3. Results—Analysis Simulation
The behavior of Single-Electron Transistors (SETs) has been comprehensively explored through theoretical analysis and simulation, with a focus on various parameters. To simplify our understanding, we envision our nanodevice as comprising a semiconducting channel separated by an insulator layer, surrounded by a metallic gate. The application of a positive gate voltage
is observed to lower the energy levels in the channel, rendering it an n-type channel. Conversely, a negative gate voltage operation reduces the number of electrons, resulting in a p-type channel. The key mathematical equations governing the current–voltage SET curves in this study are extensively detailed in [
28].
In
Figure 1, the schematic representation of an SET is depicted, where the top and bottom terminals are designated as the source and the drain terminals, respectively. Various parameters play crucial roles in shaping the device’s characteristics, including
(gate capacitance),
and
(junction resistances), as well as
and
(junction capacitances) [
11].
The intricate interplay of these parameters contributes to the overall behavior of an SET. Positive and negative gate voltage operations induce distinct changes in the channel, influencing its conductivity and electron population. The equations described in [
28] serve as fundamental tools for understanding and predicting the current–voltage characteristics of SETs under different conditions.
This theoretical framework, coupled with the simulated results, provides a valuable foundation for exploring the potential applications and optimizations of SETs in nanoelectronics. The ability to control electron transport through the manipulation of gate voltages and other device parameters opens up avenues for advancements in electronic circuits and systems at the nanoscale.
To comprehend the current flow at the source and the drain, we consider a single-level device and its corresponding rate equations, as illustrated in
Figure 2.
The current flows into and out of the source and drain ends, characterized by Fermi levels
and
for the source and the drain, respectively. It is important to note that energy
lies between the Fermi levels in the two contacts. The Fermi functions of the source and the drain, denoted as
and
, respectively [
28], indicate that at the source,
electrons occupy the corresponding state, and at the drain,
electrons occupy this state. The average number of electrons is represented by
N.
To provide a more precise description of the electron flow between the contacts, our classical approach incorporates the quantum of conductance. When small voltage is applied to this device, the Fermi energies of the source and drain become split. Initially, before the channel couples with the source and the drain, one level exists in the channel while both the source and the drain maintain a continuous distribution of states. Upon coupling, the channel undergoes broadening [
28], and this broadening is characterized by a Lorentzian function centered around
.
We begin by considering a one-level device with a broadened density of states
given by:
where
E corresponds to the energy of a specific energy state of the device;
is a single discrete energy level;
and
are the rates at which an electron, placed initially at level
, will escape into the source and drain contacts, respectively, and
is given by:
The current
is given by
where
q is the charge per electron;
;
h is Planck’s constant;
U is the total potential energy and is given by
where
is the gate capacitance;
is the drain capacitance;
is the total capacitance [
28,
29] and is given by
where
is the source capacitance.
In Equation (
3),
and
are the Fermi distributions functions for source and drain, respectively [
30], as mentioned before. The term
in Equation (
4) expresses the change in the number of electrons and is calculated with respect to the reference number of electrons, originally in the channel, corresponding to which its energy level ε is known [
28].
Initially, we assume that
/
= 0.25 eV,
=
= 0.005 eV,
= 0.2 eV, gate voltage
= 0 V,
= 0.025 eV where
is the Boltzmann constant and
T is the absolute temperature,
/
= 0.5. It is anticipated that the current will increase when the applied drain voltage becomes sufficiently large, causing the energy level to fall within the energy window between Fermi levels
and
, as illustrated in
Figure 2.
If we set
= 0.005 eV for
> 0 and
= 0.0 eV for
< 0, then the current–voltage characteristic needs parametric investigation. In our assumptions,
is independent of energy and equal to 0.005 eV [
28]. First of all, we study the electrical behavior of Single-Electron Transistors and we investigate the influence of gate voltage
values in
-
curves. The parameters are
= −0.2 eV,
=
,
,
W = 1 µm,
L = 10 nm (
W and
L are the transistor’s dimensions; see
Figure 3), insulator thickness
t = 1.5 nm [
30], where
= 4 is the dielectric constant;
is the electrical permittivity of the vacuum. In
Figure 3, the corresponding structure of the physical layers of a typical SET is presented. The correspondence to the presented SET schematic structure of
Figure 1 is evident.
In
Figure 4a,b, the SET
-
curves under forward bias conditions unveil intriguing insights into the behavior of the device, particularly under the influence of positive gate voltage (
) values.
Figure 4a elucidates a noteworthy progression, wherein current
experiences a linear upswing with the drain–source voltage (
) as
values increase, even at lower voltage levels. This linear trend persists until a critical
threshold is reached, beyond which the current saturates. Notably, the saturation value of drain–source voltage
, marking the point of current saturation, escalates proportionally with positive gate voltage
, reaching higher positive magnitudes. Concurrently, the saturation current value exhibits a corresponding increase.
As positive gate voltage
continues to escalate,
Figure 4a indicates that current
maintains a relatively low level, initiating a subsequent linear rise with
after surpassing a discernible threshold. Significantly, this threshold value demonstrates an elevation in tandem with increasing
, eventually leading to another common saturation point, as illustrated in
Figure 4b.
The observed behavior, where current surges with increasing positive gate voltage , can be attributed to the positively charged gate. The presence of positive charges on the gate attracts electrons of intrinsic conductivity within the p-channel, augmenting the carrier concentration in the channel. With each incremental increase in , the electron concentration further intensifies, resulting in a notable reduction in channel resistance. This reduction in resistance is pivotal, contributing to the observed increase in current through the device, particularly for specific values of drain–source voltage .
This intricate interplay between and the device’s electrical characteristics highlights the impact of positive gate voltage on carrier concentration, channel resistance, and ultimately the overall current flow through the device. A deeper exploration of these phenomena promises valuable insights into the device’s functionality and may pave the way for optimized applications in electronic systems.
In
Figure 5a and
Figure 4b, the reverse bias characteristics of the device are elucidated through the SET
-
curves for positive and negative gate voltage (
) values, correspondingly.
Figure 5a unveils a distinct trend as
increases to higher positive values, where current
flowing through the device initiates a linear rise with negative drain–source voltage (
) after surpassing a discernible threshold. Notably, this threshold value exhibits an increase in tandem with the applied gate voltage,
.
A parallel behavior is observed in
Figure 5b, where
-
curves in reverse bias for negative
values are presented. Here, current
for specific
values escalates as
takes higher negative values. Intriguingly, unlike the positive
scenario, the threshold value of
, beyond which the current rises linearly, remains unaffected by
. This behavior is indicative of the intricate influence of gate voltage polarity on the device’s electrical characteristics. In
Figure 5a, the positive gate voltage attracts carriers, resulting in an increased electron concentration in the channel, leading to the observed linear rise in current
with negative
. The fact that the threshold value increases with
suggests a nuanced interplay between gate voltage and the initiation of this linear rise.
Similarly, in
Figure 5b, the negative gate voltage induces a rise in current
for specific
values, indicating the significance of carrier dynamics under negative
conditions. However, the consistent threshold value of
implies that, unlike the positive
scenario, the initiation of the linear rise in current is not affected by the magnitude of negative gate voltage.
These findings underscore the polarity-dependent response of the device to gate voltage variations in reverse bias conditions. Further exploration and analysis of these behaviors hold promise for a comprehensive understanding of the device’s characteristics, paving the way for tailored applications in electronic systems.
The impact of temperature (
T) on the electrical behavior of the Single-Electron Transistor (SET) is further elucidated in
Figure 6, which provides an overview of
-
curves with gate voltage
held at 0 V. Across all temperatures, a distinctive N-type negative differential resistance (NDR) region becomes apparent in the
-
curves. This intriguing effect stems from the enhancement of one of the two tunneling barriers within the transistor, facilitated by the source–drain electric field.
Notably, the temperature-dependent behavior is highlighted in
Figure 6, where it is evident that the value of
at which the negative differential resistance region manifests undergoes modifications with changes in temperature. Specifically, with an increase in temperature (
T), the critical
value (where NDR appears) takes on lower positive magnitudes. This observation underscores the dynamic influence of thermal energy on the tunneling processes within the transistor, shaping its electrical characteristics.
Furthermore,
Figure 6 reveals that as the temperature rises, the value of the current’s peak also increases. This temperature-dependent enhancement in the current’s peak further adds to the complexity of the N-type NDR phenomenon. It is noteworthy that this behavior is not only inherent in a variety of materials [
30] but is also observed at the atomic level [
31].
Figure 7 presents
-
temperature dependence at reverse bias. It is clear that the N-type NDR region is not present at this temperature range.
In summary, the intricate interplay between temperature, tunneling barriers, and the resulting N-type NDR phenomenon is elucidated through detailed - curves. This deeper understanding may open avenues for temperature-sensitive applications and optimizations in electronic systems employing SET configurations.
In the realm of fabricated nanoelectronic devices [
32], analog results have been meticulously observed and recorded. The intriguing influence of temperature on these devices aligns with a model simulation akin to the Fermi Golden Rule [
33]. In an experiment where temperature
T is held constant at an extremely low value of 0.025 K (almost zero, 0 K), variations in gate voltage
values unveil a compelling phenomenon: as
ascends from 0 V to 10 V, the characteristic N-type negative differential resistance region in the
-
curves diminishes, and
begins to rise at higher
.
This distinctive behavior, vividly illustrated in
Figure 8a,b, delineates the impact of negative gate voltage
values on the N-type negative differential resistance region, a phenomenon previously documented [
13]. Notably, as
takes on progressively higher negative values, the region of N-type negative differential resistance gradually fades, and the
-
curves transform into the typical
-
curves of a symmetric SET junction.
This insightful observation underscores the pivotal role of negative gate voltages in modulating the electrical characteristics of the fabricated nanoelectronic device. The disappearance of the N-type negative differential resistance region and the transition to symmetric - curves reveal the nuanced influence of gate voltage polarity on the tunneling processes and overall behavior of the SET junction.
The findings not only corroborate experimental results, but also contribute to a deeper understanding of how gate voltage variations, especially in the negative range, can be leveraged to manipulate the electrical response of nanoelectronic devices. Such insights may prove instrumental in the design and optimization of SET-based circuits and systems for various applications in nanoelectronics.
4. Discussion
The acknowledgment that the operation of Single-Electron Transistors (SETs) involves pure quantum mechanical processes and surpasses classical models is crucial in understanding their intricate behaviors. While classical models, including the one presented in this study involving broadening functions and Landauer’s formula, may not capture the full quantum complexity, they offer valuable insights into the impact of external parameters such as temperature on the Negative Differential Resistance (NDR) effect.
The inclusion of a Lorenzian density of states broadening discrete level ε is a significant aspect of the model. Acknowledging the existence of multiple levels, even though only one independent level is studied, is essential. This recognition prompts consideration of Coulomb oscillations arising from these multilevels, which can profoundly affect the I-V characteristics of the SET.
The study reveals that the NDR effect of an SET is intricately tunable by both temperature and drain–source voltage. As the temperature escalates from 0 K to room temperature, the emergence of N-type negative differential resistance becomes evident at lower drain–source voltage values. Simultaneously, the choice of gate voltage () exhibits an opposite effect on the N-type negative differential resistance behavior.
An essential outcome is the realization that, with a judicious selection of temperature and gate voltage, a Single-Electron Transistor can manifest diverse behaviors, ranging from negative differential resistance to typical on–off transistor behavior or simple resistance behavior. This heightened controllability positions the SET as a highly versatile and manipulable nanodevice.
The appearance, form, and potential hysteresis effect of NDR make the SET an ideal candidate for applications in negistor and memristive devices. The intricacies unveiled in this study not only contribute to a better understanding of SET behavior, but also open avenues for exploiting these nanodevices in emerging technologies with implications for computing, memory, and beyond. Further research and exploration in this direction promise to unveil even more sophisticated and nuanced functionalities of Single-Electron Transistors.
The intricacies uncovered in this study lay the foundation for deeper exploration into the potential applications and further refinement of the control mechanisms of Single-Electron Transistors (SETs). The dynamic interplay between temperature, gate voltage, and the resulting I-V characteristics introduces a spectrum of possibilities for tailoring the behavior of these nanodevices.
The acknowledgment that the model explores only one independent level, while there may be multiple discrete levels, prompts a more comprehensive examination. Future research could delve into the impact of these multilevel structures on Coulomb oscillations and how they collectively contribute to the overall performance of SETs. Understanding and controlling these oscillations could provide a pathway for even finer control over device behavior. Expanding on the idea of tuning the NDR effect through temperature and gate voltage, researchers may explore optimization strategies. Investigating specific temperature and gate voltage combinations that yield desired behaviors could pave the way for tailored applications. This optimization could be crucial for achieving reliable and reproducible outcomes in practical applications.
The controllability demonstrated in the study positions SETs as promising candidates for various emerging technologies. Beyond negistor and memristive applications, SETs could find applications in quantum computing, neuromorphic computing, and other advanced computational paradigms. The ability to manipulate their behavior with external parameters makes them adaptable to diverse requirements. The observed hysteresis effects in the NDR behavior of SETs introduce interesting possibilities for memory applications. Investigating the characteristics of this hysteresis, such as its dependence on temperature and gate voltage, could pave the way for developing robust and energy-efficient memory devices. This could be particularly valuable in the era of low-power and high-density memory solutions.
While the theoretical model and simulations provide valuable insights, experimental validation is essential. Researchers should consider translating these findings into practical experiments, fabricating SET devices with the suggested parameters, and empirically confirming the predicted behaviors. This step is crucial for bridging the gap between theoretical studies and real-world applications. The study’s findings have implications for the design of nanoelectronic circuits. Engineers and designers may leverage the controllability of SETs to enhance the performance and functionality of circuits at the nanoscale. The integration of SETs into existing or novel circuit architectures could lead to advancements in computational efficiency and energy consumption.
In conclusion, the study’s insights into the temperature-dependent behaviors of Single-Electron Transistors open up a rich landscape for exploration and application. By refining theoretical models, optimizing external parameters, and bridging the gap between theory and experimentation, researchers can unlock the full potential of SETs for transformative contributions to the field of nanoelectronics. The journey toward harnessing the quantum mechanical intricacies of SETs is poised to bring about paradigm-shifting advancements in technology.