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1-4 DEMULTIPLEXER

                    A Single Electron Demultiplexer  is presented in this paper .Modern techniques of  lithography make it possible to confine electrons to sufficiently small dimensions that the quantization of both their charge and their energy are easily observable. When such confined electrons are allowed to tunnel to metallic leads a single electron transistor (SET) is created. This   transistor turns on and off again every time one electron is added to the isolated region . A demultiplexer, sometimes abbreviated dmux, is a circuit that has one input and more than one output. It is used when a circuit wishes to send a signal to one of many devices. This description sounds similar to the description given for a decoder, but a decoder is used to select among many devices while a demultiplexer is used to send a signal among many devices. The logic operation of 2 bit multiplier is verified using simulation software “SIMON”.

 

INTRODUCTION TO SINGLE ELECTRON CIRCUITS:

The spectacular evolution of micro electronics has demonstrated the power of the circuit paradigm. During the last decade , a broad class of nano electronic discrete devices has been proposed and successfully demonstrated, however, there still exists a gap between physics and nano electronic integrated circuit design.

Single electron devices are a group of promising devices for the next generation circuits. These new perspective also helps in conceiving novel nano devices.

The basic physical phenomena under consideration is the quantum mechanical tunneling of electrons through small insulating gap between two metal leads, coulomb blockade and coulomb oscillations.

 

BASIC PRINCIPLE OF SETS:

The basic principle of single electronics is that one needs a coulomb energy Ec to charge an island with an electron. This energy is given by

            Ec  =  e2/ ( 2*Ci)

Here ‘e’ is the charge on a single electron and  Ci is the capacitance of the island.

To ensure that the electron states are localized on islands all thermal resistances must be larger than the fundamental Resistance Rq:

R  >  Rq = h/e2   ~ 25813 ohms

Here ‘h’ is the plank’s constant.

To simulate the tunneling of the electrons from island to island in a single electron circuit one has to determine the rates of all tunnel events.

The tunnel rate of a possible tunneling event depends on the change in the circuits free energy caused by these particular event.

            E(free energy) =  U – Wc

Here E(free energy) is the free energy of single electron circuit U is the energy stored in its capacitance and Wc is the work done by voltage source of the circuit.

 

 

And the tunneling rate is given by

*   =  Fv /e2 Rt(1-exp(-F/KT)

F is the change in free energy caused by the tunneling event.

Rt is the resistance of the junction through which electron is transported.

K is the boltzman constant  and T is the temperature.

Once the tunneling rates are determined for all possible tunnel events, the actually occurring tunnel event is determined by using a Monte Carlo analysis with the exponential distribution of tunnel events.

 

SIMON  SOFTWARE VERSION 2.0:

Simon (Simulation Of Nano Structures) is a single electron tunnel device and circuit simulator.

            The program SIMON is a single electron device circuit simulator, with which it is possible to simulate a wide variety of single electron circuits. SIMON simulates the propagation of electrons through a network consisting of small tunnel junctions, capacitors, resistors and ideal voltage sources (constant voltage sources, piece-wise linearly time dependent voltage sources and voltage-controlled voltage sources) including co-tunneling if so desired.

          The graphical circuit editor lets you pick and place the various objects which you can connect via nodes to your custom circuit. In addition we can specify many parameters like the temperature or order of tunneling. The incorporated visualization tool allows a quick comparison of simulation results. We can also study the consequences of single tunnel events with the interactive analysis. That is, we can force a single electron to tunnel through a particular junction and study how energy, voltage and charge changes throughout the network.

            Electrons can tunnel through tunnel junctions. The energy sources are voltage sources, current sources and thermal energy. Simon basically calculates probabilities for every possible tunnel event using Monte Carlo scheme to choose one event from a set of possible events. In general the probabilities for possible tunnel events will change over time, because the voltage source can vary with time and the distribution of electrons in the network changes with every tunnel event, which in turn influences the propagation of electrons. Thus for every tunnel step a new set of probabilities has to be calculated.

 

 

1-4 DEMULTIPLEXER:

                         In electronics, a multiplexer or mux is a device that performs multiplexing; it selects one of many analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output.

                          An electronic multiplexer makes it possible for several signals to share one device or resource, for example one A/D converter or one communication line, instead of having one device per input signal.

                      On the other end, a demultiplexer is a device taking a single input signal and selecting one of many data-output-lines, which is connected to the single input. A multiplexer is often used with a complementary demultiplexer on the receiving end.

                    An electronic multiplexer can be considered as a multiple-input, single-output switch, and a  demultiplexer as a single-input, multiple-output switch. The schematic symbol for a multiplexer is an isosceles trapezoid with the longer parallel side containing the input pins and the short parallel side containing the output pin. The schematic on the right shows a 2-to-1 multiplexer on the left and an equivalent switch on the right. The sel wire connects the desired input to the output.

                  In telecommunications, a multiplexer is a device that combines several input information signals into one output signal, which carries several communication channels, by means of some multiplex technique. A demultiplexer is in this context a device taking a single input signal that carries many channels and separates those over multiple output signals.

              The simulation shows that there are no permanent currents crossing the tunnel junctions J1 to J8 but only small current pulses that are due to the movement of a single electron crossing a tunnel junction. This proves the single electron operation of the circuit. Fig. 8 shows the free-energy diagram ofthe circuit that is provided by SECS, during the time of operation of the device. The maximum energy that is stored in the circuit is about 5_10_19 J or 3.1 eV. This derives from the fact that there are current pulses, from a single electron tunneling, whenever there is a change at the input voltages. This fact ensures the low power consumptionof the circuit.



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