# Low Power Single Electron Or/Nor Gate Operating at 10GHz.

**ABSTRACT** The design and simulation of a single-electron OR/NOR gate is being presented using a Monte Carlo based tool. Both the OR/NOR behavior and the stability were verified while the free energy behavior of the circuit was also examined. The results confirmed that the circuit behaved as an OR/NOR gate, depicting improved characteristics than previously published single electron OR circuits, achieving a really fast operational speed at low power. Moreover, the noise through the circuit was nearly diminished, while a stable behavior of the circuit was verified without any noise present at the output points.

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**ABSTRACT:**This work presents a 4-bit shift-register designed with single-electron tunneling devices. Firstly, a single-electron D flip-flop based on NAND gates was designed and simulated. Based on D flip-flops, the shift-register architecture was also designed and successfully simulated at room temperature. Some considerations about noise margin were made. Moreover, stability analyses for the SET NAND, SET D flip-flop and SET shift-register were carried out.Microelectronics Journal 04/2013; 44(4):332–338. · 0.91 Impact Factor -
##### Article: Reconfigurable gate array architecture for logic functions in tunneling transistor technology

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Page 1

LOW POWER SINGLE ELECTRON OR/NOR GATE

OPERATING AT 10GHz

T. Tsiolakis and G. Ph. Alexiou

Dept. of Computer Engineering & Informatics,

University of Patras

Patras, GR-26500 Greece,

tsiolak@ceid.upatras.gr, alexiou@ceid.upatras.gr

N. Konofaos

Dept. Information & Communications Systems Engineering

University of the Aegean

Karlovassi Samos, GR-83200 Greece,

nkonofao@aegean.gr

may drive the electron outside the island. Hence, the necessary

condition is:

Abstract—the design and simulation of a single-electron

OR/NOR gate is being presented using a Monte Carlo based tool.

Both the OR/NOR behavior and the stability were verified while

the free energy behavior of the circuit was also examined. The

results confirmed that the circuit behaved as an OR/NOR gate,

depicting improved characteristics than previously published

single electron OR circuits, achieving a really fast operational

speed at low power. Moreover, the noise through the circuit was

nearly diminished, while a stable behavior of the circuit was

verified without any noise present at the output points.

Index Terms— Circuit free energy, circuit stability, Coulomb

blockade, Monte Carlo method, single electron gate, single

electron design and simulation, single electron transistor (SET),

single electron tunneling, single electronics, noise.

I. INTRODUCTION:

Single-electronics technology (SET), which is based on the

fundamental physical principle of the tunnelling effect through

a Coulomb blockade, seems to be a very promising

nanoelectronic technology [1, 2]. During the last five years,

several single-electronics circuits that combine large integration

and low power dissipation have been published and analysed,

such as logic gates, adders, decoders and several more [3-7].

Recently, hybrid structures have been proposed, making CMOS

and SET technologies more compatible [8-10]. The key

elements of such circuits are the fundamental gates which are

built using single-electron logic.

The basic physics behind the design, construction and

functioning of the single-electron logic are based on the

concept of a phenomenon called the Coulomb blockade. This

can be described as follows: Consider an electrical neutral

small conductor, which is called an island or a node, having

exactly as many electrons as it has protons in its crystal lattice.

This neutral island does not generate any appreciable electric

field beyond its borders and an additional electron located

outside the island can be brought into it by a weak external

force. The energy to charge an island with an electron is called

the Coulomb energy (Ec) and is given by:

C

e

Ec

2

2

=

where C is the capacitance of the island. Though the extra

charge in the island is very small, the electric field generated by

this charge is inversely proportional to the square of the island

size and may became very strong in nanoscale structures. This

strong electric field inhibits further electron transfer into the

island, giving thus rise to the Coulomb blockade effect.

Single electronics exploits the Coulomb blockade by

representing bits of information by the presence or absence of a

single electron at conducting islands. In order to keep an extra

electron confined into an island the Coulomb energy must be

greater than the thermal energy, otherwise thermal fluctuations

Tk

C

e

Ec

B

>=

2

2

,

where kB is Boltzmann’s constant and T is the absolute

temperature.

The basic principle of single-electronics is that one needs

Coulomb energy Ec to charge an island with an electron.

Electrons tunnel independently from island to island through

tunnel junctions. To assure that electron states are localized on

islands all tunnel resistances must be larger than the

fundamental resistance:

Ω≈=

25813

2

e

h

Rq

where h is Planck’s constant.

Figure 1. The designed OR/NOR gate.

To simulate the tunnelling of electrons from island to island

in a single-electron circuit one has to determine the rates of all

possible tunnel events. The tunnel rate of a possible tunnel

event depends on the change in the circuit’s free energy caused

by this particular event. The free energy, F, of a single-electron

circuit is the difference of the electrostatic energy, U, stored in

2010 IEEE Annual Symposium on VLSI

978-0-7695-4076-4/10 $26.00 © 2010 IEEE

DOI 10.1109/ISVLSI.2010.78

273

Page 2

its capacitances and the work done by the voltage sources of

the circuit, W:

F= U-W,

∑∫

n

where

=

nn

dttItVW)()(

is the summary of each work done by the n voltage sources of

the circuit, and the electrostatic energy U, stored in its

capacitances is given by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

Q

V

qU),(

2

1

υ

where, q and υ are the unknown parts of the island charge and

voltage matrices, and Q and V are the known parts of the

island charge and voltage matrices, respectively.

This paper demonstrates a complementary OR/NOR gate

capable to work in low temperature in a stable matter. The aim

of this work is to present such a circuit that is suitable for use

in more complex circuits than those presented so far and with

better and superior performance. In particular, this article

examines possible advantages of the new design in the metrics

most important for single-electron circuits, namely, the noise

margins for device parameters at a fixed operation

temperature, in this case at 4 K. Moreover, low free energy,

stability and complementarity, especially for circuits as

complex as multiplexers are essential parts of a quality design.

Hence, the need to design effective, complementary gates with

low noise and stable operation has emerged. In the following

paragraphs, we present such a task using a Monte Carlo based

tool [11] and compare with other similar gate designs

appearing in the literature [5]. In the final paragraphs we

conclude the benefits of the design which are concentrated on

the low power, the high speed, the diminished noise, and the

stable operation of the circuit in a network of gates.

II. CIRCUIT DESIGN AND SIMULATION

Figure 1 depicts the circuit of the proposed OR/NOR gate.

The circuit comprises of 6 junctions. Junctions J1-J4 are

identical, with their resistance being equal to 105 Ohm.

Junction J5 has a resistance of 2.6 x 104 Ohm, while J6 has a

large resistance of 1014 Ohm in order to prevent electron

transport from the ground. The Vdd voltage is constant at 0.1 V.

The two inputs (Input1 and Input2) can only take two values,

one equal to 0.0 V, which corresponds to the logic ‘1’, and the

other equal to -0.1 V, which corresponds to the logic ‘0’. These

inputs are applied to islands N1 and N2 through the capacitors

C1 and C2, which are identical, having a capacitance equal to

10-18 F each. The gate has two complementary output points,

the islands N3 and N4 respectively. The presence of a positive

charge in one of these islands corresponds to the logic ‘1’,

whereas the absence of a positive charge corresponds to the

logic ‘0’. Among the other elements of the circuit, the

capacitors C3 and C4 improve the performance of the OR gate

and offer its complementary output, hence becoming the vital

elements of transforming the circuit into a NOR/OR gate. Their

existence, especially for C4, depends on the whole circuit

layout in which this logic gate is considered to be part of it.

The values of their capacitance are identical and equal to10-18 F

each.

Using SIMON [11], the gate was tested for its operation

characteristics. Figure 2 depicts the inputs and the

complementary outputs of the gate. In particular, figures 2a

and 2b show the “Input1” and “Input2” input signals, whereas

figures 2c and 2d show the time variation at nodes N3 (2c)

denoted as “Output~” and N4 (2d) denoted as “Output” which

correspond to either a reading of logic “0” or to a reading of

logic “1”, always with respect to the lower and upper values.

It is concluded from the output signals that exactly one

electron is allowed to leave the output nodes providing the

positive charge at the output nodes of one electron. This offers

compatibility with previously published circuits. Indeed, the

depicted behavior is that of an OR/NOR gate, with node N4

providing the OR function and node N3 providing its NOR

complementary function. When both “Input1” and “Input2” are

equal to -0.1V, hence the input vector [0,0] is applied, N3 is

charged positively, because an electron from N3 travels to the

voltage source. Hence N3=1 and N4=0 in charge terms. Then,

if any other vector is applied, an electron jumps from N4 to N3

and the situation becomes N4=1 and N3=0. This happens every

time the outputs change while the island N3 is discharged via

N4 and the island N4 is discharged via the ground. All of the

above verify the NOR/OR functioning of the circuit.

Figure 2. The inputs and the complementary outputs of the gate.

Figure 3 depicts the free energy history diagram which is

used to test and verify the proper operation of the circuit. The

free energy of the designed circuit is calculated at each step of

an electron path, starting either from the ground, an island or a

voltage source, and ending at the ground, at an island or at a

voltage source respectively. The results are plotted as energy

versus time or time steps. This plot allows the calculation of

the total free energy [1-5] of the circuit, during a time period

of 1 sec, while the input vectors are applied as shown in figure

2. It has to be mentioned here that this time period is virtual,

because SIMON offers quasi time simulation (11). For real

time simulation we use another tool later in this paper.

Therefore, the free energy calculation can be made as:

∑

=

i

==

1

0

14. 0

t

eVEF

i

,

274

Page 3

where ti is the ith time step, resulting a quite low value.

Further than that, the plot in figure 3, depicts a stable

operation of the circuit, with an energy behavior as predicted

by the theory for single electron circuits, verifying the validity

of the designed gate.

To confirm this stable operation of the circuit, the stability

plot of the OR/NOR gate has been constructed and it is shown

in figure 4. Each point of this Cartesian space corresponds to a

combination of input voltage values. The free energy of the

circuit is calculated at each point of the stability plot. Points

that correspond to local minima of the circuit’s free energy are

colored white. Therefore, at these points, the corresponding

combination of input voltage values prohibits electron

tunneling, hence keeping the electrons into the islands. On the

other hand, points that correspond to local maxima of the

circuit’s free energy are colored black and are unstable points.

For these points, the corresponding combination of input

voltage values enhances electron tunneling and the number of

electrons into the islands can not be clearly determined.

Figure 3. The free energy diagram of the gate.

Figure 4. The stability diagram of the gate at the temperature of 4 K.

The vertices A, B, C and D of the square drawn on the

stability plot correspond to the applied vectors at the input

points. As shown, the transition of the output from low to high

and vice versa, does not drive the gate to instability or local

energy maxima.

However, a significant temperature increment drives these

gates to instability as it does in most Single Electron Circuits,

so any other operating temperatures but values near the

absolute zero are not yet recommended for these circuits. The

present circuit has been simulated for different values of

temperature and the results showed that it can achieve an

optimum performance at a temperature of 4 K.

Another design and simulation tool similar to SIMON is

the so called SECS [12]. A major difference between them is

that SECS offers real time simulation. For the extraction of an

operational speed we took notice of the time that is necessary

for a Coulomb effect to occur and the gate as tested can

operate very well at 10GHz. In SECS the simulation system

incorporates the stochastic nature of tunneling into its model

using the Monte Carlo method. We may not know exactly

when a tunnel event will occur, but we can determine, through

a probability distribution and via random sampling, the time

interval until the next tunnel event. The circuit presented here

can perform very well and without any change at its output or

any noise addition at all at 10 GHZ as a result of its good

stability.

Figure 5 shows the rise and fall times and the propagation

delay of the gate. As shown the rise and fall times of both the

complementary outputs are equal to 1.5x10-13sec. The lower

part of the picture depicts the input transition from the logical

low to the logical high value. We have chosen this transition to

be a linear one in order to be more realistic. The time needed

for the gate to completely obtain a constant value until the

input starts to change is 10-12 sec while the time the time

needed for the output to begin changing until the end of the

inputs transition is 1.4x10-13 sec. Therefore the results are more

than satisfactory. However the events that occur in each

simulation run are randomized so the propagation delay may

vary between 0.5x10-12 and 2.5x10-12 sec.

Comparing with similar designs appearing in the literature,

the closest circuit is the one proposed by Karafyllidis [5]. In

order to do so, we first calculated the free energy of our circuit

in the same way as that in the circuit depicted in [5].

Therefore, we need to apply the same sequence of the applied

input vectors over time as that in [5] with respect to the lower

and upper values. Doing so, we achieve

∑

=

i

==

1

0

09 . 0

t

eVEF

i

This value is substantially lower than that which is

calculated for the same function of the circuit in [5], which

was equal to 0.396 eV, thus an approximately 80% reduction

is calculated for our circuit.

Finally, the output noise of the OR gate that was present in

the circuit presented in [5], has been vanished in our case due

to the stability of the circuit. In more detail, the noise has been

diminished due to the small changes of the free energy during

the outputs transitions compared to that appearing in [5].

Table I summarizes the comparison between the circuit

presented here and the one depicted in [5]. The values depicted

275

Page 4

for the various parameters have been all calculated under the

same conditions.

Figure 5. Speed simulation of the circuit.

TABLE I

CIRCUIT CHARACTERISTICS AND COMPARISON.

Circuit Characteristics Circuit in [5] Circuit in this work

Junctions

Capacitors

Islands

Voltage input for logic 1 (V)

Voltage input for logic 0 (V)

Vdd (V)

Operational speed

Free energy (eVx10-1)

Noise

Complementary output

5

2

3

6

4

4

0.2

0.0

0.22

N/A

3.96

present

absent

0.0

-0.1

0.1

10GHz

0.82

diminished

present

III. CONCLUSIONS

In this article, the design and simulation of a single

electron OR/NOR gate was presented and analysed using the

Monte Carlo based tool known as SIMON. The proposed

circuit has proven its complementary functioning, it showed a

very low energy consumption value and a stable operation for

a temperature of 4K. Its operating frequency was been

determined using a real time Monte Carlo simulator known as

SECS. The noise at the outputs was minimised and a stable

operation was achieved for the whole operation. This kind of

gate is suitable for use in complex circuits and it is a well

improved design compared with similar ones appearing in the

literature.

Future work involves the design of complex circuits using

these complementary gates, such as multiplexers.

IV. REFERENCES

[1] K. Likharev: “Single-electron devices and their applications”, Proc.

IEEE 87, pp. 606–632, 1999.

[2] C. Wasshuber: “Computational Single-Electronics (Computational

Microelectronics)”, Springer, New York, 2001.

[3] G. Zardalidis, I. Karafyllidis: “Design and simulation of a nanoelectronic

single-electron control—not gate”, Microelectron. J., 37, pp. 94–97,

2006

[4] I. Tsimperidis, I. Karafyllidis, A. Thanailakis: “A single-electron three

input AND gate”, Microelectron. J., 33, pp. 191–195, 2002.

[5] I. Karafyllidis, “Single-electron OR gate,” Electron. Lett., 36, pp. 407–

408, 2000.

[6] G. Zardalidis, I. Karafyllidis: “A single-electron full adder”, IEE Proc.

Circuits, Dev. Syst., 150, pp. 173–177, 2003.

[7] T. Tsiolakis, N. Konofaos, G. Ph. Alexiou: “Design, simulation and

performance evaluation of

Microelectronics Journal 39(12), pp. 1613-1621, 2008.

[8] B. H. Lee, Y. H. Jeong: “A Novel SET/MOSFET Hybrid Static Memory

Cell Design”, IEEE Trans. Νanotech., 3, pp. 377-382, 2004.

[9] S. Mahapatra, V. Vaish, C. Wasshuber, K. Banerjee, K. Ionescu:

“Analytical Modeling of Single Electron Transistor for Hybrid CMOS-

SET Analog IC Design”, IEEE Trans. Electr. Dev. 51, pp. 1772-1782,

2004.

[10] A. Venkataratnam, A. K. Goel: “Design and simulation of logic circuits

with hybrid architectures of single-electron transistors and conventional

MOS devices at room temperature”, Microelectronics Journal 39 (12),

pp. 1461-1468, 2008.

[11] C. Wasshuber, H. Kosina, S. Selberherr: “SIMON-a simulator for single-

electron tunnel devices and circuits,” IEEE Trans. Computer-Aided

Design 16, pp. 937–944, 1997.

[12] G. Zardalidis, I. Karafyllidis "SECS: A new single electron circuit

simulator", IEEE Transactions on Circuits and Systems I, vol. 55, no 9,

2008.

a single-electron 2-4 decoder”,

276