Motivation of the Project
The ambitious objective of this project is the introduction of a completely new class of electronic devices characterized by the following features: nanoscale physical dimensions combined with nonlinear dynamics characteristics providing noise enhanced functioning. The physics of nanoscale electronic devices is a very active and competitive field internationally. Small signal to noise ratios in nanoelectronic circuits limit the signal processing capabilities if deterministic and stochastic signals are amplified non selectively and equally. On the other hand, coupling schemes of nonlinear dynamical systems summarized as stochastic resonance (SR) have received a lot of attention as a novel technique to improve the signal resolution at low switching voltages. Up to now there are no experimental investigations of semiconductor based nanoelectronic SR devices, although nanoelectronic devices have pronounced nonlinear transport properties, which can be tailored efficiently via the design and size of the structures. We plan to study the potential of Stochastic Resonance principles for nanoelectronic applications.
SR has been proposed for the first time in 1981 to explain the periodic recurrence of ice ages. Since then the same principle has been applied in a wide variety of systems and nowadays SR is commonly invoked when noise and nonlinearity concur to determine an increase of order in the system response. Strictly speaking SR occurs in bistable systems, when a small periodic (sinusoidal) force is applied together with a large wide band stochastic force (noise). The system response is driven by the combination of the two forces that compete/cooperate to make the system switch between the two stable states. The degree of order is related to the amount of periodic motion that it shows in the system response. When the periodic force is chosen small enough in order to not make the system response switch, the presence of a non-negligible noise is required for it to happen. When the noise is small very few switches occur, mainly at random with no significant periodicity in the system response. When the noise is very strong a large number of switches occur for each period of the sinusoid and the system response does not show remarkable periodicity. Quite surprisingly, between these two conditions, there exists an optimal value of the noise that cooperatively concurs with the periodic forcing in order to make almost exactly one switch per period (a maximum in the Signal-to-Noise Ratio). Such a favorable condition is quantitatively determined by the matching of two time scales:
the period of the sinusoid (the deterministic time scale) and the Kramers rate (i.e. the inverse of the average switch rate induced by the sole noise: the stochastic time scale). Thus the name “Stochastic Resonance”.
In our project SR principles will be widely applied to nanoelectronic devices and circuits in order to drive the nonlinear system dynamics in accord with the required conditions for optimal device performances.
Goals of the Project
The goal of the SUBTLE project is to realize nanoelectronic devices exploiting noise enhanced switching and to introduce sub thermal signal resolution in nanoelectronic circuits. Only if noise is added to the input the small signal train can be detected.
Schematic diagram of noise enhanced signal processing using an inverter with a bistable transfer characteristics shown on the left. If e.g. Vin is a sinusoidal signal train with an amplitude smaller than the hysteresis voltage and the starting point is VH (black dot in hysteresis) the output signal remains constant at Vout=VH. If noise is added to the input, the input signal is eventually larger than the switching threshold and the signal train can be detected. For this mode of operation steep thresholds, bistable switching and noise are requested to sense the small input signal.
Device limits will be studied with respect to the role of noise and nonlinear device response. Our project applies SR based phenomena for single nanoelectronic FETs with intrinsic feedback to realize sensors and switches with subthermal resolution of non periodic, periodic as well as static signals. We will also integrate bistable FET and on chip noise source, which will control the combinations of stochastic and deterministic control signals. Furthermore, multiterminal SR nodes will be realized and node integration will be explored. It is the main objective of the SUBTLE project to evaluate the potential of nanoelectronic SR devices by a coordinated effort of leading European groups including physics and device modeling, nanodevice fabrication techniques as well as static and dynamic device characterization.
The project includes the realization of a number of nanoelectronic key devices, which are based on SR and EC for enhanced switching in order to resolve sub thermal signals hidden by thermal noise if classical amplification schemes are applied. The devices are characterized by the following features:
- they are nonlinear
- they take advantage of (ambient) noise instead of trying to avoid it
- they can be integrated
The technological objectives will result in the first implementations of SR in semiconductor nanostructures for switches and sensors. Internal couplings will allow the tuning of nanoelectronic field effect transistors (FETs) controlled by low dimensional gates in such a manner that steep thresholds and bistable switching without external feedback occur. The technology will focus on the realization of nanoelectronic devices in GaAs and Si with on-chip integration of tunable noise sources. Our approach is based on the enhancement of signals directly by noise in single FETs. This can be realized by a proper design of the gate-channel region in such a way, that the electrochemical capacitance leads to a positive feedback. We refer to this mode of EC operation as the feedback transistor (FBFET).
Upper left: Schematic sketch of two n-channel FETs with different channel-gate separation D (large D: blue and small D: red). Lower left: A large D leads to a channel threshold reduction compared to small D. Upper right: With electrochemical capacitance coupling (EC) between the channel and the gate the effective channel separation D depends on the number of electrons in the channel and is thus dynamic. Lower right: Due to EC the channel threshold shifts with increasing gate voltage to lower values, which leads to steep thresholds and bistable switching.
The approaches of the project are based on high resolution nanofabrication and use low dimensional, high mobility carrier systems aligned either vertically or laterally in GaAs and Si based semiconductor heterostructures. For subthermal signal sensing and switching the following devices and circuits will be studied experimentally as well as theoretically:
- Feedback transistors (FBFETs) with steep threshold and multi-stable EC switching. The FBFET advantages include subthermal thresholds, large scale integration potential and possibility to reduce the number of interconnects.
- Combined with on chip noise sources the FBFETs form the basic switching elements to observe SR effects. FB design will be optimized and the maximum signal to noise ratio studied.
- Starting from theoretical descriptions of the physics and device characteristics of single SR devices stochastic resonance nodes (SRNs) will be investigated.
- Integrated SR nodes will be realized based on interconnected bistable FBFETs. Neuron nodes for sub-thermal signal processing and the potential for self-optimized switching will be studied. Recently proposed detection schemes will be applied in nanoelectronic semiconductor devices.
In particular, three key properties will be addressed within the frame of the project:
- Subthermal signal resolution
- Noise activated switching
- Noise enhanced signal processing
These properties will be investigated using the following devices and circuits:
- Electrochemical capacitance induced feedback transistors (FBFETs)
- Noise activated nonlinear devices (NADs)
- Noise enhanced signal processing nodes (NESNs)