Design of silicon devices for pass-transistor-logic circuits

"Student Paper"

DESIGN OF SILICON DEVICES FOR PASS-TRANSISTOR-LOGIC CIRCUITS Fartash Vasefi Z. Abid Abstract The continued scaling of bulk MOSFET device have resulted in an increase in leakage currents and deteriorated performance of various VLSI circuits. Pass Transistor Logic (PTL) family achieves lowest-number-transistor digital circuits targeting low power dissipation and reduced chip area [2]. The leakage current in PTL circuits is mainly due to the output voltage level degradation. Conventional MOSFET transistors have high Body-bias effect resulting in an increase in the threshold voltage and making the voltage level degradation worse. A channel doping profile, Low-body-factor (LBF) MOSFET device, is suggested and simulated for 180nm CMOS technology. The (LBF) device was designed to reduce such voltage level degradation. The designed device performance has been tested by the physics-based device and circuit simulator, ISE TCADTM [1].

Low Body Factor CMOS Devices The Voltage level degradation may occur in the drain voltage output in PTL digital circuits. Moreover, the threshold voltage is affected by the body bias and increases unfavorably because of the reverse biasing of the substrate-source pn junction. Therefore, a solution to reduce the body effect in MOSFET devices is presented. A highlow doping of the channel, of the MOSFET transistor, is suggested and compared with a uniform doping. Figure 1 shows the effect of substrate voltage (body bias) on the threshold voltage for High-Low and Uniform doping design. 0.65 Threshold Voltage

0.6 0.55 0.5 0.45 0.4 0.35 0.3 -0.5

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Substrate Voltage nMOS, High-Low doping,

nMOS, Unioform doping

Figure 1: Threshold voltage variation vs. different body bias in High-Low and Uniform doping structure. High-Low channel doping results in Low-body Factor (LBF) MOSFET.

In Uniform doping structure, increasing the substrate voltage (body-bias) increases the depletion width. In High-Low doping structure, body bias has no effect on the depletion width, since the depletion region is already extended to the Punch-through region. Therefore, reverse body bias has no significant effect on the depletion region width and consequently on threshold voltage value in the high-low doping case, A part of an XNOR gate (Fig. 2), consisting of n-type MOSFETs, is simulated to test the performance of the low-body-factor devices. The output voltage level, for shown input

1-4244-0084-8/05/$20.00/©2005 IEEE

"Student Paper"

Figure 2: The presented circuit for evaluating the MOSFET body effect

signals, is shown in Fig. 3. Case (a) is the transistor with High-low channel doping structure (low body factor design), and the Case (b) is the nMOSFET with uniform channel doping (high body factor transistor). The output voltage level is supposed to be equal to VDD-VTN (which is 1.8 - 0.5 =1.3 V). However, it is 1.26V for case (a) and 1.02V for case (b). Since both inputs are at logic “1” (high level), both transistors are subject to a maximum reverse body bias VBS= -VDD which increases the value of threshold voltage (VTN) to a larger value than 0.5V. VTN increases from 0.5V to 0.54 for Low body factor design and from 0.5V to 0.78 volt for high body factor transistor.

Figure 36: output waveform for the sample circuit using (a) Low body factor (b) High Body factor nMOSFET Transistors

Low-body-effect MOS devices results in lower voltage level degradation when used in Pass Transistors Logic circuits. Therefore, the suggested channel doping profile can be used to improve the performance of PTL digital circuits.

References [1] ISE-TCAD User Manual, ISE TCAD Release 10, Zurich, Switzerland, 2004. [2] F. Vasefi and Z. Abid, “Low Power n-bit Adders and Multiplier using Lowestnumber-of-Transistor 1-bit Adders,” 18th CCECE05, May 1-4, 2005.