction in CMOS Circuits Design Techniques for Gate-Leakage Reduction in CMOS Circuits Rafik S. Guindi and Farid N. Najm Department of Electrical and Computer Engineering University of Toronto Toronto, ON, Canada, M5S 3G4 [email protected] – [email protected] Abstract as oxides get thinner [3]. In this paper we look at the oxide tunneling current as Oxide tunneling current in MOS transistors is fast be- a static leakage current component in CMOS logic circuits. coming a non-negligible component of power consumption We restrict the analysis to static CMOS circuits, however as gate oxides get thinner, and could become in the future the results can be extended to other design styles, such as the dominant leakage mechanism in sub-100nm CMOS cir- dynamic CMOS or domino logic. We first consider the de- cuits. In this paper, we present an analysis of static CMOS pendence of the gate current on various conditions for a sin- circuits from a gate-leakage point of view. We first con- gle transistor. We then consider how the gate of a transis- sider the dependence of the gate current on various con- tor leaks in the context of a given logic structure. Also, ditions for a single transistor, and identify 3 main regions given a specific structure, we present state-dependent gate- in whic h a MOS transistor will operate between clock tran- leakage tables that can be used to estimate the total amount sitions. The amount of gate-current differs by several or- of gate-current for a large circuit. Finally, we suggest guide- ders of magnitude from one region to another. Whether a lines aimed at reducing the amount of oxide-leakage current transistor will leak significantly or not is determined by its based on the presented structure and state dependencies. position in relation to other transistors within a structure. By comparing logically equivalent but structurally differ- 2 Gate-Leakage Current in a Single Transis- ent CMOS circuits, we find that the gate current exhibits tor a ‘structur e dependence’. Also, the total gate-leakage in a given structure varies significantly for different combi- There are many published studies concerning gate- nations of inputs, from which we derive “state-dependent currents in MOS transistors [1, 4, 5, 9]. In this work we gate-leaka ge tables” that can be used to estimate the to- consider only the parameters that affect the magnitude of tal amount of gate-current for a large circuit. Finally, we gate-current in a transistor as it operates in relation to other suggest guidelines aimed at reducing the amount of oxide- transistors at the circuit level. We are assuming that Vdd, leakage current based on the presented structure and state Vt, and the oxide thickness are fixed and depend on the dependencies. technology used. Variables considered are the applied bias, the transistor type (NMOS or PMOS), and the transistor size. 1 Intr oduction 1. Applied bias: As MOS transistors get smaller and the gate oxide gets The magnitude of the gate-current is a strong function of the thinner , tunneling current through the insulator becomes a applied bias [6]. Since the gate-current is a static leakage non-ne gligible component with a potential impact on circuit current, we will consider the transistors at steady-state, i.e. operation and performance. With CMOS technology ap- in-between transitions only and not during transient switch- proaching the 100-nm regime, the current leaking through ing. In general, a transistor in a static CMOS logic gate will the gate oxide is becoming an important component of be operating in one of 3 regions of operation, each with a power consumption [1, 2]. Until a few years ago, oxide significantly different amount of gate leakage: tunneling current was not considered to be an issue, how- – Strong inversion, with jVGSj = Vdd. ever it was predicted that it could surpass weak inversion Gate-leakage current density for an NMOS in strong inver- 3 2 and DIBL as a dominant leakage mechanism in the future sion can be as high as 10 A=cm for an oxide thickness of 0-7695-1881-8/03 $17.00 Ó 2003 IEEE 1:5 nm at Vdd = 3 V [7]. For such a thin oxide, a more that a PMOS transistor leaks exactly one tenth the amount realistic value for Vdd is 1:2 V [2], in which case the gate- in an NMOS. leakage current density is around 20 A=cm2 [7]. – Threshold, with jVGSj = Vt. A transistor operating at the threshold will leak significantly Table 1. Normalized gate currents in NMOS less that in strong inversion, typically 3 to 6 orders of mag- and PMOS transistors. nitude less, depending on the value of Vdd and the oxide thickness [8]. Channel condition NMOS PMOS – Off, with jVGSj = 0. Off 0 0 For an NMOS device, there is no leakage if Vdrain = 0 V . Threshold ≈ 0 ≈ 0 W:L)n W:L)p However, if the drain is pulled up to Vdd, a small leakage Strong inversion 1 × 0:1 × W:L)min W:L)min component in the reverse direction (from drain to gate) may be present, due to the gate-drain overlap area. This current depends of course on the transistor geometry, and is around 10 orders of magnitude smaller than the gate-leakage cur- 3 Structure Dependence rent in the strong inversion case [6]. The above 3 regions represent 3 distinct conditions or The position of a specific transistor in relation to other states for the channel of a MOS transistor. Whether an transistors in a given structure will determine whether it “on” transistor will operate in strong inversion or at the will operate in strong inversion or at the threshold when threshold is determined by its position inside a structure, switched on. In the following analysis, we will assume that as well as by the states of other transistors in the structure. the gate-leakage is significant only if a given transistor is These factors are discussed in Section 3. operating in strong inversion. 2. Transistor type: 3.1 Structure-dependent channel states PMOS transistors in a static CMOS pull-up structure will also be operating in one of the same three modes as An NMOS transistor will be in strong inversion when its their NMOS counterparts. However, the main tunneling source is pulled to ground either through a direct connec- component in a PMOS device in inversion is hole tunneling tion or through another NMOS device, as illustrated in Fig- from the valence band, as opposed to electron tunneling ure 1 (a) and (b). This is the default state of an “on” transis- from the conduction band in NMOS devices, which results tor. Alternatively, the drain may be pulled low (through an- in PMOS gate currents being roughly 10 times smaller than other device), as in Figure 1 (c). Also, the NMOS transistor NMOS currents [9]. This fact has important implications will be in strong inversion if the gate is high, with both the in assessing a static CMOS structure from a gate leakage source and drain floating low and the substrated grounded point of view. (Fig. 1 (d)). In general, an NMOS will be in strong inver- sion when switched on unless the channel is actively pulled 3. Transistor size: towards Vdd, as in the following. Since gate leakage currents are measured as current den- 0 V 0 V ~ 0 V sities, it follows that the leakage current will be directly proportional to the gate area (W:L). Transistor sizing 0 V Vdd Vdd Vdd therefore has a direct impact on the amount of gate leakage 0 V 0 V Vdd ~ 0 V in a CMOS circuit. Vdd Vdd In static CMOS structures, the signals at transistor gates (a) (b) (c) (d) will always be strong (forcing), coming from either a signal source, or a previous output. In estimating the amount of Figure 1. NMOS in strong inversion with gate-current in a given CMOS structure, we can therefore Vgate = Vdd, and (a) source tied to ground, assume that a minimum-size transistor in strong-inversion (b) source brought down through another will leak a specific amount that can be measured for a given transistor, (c) drain brought down, and (d) technology, at a specific oxide thickness, with jVGSj = Vdd. source and drain floating low. This is the basis for the values shown in Table 1, where gate- currents are normalized relative to the amount measured in a minimum-size NMOS transistor in the technology at hand, An NMOS transistor will be operating at the threshold for a given Vdd. Without loss of generality, we will consider when the drain is pulled high through the PMOS pull-up network. This usually occurs when the transistor is in a of 8 possible cases, corresponding to the inputs (A,B,C) = stack, with one or more transistors in lower positions turned (0,0,1), (0,1,1), and (1,0,1). off. Depending on the transistor’s position in the stack, the voltage at the drain may be either Vdd or Vdd −Vt, as shown in Fig. 2 (a) and (b). Alternatively, in more complex struc- C tures, the source of the transistor may be pulled to Vdd − Vt B through another NMOS device, as in Figure 2 (c). A B C A Vdd Vdd Vdd PMOS PMOS PMOS on on on Figure 3. NOR and NAND pull-down struc- V V V dd dd dd tures. Vdd Vdd - Vt Vdd Vdd - Vt Vdd V - V dd t Vdd Vdd Table 2. Gate leakage in NOR and NAND pull- Vdd - Vt V - V dd t down networks.
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