Variable Threshold MOSFET Approach (Through Dynamic Threshold MOSFET) For Universal Logic Gates

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K. Ragini,1
Dr. M. Satyam,2
and Dr. B.C. Jinaga,3
1
G. Narayanamma Institute of Technology & Science , Department of Electronics and
Communication Engineering, Hyderabad, India
ragini_kanchimi@yahoo.co.in
2
International Institute of Information Technology, Hyderabad, India
satyam@iiit.ac.in
3
School of Information Technology, Jawaharlal Nehru Technology University,
Hyderabad, India
jinagabc@gmail.com
ABSTRACT
In this article, we proposed a Variable threshold MOSFET(VTMOS)approach which is realized from
Dynamic Threshold MOSFET(DTMOS), suitable for sub-threshold digital circuit operation .Basically the
principle of sub- threshold logics is operating MOSFET in sub-threshold region and using the leakage
current in that region for switching action, there by drastically decreasing power .To reduce the power
consumption of sub-threshold circuits further, a novel body biasing technique termed VTMOS is introduced
.VTMOS approach is realized from DTMOS approach. Dynamic threshold MOS (DTMOS) circuits provide
low leakage and high current drive, compared to CMOS circuits, operated at lower voltages.
The VTMOS is based on operating the MOS devices with an appropriate substrate bias which varies with
gate voltage, by connecting a positive bias voltage between gate and substrate for NMOS and negative bias
voltage between gate and substrate for PMOS. With VTMOS, there is a considerable reduction in operating
current and power dissipation, while the remaining characteristics are almost the same as those of DTMOS.
Results of our investigations show that VTMOS circuits improves the power up to 50% when compared to
CMOS and DTMOS circuits, in sub- threshold region..
The performance analysis and comparison of VTMOS , DTMOS and CMOS is made and test results of
Power dissipation, Propagation delay and Power delay product are presented to justify the superiority of
VTMOS logic over conventional sub-threshold logics using Hspice Tool. . The dependency of these
parameters on frequency of operation has also been investigated.
KEYWORDS
Sub- threshold, Dynamic threshold MOS Inverter, Propagation delay, Noise-margin, Variable
threshold MOS Inverter, Power dissipation.

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1.0 INTRODUCTION
There have been continual research efforts in recent years to explore the ultra-low power region of
operation. These efforts have been effective in applications where ultra low power consumption is
a primary requirement and speed is of secondary consideration .The proposed method to achieve
ultra low power is to operate the transistor’s in sub-threshold region[1,2].Research in sub-threshold
region of operation highlights the advantage of operating transistor’s below their threshold
voltage. This advantage of ultra low power is especially attractive for deployed medical
devices(Hearing aids, pacemakers etc),sensors and devices with low processing needs[3].Sub-
threshold current has been viewed as a continually increasing source of wasted power in circuits.
Instead of wasting the power, the leakage currents can be used as computational current in circuits.
This is the basis for sub-threshold logic circuits[4] .Some applications demand a low energy
approach, others medium speed and medium power dissipation Many researchers focused on
achieving low power with medium speed.[5,6] Assaderaghi.F[7] I..Chung[8] , A.Drake[9] and
M.R.Casu [10] has proposed the circuits, Fig 1(a),1(b) and 1(c) to improve the current drive and
showed that the delay has come down compared to CMOS inverters. This in turn has resulted in
some increase in the power dissipation
Fig 1(a) Standard DT-CMOS
Fig 1(b) DT-MOS with augmenting devices
Fig 1(c) Another topology of DT- CMOS with augmenting devices
Fig 1(d) DT-CMOS with Drains connected to substrate
The additional transistor’s lead to more complexity in the case of circuits 1(b) and 1(c). Hence
Soleimani[11] proposed a new inverter scheme, 1(d) with drains connected to substrate. He claims
that he achieved better power delay product compared to 1(b) and 1(c). In view of this, it is
thought that operating the DTMOS architecture , with a proper bias voltage applied between gate
and substrate (VTMOS), might result in lowering of operating currents and power dissipation
.With a view to examine this conjuncture, circuits based on VTMOS have been conceived and
their performance has been analyzed .The aim of this paper is to communicate the results obtained
through the simulations on VTMOS circuits .In the first instance dc transfer characteristics of the
MOS devices under VTMOS operation have been obtained through simulations. It has been found
that VTMOS circuits do result in lower power dissipation compared to CMOS and DTMOS
circuits, while other performance characteristics remain almost the same. Later using these
characteristics, VTMOS inverter has been constructed and its performance has been measured
through simulation. The analysis is extended to Universal logic gates ( Two-input NAND and
Two-input NOR logic gates). Preliminary Investigations carried out on all inverter circuits have
been indicated .It has been concluded that the VTMOS can provide the benefits of DTMOS and
also results in lower power dissipation with marginal increase in propagation delay.

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2 . 0 STRUCTURE OF CMOS, DTMOS AND VTMOS CONFIGURATION
Typical schematic structures of CMOS, DTMOS and VTMOS are given in Fig 2(a), 2(b) and 2(c).
In conventional NMOS circuit, Fig. 2(a), the substrate is normally connected to ground or lowest
potential in the circuit and in PMOS circuits, the substrate is connected to supply voltage or the
highest potential in the circuit. In DTMOS, Fig 2 (b), the substrate is always kept at gate potential.
Also, the voltage of each transistor substrate is dynamically adjusted depending on the gate
voltage, causing the threshold voltage of the device to adjust dynamically [12,13]. DTMOS
devices are efficient because they function as dual threshold logic gates. When a DTMOS
transistor is ON, its threshold is lowered increasing the current and decreasing propagation delay.
Likewise when the transistor is OFF, the threshold is raised, reducing leakage current and
minimizing power and energy dissipation. DTMOS is an excellent scheme to provide ultra-low
power with increased speed compared to traditional body biasing in the sub-threshold region
[14,15].VTMOS is nothing but an extension of DTMOS in the sense that the substrate voltage
differs always by a constant voltage from the gate voltage. As shown in Fig 2(c),by connecting
positive bias between gate and substrate for NMOS and negative bias between gate and substrate
for PMOS , there is a good reduction of power dissipation in sub-threshold when compared to
DTMOS and traditional CMOS. The circuit is named as VTMOS because ,we have used the
same DTMOS with a biased voltage between gate and substrate .The voltage of each transistor is
dynamically adjusted depending on gate voltage ,causing the threshold voltage of device to adjust
dynamically.
Fig 2(a) - CMOS Structure
Fig 2(b) - DTMOS Structure

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Fig 2(c) - VTMOS Structure
2. 1 I-V characteristics of MOS devices
To evaluate the behavior of NMOS devices under VTMOS operating conditions, the I-V
characteristics are measured and are given in Fig 3(a)and Fig 3(b).It may be observed from Fig
3(a) and Fig 3(b), that general current levels (Ion and Ioff) get reduced with increase in bias voltage,
when NMOS gate is positively biased with respect to substrate and in the PMOS case, when gate is
negatively biased with respect to substrate .In order to examine the effect of substrate bias on I-V
output characteristics of VTMOS, drain current Ids for different substrate bias voltages, Keeping
gate voltage Vgs = 0.2v have been measured and are given in Fig 3(b).It may be seen that the
variation in Ids with drain voltage ,Vds becomes less as VAN is made positive (deep sub- threshold
region). The characteristics may become flat, indicating that the output resistance becomes high.
Thus the drain current is less sensitive to variations in drain voltages, which is a welcome feature
for application of device in circuits.
Fig 3(a) -Ids - Vgs Characteristics of VTMOS VAN Varying from 0(top) to 0.2 V

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Fig 3(b)- Ids-Vds Curves For VTNMOS-VAN Varying From 0(Top) TO 0.2V(Bottom)
3.0 VTMOS LOGIC GATES
The transistors for VTMOS logic are chosen from the 65 nm technology [16,17]. The threshold
voltage for these devices is 0.22V for VT- NMOS and-0.22V for VT-PMOS. The width of VT-
NMOS (WN) and VT-PMOS (WP) is chosen as 200 nm and 400 nm respectively. The supply
voltage is taken as 0.2V which is below the threshold of both the devices. For different values of
VAN starting from 0 to 0.2V, and corresponding VAP from 0 to - 0.2V, the performance of the
VTMOS Inverter, VTMOS Two-input NAND gate and VTMOS Two-input NOR gate- power
dissipation, propagation delay, power delay product and frequency response have been obtained,
through simulation[18]. When the bias voltage is increased beyond supply voltage, the logic levels
are affected. Hence there is a limitation for bias voltage and it should be always below supply
voltage In the first instance, the VTMOS Inverter has been constructed as shown in Fig (4) It
consists of a PMOS and NMOSFETS, connected in series. The substrate of PMOS is connected to
gate through VAP, which bias the gate negative with respect to substrate. The substrate of NMOS
is connected to gate through VAN, which bias the gate positive with respect to substrate. This
ensures that both transistors work in the low current region. The voltage transfer characteristics of
CMOS and VTMOS Inverter with VAN varying from 0 to 0.2V and VAP varying from 0 to -0.2V
have been obtained and is shown in Fig (5). In this case the input is taken in the form of pulse
wave varying from 0 to 0.2 V, with a rise and fall time of 25 ns. The frequency of pulse wave is
100Khz..

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Fig 4 VTMOS Inverter
Fig 5 Voltage transfer characteristics of CMOS and VTMOS Inverter (VAN varying from 0 to
0.2v) at 100Khz frequency
The universal logic gates like Two-input NAND and Two-input NOR have been built with the
same device parameters .Spice simulated output waveforms of Two-input NAND and Two-input
NOR gates are given in Fig 6(a) and Fig 6(b) at a frequency of 400Khz .

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Fig 6 (a) Simulated output waveform of VTMOS Two-input NAND gate for VAN = 0.2V at
400Khz frequency.
Fig 6 (b) Simulated output waveform of VTMOS Two-input NOR gate for VAN = 0.2V at
400Khz frequency
4.0 RESULTS AND ANALYSIS
4.1 PERFORMANCE CHARACTERISTICS OF VT MOS CIRCUITS
The variation of propagation delay of VTMOS circuits ( Inverter , Two-input NAND, Two-input
NOR) with VAN are compared with CMOS at a frequency of 100 Khz and is shown in Fig 7(a). It
may be seen from Fig 7(a), that highest propagation delay in VTMOS circuits is almost equal to
propagation delay of CMOS, but at VAN =0, the delay in VTMOS is the lowest and gradually
increases to that of CMOS at VAN =0.2V.The average power consumed by the VTMOS Circuits
(Inverter ,Two-input NAND ,Two-input NOR) with varying VAN are compared with CMOS at a
frequency of 100 Khz and is given in Fig 7(b). From the Fig 7(b), it is observed that there is a

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slight increase in power dissipation for VTMOS at VAN =0, when compared to CMOS. From VAN
=0 there is considerable reduction in power as VAN is increased. Overall 54% reduction in power
can be achieved at frequency=100 Khz in VTMOS circuits for VAN =0.2V compared to CMOS
circuits. From Fig 7 (a) and Fig 7 (b), it may be seen that, while the power dissipation decreases
with increase in VAN, the delay has been found to increase. In order to get the merit of operating
the inverter in sub- threshold region, the power delay product [19] is computed and given in the
Fig 7(c). It may be seen that, the PDP reduces as we go from CMOS to VTMOS (0.2V).Thus the
VTMOS appear to have an edge over the other configuration from the point of power and delay at
100 Khz frequency.
Fig 7(a) Variation of propagation delay for CMOS and VTMOS Circuits (VAN varying from 0 to
0.2V)
Fig 7(b) Variation of power dissipation for CMOS and VTMOS Circuits (VAN varying from 0 to
0.2V)
Fig 7(c) Variation of power delay product for CMOS and VTMOS Circuits (VAN varying from 0
to 0.2v)

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4.2 EFFECT OF FREQUENCY ON THE PERFORMANCE CHARACTERISTICS OF VTMOS
CIRCUITS
In order to get an idea of the effect of frequency on the performance characteristics, they are
measured at different frequencies ranging from 100 Khz to 8 Mhz. It has been found that the
general nature of variation of power dissipation and propagation delays are maintained as those
reported at 100 Khz. However, the power dissipation increases with frequency and the variation of
power dissipation with frequency for CMOS circuits and VTMOS circuits(0 to 0.2V) is analyzed
.The variation of power dissipation with frequency for CMOS and VTMOS two-input NAND is
plotted in Fig (8).The same trend follows for VTMOS Inverter and VTMOS two input NOR gates
.Propagation delay and logic levels remain almost constant with frequency .At frequencies lower
than 8 Mhz, even though power consumption increases with frequency, the gain in the static power
dissipation, still provides an advantage compared to CMOS circuits [20,21]. How ever beyond 8
Mhz, the dynamic power is higher than the static power dissipation and there appears to be no
advantage of VTMOS compared to CMOS circuits. Therefore it may be noted that for a given
technology (feature size) and for a specific W/L of the transistors one can obtain power saving up
to certain frequency of operation compared to CMOS. In the particular case discussed in this
article, it is found that VTMOS operation in sub threshold region is advantageous up to 8 Mhz
operation, beyond this frequency there seems to be no advantage of VTMOS circuits.
Fig 8 : Variation of power dissipation of Two-input NAND gate with frequency
5.0 PERFORMANCE ANALYSIS OF TWO INPUT NAND AND TWO-INPUT NOR LOGIC
GATES WITH RANDOM INPUT VECTORS
To evaluate the performance under normal conditions, the performance has been evaluated with
random vectors. In this case , each input of NAND and NOR gate is presented with pseudo random
inputs and variation of propagation delay, power dissipation ,power delay product and rise and fall
time delays are analyzed at 100Khz..The results indicate that with random inputs ,the variation of
propagation delay, power dissipation ,power delay product and rise and fall time delays are almost
same as in the case with normal inputs .The experiment have been repeated with different random
inputs and it is observed that the readings are almost constant .
6.0 CONCLUSIONS
This paper reports a modified DTMOS approach which is called VTMOS approach. In these
circuits the substrate is operated with a fixed bias (VAN /VAP) which results in further reduction in
the operating currents compared to DTMOS circuits. The Proposed scheme shows improved
power efficiency compared to CMOS and DTMOS circuits , up to a frequency of 8 M h z (for the
specific devices used in this Investigation).Using these concepts one may be able to build low

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power digital circuits like[22,23] which consume lower power than the conventional CMOS and
DTMOS circuits.
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BIO-DATA OF AUTHORS
K.Ragini-Received B.Tech degree from Sri Venkateswara University, Tirupathi in 1990 and M.Tech
degree in Electronics and Communication Engineering from JNTU, Hyderabad .Worked as Assistant
Professor in ECE Department at Chaitanya Bharathi Institute of Technology and
Science,Gandipet,Hyderabad from 2000-2005.Then joined as Associate Professor in ECE Department at
G.Narayanamma Institute of Technology and Science and presently working in the same institute.Now
pursueing Ph.D Under the guidance of DR.M.Satyam, Professor,IIIT,Hyderabad in Low Power VLSI
Design.
DR.M.Satyam-Received B.E.(Electronics) from Madras University in 1958.Obtained M.E. and Ph.D from
Indian Institute of Sciences in 1960 and 1963 respectively.He worked in various capacities at IISc,Bangalore
till his retirement. Presently he is working as visiting professor ,IIIT, Hyderabad. He guided 33 Ph.D
students and 11 M.S. students. He has 139 publications in various National and International Journals. He
has 10 patents to his credit. He is actively engaged in Research and Teaching.
DR.B.C.Jinaga- Born on June, 1950, graduation from Karnataka University, Dharwad, Post Graduation from
Regional Engineering, Warangal and Ph.D. from Indian Institute of Technology, Delhi. He has been with
JNT University since 21 years. Prof.B.C. Jinaga was occupied various key positions at JNT University . He
guided several students for Ph.D. He has published quite number of papers in reputed international and
National journals. His research interest includes signal Processing and Coding Techniques. He received Best
Teacher Award for the year 2002 - Awarded by Government of Andhra Pradesh.

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