Body-Effect-paper test data

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Proceedings of The National ConferenceOn Undergraduate Research (NCUR) 2005Washington and Lee UniversityVirginia Military InstituteLexington, VirginiaApril 21 - 24, 2005A Study of the Body Effect in nMOS Enhancement Mode TransistorsWei-Han Jeng, and Kanchanadet BanchusuwanDepartment of Electrical and Computer EngineeringVirginia Military InstituteLexington, VA 24450. USAFaculty Advisor: Dr. J. Shawn AddingtonAbstractMOSFET devices are characterized by a threshold voltage, which determines when the device is “on” or “off” in digital logic gate applications. A variation of this threshold voltage, caused by changes in the base to source voltage, can have significant effects on the design and performance of these digital logic gates. This variation is called the body effect. This study involves an evaluation of the body effect in the nMOS enhancement mode transistors. First, several transistor structures were designed and fabricated, following the procedures established in a previous study. Then, the researchers performed an investigation of the extent of the body effect for each of these structures under varying test conditions, and demonstrated how this effect is significant in digital logic gate design.Keywords: nMOS, Body effect, Semiconductor1. IntroductionThe project is based on a specific characteristic of n-channel enhancement MOSFET (nMOS) devices, known as body effect.As an extension of work that was started last year [1], this research investigates how non-zero source-base voltages (V SB) influence the operational characteristics of these devices. Due to the different condition of the base voltage, the cascaded nMOS transistors (logic gates) perform differently. Specifically, the voltage required to “trigger” (turn on or off) these logic gates is significantly altered by the presence of body effect in the transistors. Since nMOS logic design requires the use of varying transistor sizes for proper functionality, this project also includes an evaluation of multiple nMOS transistor structures (of varying size) in order to better understand the extent of body effect on such structures. Such data will assist in improving the performance of these logic devices by incorporating the body effect in the design process.2. nMOS Technology2.1 device structure and layout [1]Figure 1 is the schematic, edge view, and top view of a nMOS device. nMOS enhancement mode devices may operate as switches by applying a gate (G) voltage to modulate (open and close) the channel between the source (S) and drain (D) regions. When the difference between gate and source voltage (V GS) is more positive than the threshold voltage (V T), an inversion layer (p-type material converted to n-type) is created under the gate that completes the channel and allows current (I D) to flow through the circuit (Figure 2). Non-zero source-base voltages (V SB), common in certain digital designs, result in changes in this threshold voltage, as shown in Figure 3.2181Figure 1 Different views of nMOS device2.2 threshold voltage and body effectFigure 2 shows the ideal transfer characteristic curve between I D and V GS . The important information gained from this curve is the threshold voltage (V T ), which is the voltage necessary to turn the device “on.”Figure 2 Transfer characteristic of nMOS transistorFigure 3 shows the ideal transfer characteristic curve for different values of V SB . Due to the body effect, changes inthe source-base voltage result in changes in the threshold voltage.Figure 3 Transfer characteristic of nMOS transistor with body effect2.3 nMOS transistor designThe design of the nMOS transistor is based on references [2,3]. The design of nMOS transistor size is based on the design from an earlier study [4].Transistors of multiple sizes are characterized under different test conditions in order to see the extent of the body effect. The transistors are biased in the saturation region by shorting the gate to the drain and applying a drain voltage of 5 volts. The transfer characteristics (I D vs V GS) of each transistor are then measured for different values of V SB. These data result in two analyses – the changes in V T with V SB (body effect), and the influence of transistor size on the body effect.4. ResultsThe devices on the wafer are labeled into 12 positions. There are 2 or 3 devices in each position. These positions are shown as Figure 4:a bFigure 4 Device orders on single wafer; a: Device orders, b: Final productTable 1 is the ratio (W/L) of nMOS transistor gate regions. The ratio determines the size of the transistor for our study.Table 1 Ratio of gate regionL(unit) W/L(unit)Transistor W1 & 7 a 4 18 0.22b 12 4 32 & 8 a 4 18 0.22b 12 4 33 & 9 a4 26 0.15b 10 4.5 2.22c 10 4 2.54 & 10 a 4 26 0.15b 10 4.5 2.22c 10 4 2.55 & 11 a 4 34 0.12b 10 3.5 2.86c 12 3.5 3.436 & 12 a 4 34 0.12b 10 3.5 2.86c 12 3.5 3.4321822183Figures 5, 6, and 7 are the transfer characteristic curves (I D vs V GS ) under different source-base voltages (0V to 5V). We establish an “on-current” level of 0.07 mA. The intersections of the x- axis and the curves provide an estimate of the threshold voltage (V).0.070.120.170.220.270.320.370.420.470.52I (m A )I D (m A )21840.070.0950.120.1450.170.1950.22I D (m A )4.2 observations4.2.1 body effectBy observing the characteristic curves (Figure 5, 6, 7), the body effect is apparent. As expected [5], an increase in V SB results in an increase in the threshold voltage (Figure 8).V T vs V SB0.40.50.60.70.80.910.511.522.533.544.555.5V SB (V)V T (V )Figure 8 V T vs V SB2185Our results also indicate that the extent of body effect is a function of transistor size (Figure 9). An increase in theratio (W/L) of the gate region results in a more significant increase in the threshold voltage due to body effect.Increase in V T vs Transistor Size78798081828384852.42.62.833.23.43.6ratio of gate region size (W/L)I n c r e a s e i n V T (%)Figure 9 % increase in V T for different transistor size5. ConclusionThe results of our study confirm the influence of body effect on the performance of nMOS enhancement modetransistors. As introduced earlier, the effect is especially significant in digital logic design. For example, consider the inverter structure in Figure 10.Figure 10 Schematic of inverterIn this circuit, the V SB of the switching transistor, M2, is always zero. However, due to the saturated loadconfiguration, the V SB of loading transistor, M1, is never zero, and has a maximum value of V DD -V T1, where V T1 is the threshold voltage of transistor M1 [1].The inherent limitations of the saturated load configuration are clearly demonstrated by this study in that attempts to increase the output level, Vout, are countered by the corresponding increase in V T1 due to the body effect. This is especially important in cascaded systems where this output, Vout, must trigger subsequent devices.Although the body effect cannot be eliminated, an estimation of its impact on varying transistor sizes may lead to design modifications that minimize the impact of the body effect on the overall performance of the device.6. References1. Wei-Han Jeng and Kan Banchusuwan, “The Design and Realization of nMOS Digital Devices” , NCUR 2004 Proceedings, April 2004.2. Jaeger, R.C. and Blalock, T.N., Microelectronic Circuit Design, 2nd ed. (2004), McGraw Hill, New York, New York, pp. 195-96, pp. 377-407.3. Jaeger, R.C., Introduction to Microelectronic Fabrication, 2nd ed. (2002), Vol. V of the Modular Series on Solid State Devices, Prentice Hall, Upper Saddle River, New Jersey, pp. 49, pp. 53, pp. 217.4. Gau and Boet, Microelectronic Fabrication, 2004 St. Cyr French Military Academy Training at VMI5. Sung-Mo Kang, CMOS DIGITAL INTEGRATED CIRCUITS, 2nd ed. (1999), McGraw Hill, New York, New York, pp. 73-79.2186。