Patent Description:
Artificial intelligence (AI) systems using computing algorithms of deep learning neural networks are emerging rapidly. Despite the recent advancements within the field, the power budget involved in running deep learning neural networks in conventional computers is growing exponentially.

A type of current neural network systems is spiking neural networks (SNN) that uses artificial neuronal units that exchange information via spikes. An SNN uses the timing of the spikes to process information and each artificial neuron is typically only active when it receives or emits spikes. Such behavior reduces the required energy for operating the neural network. Neuromorphic hardware exhibiting a spiking behavior can be implemented using electronics, but then typically operates a low (kHz) speeds and requires several picojoule per spike.

A different type of upcoming neural networks includes photonic neural networks, and in order for such networks to function as a SNN, spiking photonic neurons have been implemented using graphene excitable lasers, distributed feedback lasers, or vertical-cavity surface-emitting lasers, to name a few. However, the footprint of such lasers is too large (> <NUM><NUM>) for compact and efficient SNNs. An alternative is to use a light-emitting diode (LED), e.g. a nano-LED. However, LEDs typically also operate at slow speeds (MHz) since they rely on the spontaneous emission rate (about <NUM> ns).

Thus, there exists a need for efficient, fast, and miniaturized light-emitting sources suitable for brain-like photonic spike-based computing such as a neural network.

Document <CIT> discloses a solid-state light emitting diode for electro-optical heterodyne converters which uses a resonant tunnelling structure in which charge carriers tunnel through an energy barrier.

It is an object of the present inventive concept to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination.

According to the invention a neuromorphic device is provided as recited in claim <NUM>.

Since the double-barrier quantum well region of the neuromorphic device allows for resonant tunneling of charge carriers, it features a non-linear N-shaped current-voltage characteristic (I-V characteristic). With increasing voltage across the double-barrier quantum well region, the current through the double-barrier quantum well region initially increases until it reaches a peak, and then rapidly decreases until it reaches a valley. Further increasing the voltage, the current again starts to increase after the valley. This I-V characteristic allows the neuromorphic device to operate in a positive differential conductance (PDC) configuration or a negative differential conductance (NDC) configuration depending on an applied bias voltage (i.e. by applying a bias voltage corresponding to the valley or the peak). The bias voltage is applied between the first and second contacts of the neuromorphic device. More importantly, since the neuromorphic device also comprises a light-emitting region, the PDC and NDC configurations allow for a light output from the light-emitting region to increase (in PDC configuration) or decrease (in NDC configuration) in response to a relatively small increase in voltage. Thus, the neuromorphic device is therefore allowed to produce optical spikes that is similar to the pulse-like (spiking) dynamic behavior of biological neurons. Further, depending on the bias voltage applied to the neuromorphic device, the optical spikes reproduces the behavior of excitatory synapses and/or inhibitory synapses. Hence, the neuromorphic device is allowed to operate as an artificial neuron.

Since the neuromorphic device comprises the double-barrier quantum well region and the light-emitting region, it is further appreciated that losses (e.g., electrical and/or optical losses) related to electrical and/or optical connections between external units (e.g. a resonant tunneling diode comprising a double-barrier quantum well region connected to a light-emitting diode or laser) are reduced. It is also appreciated that the footprint of the neuromorphic device is reduced since the neuromorphic device comprises the double-barrier quantum well region and the light-emitting region within the same device.

The collector region may comprise a p-doped material and the emitter region may comprise an n-doped material.

An associated advantage is that an efficiency of charge carrier injection and/or light confinement in the active region due to the double-barrier quantum well region may be improved. Thereby, the neuromorphic device may operate with a negative differential conductance region (NDC region as illustrated in <FIG>) and emit light at room temperature.

The p-doped material may comprise a first and a second p-doped layer, the first p-doped layer having a relatively higher degree of doping than the second p-doped layer, and the n-doped material may comprise a first and a second n-doped layer, the first n-doped layer having a relatively higher degree of doping than the second n-doped layer.

The collector region may comprise an n-doped material and the emitter region may comprise an n-doped material.

An associated advantage is that an electrical performance of the neuromorphic device may be determined mainly by electron flow, and hole-tunneling into the light emitting region may be mitigated. Hence, the non-linear N-shaped I-V characteristic of the neuromorphic device may exhibit a more pronounced peak-to-valley ratio at room temperature. Light emission may be achieved via hole generation directly in the double-barrier quantum well region and may be a result of Zenner tunnelling and impact ionization effects occurring in the double-barrier quantum well region due to the high local field in a depletion region ocurring in the double-barrier quantum well region upon a voltage drop. Thus, in case both the emitter region and the collector region comrpises an n-doped material, the light-emitting region may be an internal portion of the double-barrier quantum well region.

The emitter region comprises a central emitter layer sandwiched between two outer emitter layers, and a contact emitter layer arranged in contact with one of the outer emitter layers. The emitter region is in electrical contact with the first contact via the contact emitter layer. The central emitter layer has a relatively higher degree of doping than the outer emitter layers, and the contact emitter layer has a relatively higher degree of doping than the central emitter layer. The collector region comprises a central collector layer sandwiched between two outer collector layers, and a contact collector layer arranged in contact with one of the outer collector layers. The collector region is in electrical contact with the second contact via the contact collector layer. The central collector layer has a relatively higher degree of doping than the outer collector layers, and the contact collector layer has a relatively higher degree of doping than the central collector layer.

An associated advantage is that charge carriers accumulate in at least one of the outer emitter layers and the outer collector layers when the neuromorphic device is in use. The charge accumulation, in turn, allows the at least one of the collector region and the emitter region to function as a memory switching that change the differential conductance (or equivalently resistance) when a voltage is applied. This results in different I-V (current-voltage) curves corresponding to different differential conduction states. Put differently, accumulating (or trapping) charge carriers in at least one of the collector region and the emitter region give rise to hysteresis in the neuromorphic device. As a consequence of this hysteresis, the neuromorphic device may be used as, and/or as part of, a non-volative memory if the multiple conduction (resistive) states remain stable (i.e. that the same voltage and/or light pertubation produces the same resistive state) and only switched from one conduction state to another with the application of bias in excess of certain threshold voltages.

The light-emitting region may comprise a central layer sandwiched between two outer layers. The two outer layers may have a higher bandgap material and/or lower refractive index than the central layer of the light-emitting region. The outer layers of the light-emitting region may comprise AlGaAs. The outer layers of the light-emitting region may comprise an n-doped material.

An associated advantage is that layers comprising higher bandgap material and/or lower refractive index may provide carrier confinement for efficient carrier recombination and/or improved light confinement in the light-emitting region.

The double-barrier quantum well region and the light-emitting region may be integrally formed.

An associated advantage is that a size of the neuromorphic device may be reduced. A further associated advantage is that a more power efficient device may be formed.

The double-barrier quantum well region and the light-emitting region may be arranged one after the other along a longitudinal direction of the semiconductor nanostructure.

An associated advantage is that the characteristics of the double-barrier quantum well region and the light-emitting region may be tailored independently, thereby allowing for an enhanced output of light from the neuromorphic device.

The double-barrier quantum well region may be an internal portion of the light-emitting region along a longitudinal direction of the semiconductor nanostructure.

An associated advantage is that a reduced energy dispersion of the emitted light may be allowed. Hence, an improved control of the light emission energy may be achieved.

The neuromorphic device may further comprise a resonant cavity enclosing the semiconductor nanostructure, wherein the resonant cavity may be configured to confine the light emitted by the light-emitting region.

By enclosing the semiconductor nanostructure in a resonant cavity, an enhanced emission of light from the light-emitting region may be promoted. This may be due to an enhancement of the spontaneous emission rate via the Purcell effect. The neuromorphic device may therefore be allowed to produce inhibitory and excitatory optical spikes at multi-GHz rate in response to a relatively small change in voltage.

One or more of the light-emitting region, the emitter region and the collector region may comprise a III-V semiconducting material and/or a nitride, e.g. III-N combinations.

The double-barrier quantum well region may be arranged in between the n-doped emitter region and the light-emitting region.

The double-barrier quantum well region may be configured such that the at least one resonance energy level corresponds to a Fermi level of the emitter region at a predetermined voltage difference between the first contact and the second contact, thereby providing a local maximum of a probability of charge carriers of the emitter region to tunnel through the double-barrier quantum well region.

By adjusting the bias voltage such that it corresponds to the predetermined voltage difference, the neuromorphic device may be allowed to operate in the NDC configuration, thereby allowing for the light emission from the light-emitting region to decrease in response to a relatively small voltage change between the first and second contacts.

The neuromorphic device may further comprise: an electrically isolating layer arranged between the nanostructure and the resonant cavity, wherein the electrically isolating layer may comprise throughputs allowing for the electrical contact between the first contact and the emitter region, and the second contact and the collector region.

By this arrangement electrical short-circuiting of the neuromorphic device may be prevented.

The emitter region and the collector region may comprise GaAs and/or InP.

Preferentially, the emitter region and the collector region may comprise the same material having different or the same doping.

The double-barrier quantum well region may be formed by a stack of alternating layers comprising GaAs and AlAs.

The stack of alternating layers may be formed of a central layer comprising GaAs sandwiched between two intermediate layers comprising AlAs, the central layer and the two intermediate layers may be arranged between two outer layers comprising GaAs.

A size and/or material of the resonant cavity may be configured such that an evanescent field associated with light emitted by the light-emitting region may extend beyond an outer surface of the resonant cavity.

By configuring the resonant cavity such that an evanescent field extends beyond an outer surface of the resonant cavity, a further neuromorphic device may be arranged to receive the evanescent field as input. Two neuromorphic devices may thereby communicate and interact via the evanescent field.

One of the emitter region and collector region may comprise a light output portion, and wherein the resonant cavity may be configured such that light emitted from the light-emitting region may be predominantly conveyed to the light output portion, and the neuromorphic device may further comprise: a waveguide arranged to receive light from the light output portion, and to convey the received light to a light output region of the waveguide.

By configuring the resonant cavity such that the emitted light is conveyed to the light output portion of the waveguide, a further device (e.g. a light sensor) may be arranged at the light output portion of the waveguide to receive the emitted light as input. The emitted light may thereby be used and/or detected by the further device.

Said one of the emitter region and collector region may further comprise a light input portion configured to absorb light; and wherein the waveguide may be further configured to convey light from a light input region of the waveguide to the light input portion, thereby providing the voltage difference between the first contact and the second contact.

By the neuromorphic device comprising a light input portion configured to absorb light, the neuromorphic device may be allowed to absorb light as input which in turn allows the light-emitting region of the neuromorphic device to emit light. In other words, the absorbed light may result in a flow of charge carriers through the double-barrier quantum well region and the light-emitting region which in turn may result in light emission from the light-emitting region. The emitted light may, as described previously, then be conveyed to a light output portion of a waveguide. Thus, two or more neuromorphic device may thereby be allowed to communicate and interact.

The semiconductor nanostructure may be a nano-pillar, a nano-wire, a nano-whisker, a nano-rod, and/or a nano-column.

According to a second aspect, a neural network system is provided. The neural network system comprising: at least two neuromorphic devices according to the first aspect arranged such that the evanescent field associated with light emitted from a first neuromorphic device interacts with a material in the first contact and/or second contact of a second neuromorphic device, thereby resulting in a voltage difference between the first contact and the second contact of the second neuromorphic device.

The above-mentioned features of the first aspect, when applicable, apply to this second aspect as well. In order to avoid undue repetition, reference is made to the above.

According to a third aspect, a neural network system is provided. The neural network system comprising: at least two neuromorphic devices according to the first aspect arranged such that light conveyed to the light output region of a waveguide of a first neuromorphic device is conveyed to the light input region of a waveguide of a second neuromorphic device.

The above-mentioned features of the first aspect and the second aspect, when applicable, apply to this third aspect as well. In order to avoid undue repetition, reference is made to the above.

A further scope of applicability of the present disclosure will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred variants of the present inventive concept, are given by way of illustration only, since various changes and modifications within the scope of the inventive concept will become apparent to those skilled in the art from this detailed description.

Hence, it is to be understood that this inventive concept is not limited to the particular steps of the methods described or component parts of the systems described as such method and system may vary. It must be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Furthermore, the words "comprising", "including", "containing" and similar wordings do not exclude other elements or steps.

The above and other aspects of the present inventive concept will now be described in more detail, with reference to appended drawings showing variants of the invention. The figures should not be considered limiting the invention to the specific variant; instead they are used for explaining and understanding the inventive concept.

As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of variants of the present inventive concept.

The present inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred variants of the inventive concept are shown. This inventive concept may, however, be implemented in many different forms and should not be construed as limited to the variants set forth herein; rather, these variants are provided for thoroughness and completeness, and fully convey the scope of the present inventive concept to the skilled person.

A neuromorphic device will now be described with reference to <FIG>, <FIG>, and <FIG>.

<FIG> illustrates a neuromorphic device <NUM>. The neuromorphic device <NUM> comprises a semiconductor nanostructure <NUM>. The semiconductor nanostructure <NUM> may be a nano-pillar, a nano-wire, a nano-whisker, a nano-rod, and/or a nano-column. A width W of the semiconductor nanostructure may be in a range from <NUM> - <NUM>. A height H of the semiconductor nanostructure may be in a range from <NUM> - <NUM>. The semiconductor nanostructure <NUM> may be formed as a mesa-structure. The neuromorphic device may be arranged on a substrate <NUM> as exemplified in <FIG>. The substrate may comprise SiO<NUM>.

The semiconductor nanostructure <NUM> comprises an emitter region <NUM>, a collector region <NUM>, a double-barrier quantum well region <NUM>, and a light-emitting region <NUM>. One or more of the emitter region <NUM>, the collector region <NUM>, and the light-emitting region <NUM> may comprise a III-V semiconducting material and/or a nitride, e.g. III-N combinations. The emitter region <NUM> may have a thickness within a range from <NUM> - <NUM>. The collector region <NUM> may have a thickness within a range from <NUM> to <NUM>. The thickness of the collector region <NUM> may preferably be within a range from <NUM> to <NUM>. The thickness of the collector region <NUM> may more preferably be within a range from <NUM> to <NUM>. The double-barrier quantum well region <NUM> may have a thickness within a range from <NUM> - <NUM>. The light-emitting region <NUM> may have a thickness within a range from <NUM> to <NUM>. The thickness of the light-emitting region <NUM> may preferably be within a range from <NUM> to <NUM>. However, as will be described below, the light-emitting region <NUM> may be an internal portion of the double-barrier quantum well region <NUM>, and the thickness of the light-emitting region <NUM> may, in that case, therefore be smaller, e.g. <NUM>. The double-barrier quantum well region <NUM> may be formed in an intrinsic (undoped) region of the semiconductor nanostructure <NUM>, i.e. in an intrinsic region of a pn-junction.

The collector region <NUM> may comprise a p-doped material or an n-doped material. The emitter region <NUM> may comprise an n-doped material. In case both the emitter and collector regions <NUM>, <NUM> comprise an n-doped material (e.g. n-doped GaAs), the efficiency of charge carrier injection and/or light confinement in the double-barrier quantum well region <NUM> may be improved. This may allow for the neuromorphic device <NUM> to operate and emit light at room temperature. As exemplified in <FIG>, the collector region <NUM> may comprise a first collector layer <NUM> and a second collector layer <NUM>. The first collector layer <NUM> may have a relatively higher degree of doping than the second collector layer <NUM>. Within the art, the first collector layer <NUM> may be referred to as a p+-doped or n+-doped layer (depending on the type of doping of the collector region <NUM>) and the second collector layer <NUM> may be referred to as a p--doped or n--doped layer (depending on the type of doping of the collector region <NUM>). As shown in the example of <FIG>, the emitter region <NUM> may comprise a first emitter layer <NUM> and a second emitter layer <NUM>. The first emitter layer <NUM> may have a relatively higher degree of doping than the second emitter layer <NUM>. Within the art, the first emitter layer <NUM> may be referred to as a n+-doped layer and the second emitter layer <NUM> may be referred to as a n--doped layer.

Arrangements according to the invention of the emitter region <NUM> and the collector region <NUM> are illustrated in <FIG>. In <FIG>, additional layers and features that are similar to, or the same as, those illustrated in <FIG>, and <FIG> are represented by dots <NUM>, <NUM>, <NUM>. Hence, <FIG> should be understood as describing an alternate internal configuration of the emitter region <NUM> and the collector region <NUM>, while the technical features represented by dots <NUM>, <NUM>, <NUM> are understood from <FIG>, and <FIG> and the description of those figures.

As shown in the example according to the invention of <FIG>, the emitter region <NUM> may comprise a central emitter layer <NUM> sandwiched between two outer emitter layers 414a, 414b, and a contact emitter layer <NUM> arranged in contact with one of the outer emitter layers 414a. Put differently, one of the outer emitter layers 414a is sandwiched between the central emitter layer <NUM> and the contact emitter layer <NUM>. The emitter region <NUM> may be in electrical contact with the first contact <NUM> via the contact emitter layer <NUM>. The central emitter layer <NUM>, the outer emitter layers 414a, 414b, and the contact emitter layer <NUM> may have a thickness within a range from <NUM> - <NUM>. As is understood, the central emitter layer <NUM>, the outer emitter layers 414a, 414b, and the contact emitter layer <NUM> may each have a different thickness within this range. It is further to be understood that at least two of the layers <NUM>, 414a, 414b, <NUM> may have equal or similar thicknesses. The central emitter layer <NUM> has a relatively higher degree of doping than the two outer emitter layers 414a, 414b. The contact emitter layer <NUM> may have a relatively higher degree of doping than the central emitter layer <NUM>. Within the art, the central emitter layer <NUM> may be referred to as a n+-doped layer, the two outer emitter layers 414a, 414b may be referred to as n--doped layers, and the contact emitter layer <NUM> may be referred to as a n++-doped layer. As exemplified in <FIG>, the collector region <NUM> may comprise a central collector layer <NUM> sandwiched between two outer collector layers 424a, 424b, and a contact collector layer <NUM> arranged in contact with one of the outer collector layers 424a. Put differently, one of the outer collector layers 424a may be sandwiched between the central collector layer <NUM> and the contact collector layer <NUM>. The collector region <NUM> may be in electrical contact with the second contact <NUM> via the contact collector layer <NUM>. The central collector layer <NUM>, the outer collector layers 424a, 424b, and the contact collector layer <NUM> may have a thickness within a range from <NUM> - <NUM>. As is understood, the central collector layer <NUM>, the outer collector layers 424a, 424b, and the contact collector layer <NUM> may each have a different thickness within this range. It is further to be understood that at least two of the layers <NUM>, 424a, 424b, <NUM> may have equal or similar thicknesses. The central collector layer <NUM> has a relatively higher degree of doping than the two outer collector layers 424a, 424b. The contact collector layer <NUM> may have a relatively higher degree of doping than the central collector layer <NUM>. Within the art, the central collector layer <NUM> may be referred to as a p+-doped or n+-doped layer (depending on the type of doping of the collector region <NUM>), the two outer collector layers 424a, 424b may be referred to as p--doped or n--doped layers (depending on the type of doping of the collector region <NUM>), and the contact collector layer <NUM> may be referred to as a p++-doped or n++-doped layer (depending on the type of doping of the collector region <NUM>). In case the emitter and collector regions <NUM>, <NUM> comprise stacks of layers as described above, the neuromorphic device <NUM> may exhibit hysteresis in its I-V characteristics (discussed in connection with <FIG> and <FIG>). This may have the effect that the I-V characteristics follow the solid lines in <FIG> and <FIG> when the voltage over the neuromorphic device <NUM> is increased, but follows different lines (see the dashed lines L1 and L2 in <FIG> and <FIG>, respectively) when the voltage over the neuromorphic device <NUM> is descreased. This hysteresis behavior may allow the neuromorphic device <NUM> to be used as, and/or as part of, a non-volatile memory. The doping of the layers <NUM>, 414a, 414b, <NUM> of the emitter region <NUM> and/or the layers <NUM>, 424a, 424b, <NUM> of the collector region <NUM> may be chosen such that the emitter region <NUM> and/or the collector region <NUM> exhibit a highly variable charge distribution along a longitudinal direction of the semiconductor nanostructure (i.e. the stacking direction of the layers <NUM>, 414a, 414b, <NUM>, <NUM>, 424a, 424b, <NUM>). Typically, the highly variable charge distribution of the emitter region <NUM> and/or the collector region <NUM> may have a larger variation than a charge distribution of the double-barrier quantum well region <NUM>. For example, the doping of the n++-doped layers and/or n+-doped layers may be as high as <NUM>×<NUM><NUM> cm-<NUM>, and the doping of the n--doped layers may be as low as <NUM><NUM> cm-<NUM>. Thus, the doping of the n++-doped layers and the n+-doped layers may be similar or the same.

The first contact <NUM> and the second contact <NUM> are illustrated in <FIG>, however, their arrangement may be similar to the arrangement illustrated in <FIG>. In particular, the collector contact layer <NUM> may be in electrical contact with the second contact <NUM> in <FIG> in the same manner as the first collector layer <NUM> is in electrical contact with the second contact <NUM> in <FIG>. Further, the emitter contact layer <NUM> may be in electrical contact with the first contact <NUM> in <FIG> in the same manner as the first emitter layer <NUM> is in electrical contact with the first contact <NUM> in <FIG>. Put differently, <FIG> is intended to only describe the alternate internal configuration of the emitter region <NUM> and the collector region <NUM>, while the other technical features illustrated in <FIG>, and <FIG> may be similar or the same.

The emitter region <NUM> and the collector region <NUM> may comprise GaAs and/or InP. The emitter region <NUM> and the collector region <NUM> may comprise the same material with different or the same type of doping.

The double-barrier quantum well region <NUM> has at least one resonance energy level E0. The resonance energy level E0 will be discussed in connection with <FIG>.

The double-barrier quantum well region <NUM> may be formed by a stack of alternating layers comprising GaAs and AlAs. Alternatively, the double-barrier quantum well region <NUM> may be formed by a stack of alternating layers comprising InP and InAs. In any case, a material of layers acting as barriers in the double-barrier quantum well region <NUM> may have a higher bandgap than a material of layers surrounding the layers acting as barriers, which is shown in the example of <FIG>. As shown in the example of <FIG>, the stack of alternating layers may be formed of a central layer <NUM> comprising GaAs sandwiched between two intermediate layers <NUM> comprising AlAs, the central layer <NUM> and the two intermediate layers <NUM> may be arranged between two outer layers <NUM> comprising GaAs. In the example of <FIG>, the central layer has a thickness of <NUM>, each of the two intermediate layers a thickness of <NUM>, and each of the two outer layers a thickness of <NUM>. It is to be understood that the dimensions (e.g. thicknesses and widths) of the layers illustrated in <FIG> are examples only. For example, the central layer <NUM> and the outer layers <NUM> are shown to be of equal thickness and widths, however they may be different. For example, the central layer <NUM> of the double-barrier quantum well region <NUM> may have a thickness of <NUM>. A first outer layer 136a of the double-barrier quantum well region <NUM> may have a thickness within a range from <NUM> - <NUM>, e.g. <NUM>. A second outer layer 136b of the double-barrier quantum well region <NUM> may have a thickness within a range from <NUM> - <NUM>, e.g. <NUM>. The thickness of the two intermediate layers <NUM> may furthermore have different thicknesses and/or widths. The two intermediate layers <NUM> may have thicknesses within a range from <NUM> - <NUM>, e.g. <NUM> (as described above) and/or <NUM>. In <FIG>, a bandgap diagram of the double-barrier quantum well region <NUM> of <FIG> is illustrated.

At least one of the outer layers <NUM> of the double-barrier quantum well region <NUM> may be configured to absorb light. The outer layers <NUM> may be configured to absorb light by, e.g., comprising GaAs. Further, the thickness of the at least one of the outer layers <NUM> may be at least <NUM>, which may allow the at least one of the outer layers <NUM> to absorb light. Put differently, at least one of the outer layers <NUM> of the double-barrier quantum well region <NUM> may be configured for photodetection of input light (i.e. light which the neuromorphic device <NUM> takes as input). Which one of the outer layers <NUM> that is configured to absorb light may further depend on a voltage bias of the neuromorphic device <NUM>. For example, in case of forward voltage bias of the neuromorphic device <NUM>, outer layer 136a (i.e. the outer layer <NUM> which is closer to the emitter region <NUM>) may be configured to absorb light. The absoption of light may be enhanced by a resonant cavity <NUM> (resonant cavity <NUM> is described in more detail further below) and/or by an in-built local electric field resulting from accumulated charge carriers in the double-barrier quantum well region <NUM>. In response to absorbing the input light, the neuromorphic device <NUM> may emit light from the light-emitting region <NUM>. Thus, absorption of light in at least one of the outer layers <NUM> of the double-barrier quantum well region <NUM> may provide the voltage difference between the first contact <NUM> and the second contact <NUM>, whereby the light-emitting region <NUM> may emit light. By configuring at least one of the outer layers <NUM> of the double-barrier quantum well region <NUM> to absorb light, a footprint (size) of the neuromorphic device <NUM> may be reduced.

The light-emitting region <NUM> may comprise a stack of layers. As is exemplified in <FIG>, the light-emitting region <NUM> may comprise a central layer <NUM> sandwiched between two outer layers 144a, 144b. The central layer <NUM> of the light-emitting region <NUM> may comprise GaAs. The outer layers 144a, 144b of the light-emitting region <NUM> may comprise AlGaAs. The outer layers 144a, 144b of the light-emitting region <NUM> may comprise an n-doped material. In case the emitter region <NUM> and the collector region <NUM> comprises an n-doped material, emission of light may be allowed via hole generation directly in the double-barrier quantum well region <NUM>. The hole generation may be an effect of Zenner tunneling and impact ionization effects occuring in the double-barrier quantum well region <NUM> in response to the applied voltage difference (and/or absorbed input light). The generated holes may recombine with electrons in the double-barrier quantum well region <NUM>, whereby light may be emitted. Thus, in case both the emitter region <NUM> and the collector region <NUM> comprises an n-doped material, the light-emitting region <NUM> may be an internal portion of the double-barrier quantum well region <NUM>.

The light-emitting region <NUM> may be a double heterostructure.

As is seen in the example of <FIG>, the double-barrier quantum well region <NUM> and the light-emitting region <NUM> are arranged in between the emitter region <NUM> and the collector region <NUM>. The double-barrier quantum well region <NUM> and the light-emitting region <NUM> may be integrally formed. As shown in the example of <FIG>, the double-barrier quantum well region <NUM> and the light-emitting region <NUM> may be arranged one after the other along a longitudinal direction Z of the semiconductor nanostructure <NUM>. Alternatively, as is shown in the example of <FIG>, the double-barrier quantum well region <NUM> may be an internal portion of the light-emitting region <NUM> along a longitudinal direction Z of the semiconductor nanostructure <NUM>. Since the light-emitting region <NUM> may be an undoped region of the semiconductor nanostructure <NUM>, the double-barrier quantum well region <NUM> may be formed as an internal portion of the light-emitting region <NUM>. As is shown in the example of <FIG>, the double-barrier quantum well region <NUM> may be arranged in between two surrounding layers 142a, 142b of the light-emitting region <NUM>.

Hence, in case the emitter region <NUM> comprises an n-doped material, the double-barrier quantum well region <NUM> may be arranged in between the n-doped emitter region <NUM> and the light-emitting region <NUM>. The double-barrier quantum well region <NUM> may be within an undoped (intrinsic) region of the semiconductor nanostructure <NUM>. Thus, an electrical performance of the neuromorphic device <NUM> may be determined mainly by electrons, and hole-tunneling into the light emitting region <NUM> may be mitigated. The non-linear N-shaped I-V characteristic of the neuromorphic device <NUM> may thereby exhibit a more pronounced peak-to-valley ratio (i.e. the peak I0 at voltage V0 and valley I1 at voltage V1 in <FIG>). In other words, since electron transport may be faster (i.e. higher mobility) and the minoritory hole concentration and hole diffusion will be reduced, the peak-to-valley ratio may be higher than when holes contribute to the flow of charge carriers.

The neuromorphic device <NUM> further comprises a first contact <NUM> arranged in electrical contact with the emitter region <NUM>. The neuromorphic device <NUM> further comprises a second contact <NUM> arranged in electrical contact with the collector region <NUM>. As is shown in <FIG>, a voltage difference V is applied between the first contact <NUM> and the second contact <NUM>. The voltage difference V comprises a bias voltage Vdc.

Charge carriers of the emitter region <NUM> are caused to tunnel through the double-barrier quantum well region <NUM> at a resonant energy level of the double-barrier quantum well region <NUM> in response to a voltage difference V between the first contact <NUM> and the second contact <NUM> being within a predetermined voltage difference range. In case the charge carriers are electrons, a peak current density of the flow of charge carriers may be within a range from <NUM> kA/cm<NUM> - <NUM> kA/cm<NUM>.

As indicated in <FIG>, the double-barrier quantum well region <NUM> has at least one resonance energy level E0. A probability for tunneling through the double-barrier quantum well region <NUM> for charge carriers having an energy corresponding to the at least one resonance energy level E0 may relatively speaking be high, resulting in a large current through the double-barrier quantum well region <NUM>. Thus, a flow of charge carriers through the double-barrier quantum well region <NUM> and the light-emitting region <NUM> is formed in response to the charge carriers tunneling through the double-barrier quantum well region <NUM>.

The light-emitting region <NUM> is configured to emit light in response to the flow of charge carriers through the light-emitting region <NUM>. A central wavelength of the light emitted by the light-emitting region <NUM> may be determined by the materials and/or dimensions of the light-emitting region. Bulk GaAs may, e.g., target a light emission at approximately <NUM>. Depending on the target emission, other materials may be used, e.g. quantum wells or quantum dots. Since the double-barrier quantum well region <NUM> of the neuromorphic device <NUM> allows for resonant tunneling of charge carriers, it features a non-linear N-shaped current-voltage characteristic (I-V characteristic) which is described in connection to <FIG>.

The double-barrier quantum well region <NUM> may be configured such that the at least one resonance energy level E0 corresponds to a Fermi level of the emitter region <NUM> at a predetermined voltage difference between the first contact <NUM> and the second contact <NUM>, thereby providing a local maximum of a probability of charge carriers of the emitter region <NUM> to tunnel through the double-barrier quantum well region <NUM>. The local maximum of a probability of charge carriers of the emitter region <NUM> to tunnel through the double-barrier quantum well region <NUM> may result in a local maximum of the emitted light from the light-emitting region <NUM>. By adjusting the bias voltage Vdc such that it corresponds to the predetermined voltage difference, the neuromorphic device <NUM> may thereby be allowed to operate in the NDC configuration, thereby allowing for the light emission from the light-emitting region <NUM> to decrease in response to a relatively small change in voltage between the first contact <NUM> and second contact <NUM>. The small change in voltage may be a result of an incoming signal Vac to the neuromorphic device <NUM>. The incoming signal Vac may be within a range from <NUM> mV to <NUM> mV. The incoming signal Vac may hence be close to a shot-noise limit. The neuromorphic device <NUM> may thus be driven by noise (i.e. that the neuromorphic device <NUM> outputs optical pulses/optical spikes in response to input noise). The incoming signal Vac may typically be within a range from <NUM> mV to <NUM> mV. Different ways in which an incoming signal Vac may be coupled to the neuromorphic device <NUM> will be described later.

The double-barrier quantum well region <NUM> may be configured such that the Fermi level of the emitter region <NUM> is higher than the at least one resonance energy level E0 at a further predetermined voltage difference between the first contact <NUM> and the second contact <NUM>, thereby providing a local minimum of the probability of charge carriers of the emitter region <NUM> to tunnel through the double-barrier quantum well region <NUM>. The local minimum of the probability of charge carriers of the emitter region <NUM> to tunnel through the double-barrier quantum well region <NUM> may result in a local minimum of the emitted light from the light-emitting region <NUM>. By adjusting the bias voltage Vdc such that it corresponds to the further predetermined voltage difference, the neuromorphic device <NUM> may thereby be allowed to operate in the PDC configuration, thereby allowing for the light emission from the light-emitting region <NUM> to increase in response to a relatively small change in voltage between the first contact <NUM> and second contact <NUM>. The small change in voltage may be a result of an incoming signal Vac to the neuromorphic device <NUM>. Different ways in which an incoming signal Vac may be coupled to the neuromorphic device <NUM> will be described later.

In other words, a bandgap structure of the double-barrier quantum well region <NUM> may shift in response to an applied voltage difference between the first contact <NUM> and the second contact <NUM>, and the double-barrier quantum well region <NUM> may be configured such that the resonance energy level E0 corresponds to the Fermi level of the emitter region <NUM> at the predetermined voltage difference (i.e. the bias voltage Vdc) between the first contact <NUM> and the second contact <NUM>. The predetermined voltage difference may be within the predetermined voltage difference range.

The neuromorphic device <NUM> may further comprise a resonant cavity <NUM> enclosing the semiconductor nanostructure <NUM>. The resonant cavity <NUM> may be configured to confine the light emitted by the light-emitting region <NUM>. The resonant cavity <NUM> may be a metal cavity. The metal cavity may comprise gold (Au). The metal cavity may reflect the light emitted by the light-emitting region <NUM>. Alternatively, the resonant cavity <NUM> may be a dielectric cavity configured to concentrate the light emitted by the light-emitting region <NUM> inside the semiconductor nanostructure <NUM>. A thickness of the resonant cavity may be within a range from <NUM> to <NUM>. In case the resonant cavity <NUM> is a metal cavity, the thickness of the resonant cavity <NUM> may be within a range from <NUM> to <NUM>. In case the resonant cavity <NUM> is a dielectric cavity, the thickness of the resonant cavity <NUM> may be within a range from <NUM> to <NUM>. By enclosing the semiconductor nanostructure <NUM> in a resonant cavity <NUM>, an enhanced emission of light from the light-emitting region <NUM> may be allowed. This may be due to an enhancement of the spontaneous emission rate from the light-emitting region <NUM> via the Purcell effect. A further advantageous effect of enclosing the semiconductor nanostructure <NUM> in a resonant cavity <NUM> is that the temporal duration of the emitted light (i.e. an optical spike) may be shorter than <NUM> ps, since the bandwidth of the emitted light may be increased by the Purcell effect. The neuromorphic device <NUM> may therefore be allowed to produce inhibitory and excitatory optical spikes at multi-GHz rate in response to a relatively small change in voltage.

The neuromorphic device <NUM> may further comprise: an electrically isolating layer <NUM> arranged between the semiconductor nanostructure <NUM> and the resonant cavity <NUM>. The electrically isolating layer <NUM> may comprise throughputs allowing for the electrical contact between the first contact <NUM> and the emitter region <NUM>, and the second contact <NUM> and the collector region <NUM>. A The neuromorphic device <NUM> may further comprise a further electrically isolating layer <NUM> arranged between the first contact <NUM> and/or the second contact <NUM> and the resonant cavity <NUM>. This is exemplified in <FIG> by the further electrically isolating layer <NUM> arranged between the second contact <NUM> and the resonant cavity <NUM>. The electrically isolating layer <NUM> and/or the further electrically isolating layer <NUM> may comprise silicon dioxide (SiO<NUM>). A thickness of the electrically isolating layer <NUM> and/or the further electrically isolating layer <NUM> may be within a range from <NUM> to <NUM>. In case the resonant cavity <NUM> is a metal cavity, it may form a reflector in case the electrically isolating layer is thick or a plasmonic enhancer in case the electrically isolating layer is thin. In this context, "a thick electrically isolating layer" may have a thickness above <NUM> % to <NUM> % of a wavelength (e.g. a central wavelength) of the emitted light. Hence, "a thick electrically isolating layer" may have a thickness in a range from <NUM> and larger. In this context, "a thin electrically isolating layer" may have a thickness below <NUM> % of the wavelength (e.g. a central wavelength) of the emitted light. Hence, "a thin electrically isolating layer" may have a thickness smaller than or eqaul to <NUM>.

One of the emitter region <NUM> and collector region <NUM> may comprise a light output portion <NUM>, and wherein the resonant cavity <NUM> may be configured such that light emitted from the light-emitting region <NUM> may be predominantly conveyed to the light output portion <NUM>, and the neuromorphic device <NUM> may further comprise: a waveguide <NUM> arranged to receive light from the light output portion <NUM>, and to convey the received light to a light output region <NUM> of the waveguide <NUM>. A thickness of the waveguide <NUM> may be adapted such that the received light is conveyed within the waveguide <NUM>. The waveguide <NUM> may comprise a core layer and a cladding layer. It is to be understood that the substrate <NUM> and the waveguide <NUM> may be integrally formed.

By configuring the resonant cavity <NUM> such that the light emitted by the light-emitting portion <NUM> is conveyed to the light output portion <NUM> of the waveguide <NUM>, a further device <NUM> (e.g. a light sensor) may be arranged at the light output portion <NUM> of the waveguide <NUM> to receive the emitted light as input. The emitted light may thereby be used and/or detected by the further device.

Said one of the emitter region <NUM> and collector region <NUM> may further comprise a light input portion <NUM> configured to absorb light. The waveguide <NUM> may be further configured to convey light from a light input region <NUM> of the waveguide <NUM> to the light input portion <NUM>, thereby providing the voltage difference between the first contact <NUM> and the second contact <NUM>. By the neuromorphic device <NUM> comprising a light input portion <NUM> configured to absorb light, the neuromorphic device <NUM> may be allowed to absorb light as input which in turn allows the light-emitting region <NUM> of the neuromorphic device <NUM> to emit light. The emitted light may, as described previously, then be conveyed to a light output portion <NUM> of a waveguide <NUM>. Thus, two or more neuromorphic devices <NUM> may thereby be allowed to communicate via the waveguide <NUM>, as will be described in connection to <FIG>. In case at least one of the outer layers <NUM> of the double-barrier quantum well region <NUM> are configured to absorb light, the light input portion <NUM> may be arranged to convey light (i.e. input light) to at least one of the outer layers <NUM> of the double-barrier quantum well region <NUM>, whereby the light (i.e. input light (may be absorbed). The light (i.e. input light) may be conveyed by the light input portion <NUM> to the at least one of the outer layers <NUM> via, e.g., evanescent coupling from a waveguide <NUM> (waveguide <NUM> is described in more detail below) to the semiconductor nanostructure <NUM>.

A size and/or material of the resonant cavity <NUM> may be configured such that an evanescent field associated with light emitted by the light-emitting region <NUM> may extend beyond an outer surface of the resonant cavity <NUM>. Further, a size and/or material of the electrically isolating layer <NUM> may be configured such that an evanescent field associated with light emitted by the light-emitting region <NUM> may extend beyond an outer surface of the resonant cavity <NUM>. By configuring the resonant cavity <NUM> (and the electrically isolating layer <NUM>, if present) such that an evanescent field extends beyond an outer surface of the resonant cavity <NUM>, a further neuromorphic device <NUM> may be arranged to receive the evanescent field as input. Two neuromorphic devices <NUM> may thereby communicate via the evanescent field, as will be described in connection to <FIG>.

<FIG> illustrates a schematic diagram <NUM> of the voltage-current (I-V) characteristic of the neuromorphic device <NUM>. With increasing voltage V across the double-barrier quantum well region <NUM>, the current I through the double-barrier quantum well region <NUM> initially increases until it reaches a peak current I0 at a first voltage V0, and then rapidly decreases until it reaches a valley. The current I passing through the double-barrier quantum well region <NUM> is I1 in the valley at a second voltage V1. Further increasing the voltage V, the current I through the double-barrier quantum well region <NUM> again starts to increase. This characteristics can be understood by studying the bandgap diagram of <FIG>. Without an applied voltage, the differences between the resonant energy level E0 of the double-barrier quantum well region <NUM> and the energies of the charge carriers are large, thereby resulting in a low tunneling probability whereby the flow of charge carriers (the current I) is low. Increasing the voltage, the bandgap structure starts to shift, and the differences between the resonant energy level E0 and the energies of the charge carriers start to decrease, thereby resulting in an increasing tunneling probability and an increasing flow of charge carriers through the double-barrier quantum well region <NUM>. The voltage at which the neuromorphic device <NUM> starts to emit light may be a threshold or an activation voltage. At a point where the applied voltage is such that the energies of the charge carriers correspond to the resonant energy level E0 (i.e. at voltage V0 in <FIG>), the probability for tunneling has a maximum (due to the resonant tunneling of charge carriers through the double-barrier quantum well region <NUM>), thereby resulting in a maximum flow of charge carriers. Further increasing the voltage again starts to increase the differences between the energies of the charge carriers and the resonant energy level E0, thus leading to a reduced tunneling probability and a reduced current. The applied voltage can be increased such that the bandgap structure is shifted to a degree such that the energies of the charge carriers in the emitter region <NUM> is above the energy of one of the barrier regions <NUM>. In such case, the tunneling probability is increasing with increasing voltage, which is seen at the voltage V1 in <FIG>. At sufficiently high voltages, the bandgap structure is shifted to an extent such that the Fermi level is above both barrier regions <NUM>, leading to a large flow of charge carriers. Thus, the double-barrier quantum well region <NUM> of the neuromorphic device <NUM> exhibits a non-linear N-shaped I-V characteristic as shown in <FIG>.

This I-V characteristic allows the neuromorphic device <NUM> to operate in positive differential conductance (PDC) configurations or a negative differential conductance (NDC) configuration depending on the applied bias voltage Vdc (i.e. by applying a bias voltage Vdc corresponding to the voltage corresponding to the valley or the peak). The bias voltage Vdc is applied between the first contact <NUM> and the second contact <NUM> of the neuromorphic device <NUM>. More importantly, since the neuromorphic device <NUM> also comprises a light-emitting region <NUM>, the PDC and NDC configurations allow for a light output from the light-emitting region <NUM> to increase (in the PDC configuration) or decrease (in the NDC configuration) in response to a relatively small increase in voltage V. This is shown in the optical output-voltage (P-V) characteristic shown illustrated in <FIG>. Thus, the neuromorphic device <NUM> is therefore allowed to produce optical spikes in response to a small increase in voltage. The small increase in voltage V may be a result of an incoming signal Vac to the neuromorphic device <NUM>. The input signal Vac may, e.g., be a result of the absorption of an incoming optical pulse and/or the coupling of an evanescent field from a different neuromorphic device <NUM>, which is described in connection to <FIG>. The input signal Vac may be within a range from <NUM> mV to <NUM> mV. The incoming signal Vac may typically be within a range from <NUM> mV to <NUM> mV. Hence, an input signal Vac causing the neuromorphic device <NUM> to output optical spikes may be within a range from <NUM> mV to <NUM> mV.

Now, according to <FIG> and <FIG>, operating the neuromorphic device <NUM> at a bias voltage Vdc corresponding to the first voltage V0 (i.e. at the boundary between the PDC and NDC configurations), a current I0 passes through the semiconductor nanostructure <NUM> and the light-emitting region <NUM> thereby emits light with an output power P0. As is seen in <FIG>, a small change in the voltage V would abruptly decrease the power P of the emitted light. Thus, in this configuration, the neuromorphic device <NUM> is functioning similar to an inhibitory synapse in that the neuromorphic device <NUM> is constantly firing (i.e. emitting light), and in response to an input (i.e. a small voltage drop) the current flow is abruptly decreased which will be seen as an abrupt decrease in the emitted light.

If the neuromorphic device <NUM> instead is operated at a bias voltage Vdc corresponding to the second voltage V1, a current I1 passes through the semiconductor nanostructure <NUM> and the light-emitting region <NUM> thereby emits light with an output power P1. In this case, a small decrease in the voltage V would abruptly increase the power P of the emitted light. Thus, in this configuration, the neuromorphic device <NUM> is functioning similar to an excitatory synapse in that the neuromorphic device <NUM> is constantly emitting light at a relatively low level, and in response to an input (i.e. a small voltage drop) the current flow is abruptly increased which will be seen as an abrupt increase in the emitted light (i.e. as an optical spike). This spiking behavior is similar to that of biological neurons, i.e. that the device reacts strongly to a weak input. In other words, depending on the applied bias voltage Vdc, the neuromorphic device <NUM> may function as an inhibitory artificial neuron or as an excitatory artificial neuron.

Turning to <FIG> which illustrates the output power of light emitted by the neuromorphic device <NUM> operating at the second voltage V1 (i.e. in the valley) in response to voltage drops ΔV. In other words, <FIG> illustrates the output power of light emitted by the neuromorphic device <NUM> operating as an excitatory artificial neuron. In this example, the neuromorphic device <NUM> is constantly emitting light having a power of approximately <NUM>µW, and in response to the voltage drops the neuromorphic device <NUM> emits optical spikes. The temporal duration of each optical spike may be within a range from <NUM> ps - <NUM> ns. An energy of each optical spike may be within a range from <NUM> fJ - <NUM> fJ. A voltage drop ΔV here may be due to an incoming signal Vac. On the left-hand side of <FIG>, different voltage drops ΔV (e.g. resulted from input signals Vac) are illustrated, and on the right-hand side the corresponding output power P of light emitted by the neuromorphic device <NUM> are illustrated.

In the top row of <FIG>, three different voltage drops ΔV with linearly decreasing amplitude is shown, however the corresponding output power P of the emitted light increases exponentially. Hence, the output powers P increase exponentially with linearly decreasing voltage drops ΔV, which can be realized when studying the steep slope of the NDO region of <FIG>.

In the middle row of <FIG>, three different voltage drops ΔV with increasing temporal duration is shown, however temporal durations of the corresponding emitted light remain substantially constant. Hence, the temporal durations of the emitted light remain substantially constant increasing temporal duration of the voltage drops ΔV. However, if the temporal duration of the voltage drop ΔV is sufficiently long, the neuromorphic device <NUM> will emit two optical pulses, as illustrated in the bottom row of <FIG>. This is an effect of a refractory period of the neuromorphic device <NUM>. An excitable system, such as the neuromorphic device <NUM>, may provide a large response to a small input (in this case the emission of light in response to a small voltage change) followed by a lapse (i.e. the refractory time) during which the system (i.e. the neuromorphic device <NUM>) does not respond to new stimuli. The refractory period of the neuromorphic device <NUM> may be limited by the particular materials and/or construction of the neuromorphic device <NUM>. The refractory period of the neuromorphic device <NUM> may be limited by the switch on-off time of the resonant tunneling escape time in the double-barrier quantum well region <NUM>, which typically is on the picosecond timescale. Hence, in case the temporal duration of the voltage drop ΔV (as is the case for the bottom row of <FIG>) is longer than the refractory period, the neuromorphic device <NUM> provides two optical pulses in response to a single input signal. In this way, the neuromorphic device <NUM> may be allowed to produce excitatory optical spikes (pulses) or inhibitory optical spikes (pulses), depending on the applied bias voltage Vdc, in response to a small increase in voltage.

The behavior of the neuromorphic device <NUM> as illustrated in <FIG> is similar to the behavior of biological neurons. Hence, the neuromorphic device <NUM> is thereby designed to function as an artificial neuron replicating the behavior of biological neurons.

<FIG> is a schematic illustration of a first neural network system <NUM>. The first neural network system <NUM> comprises at least two neuromorphic devices <NUM>. In the example of <FIG>, only two neuromorphic devices 10a, 10b arranged on a substrate <NUM> are illustrated, however, it is understood that the first neural network system <NUM> may comprise more than two neuromorphic devices <NUM>. Further, each of the neuromorphic devices <NUM> in the example of <FIG>, comprises a resonant cavity <NUM>, and a size/dimensions and/or material of the resonant cavity is configured such that an evanescent field associated with light emitted by the light-emitting region <NUM> extends beyond an outer surface <NUM> of the resonant cavity <NUM>.

The neuromorphic devices <NUM> are arranged such that the evanescent field <NUM> associated with light emitted <NUM> from a first neuromorphic device 10a interacts with a material in the first contact <NUM> and/or second contact <NUM> of a second neuromorphic device 10b, thereby resulting in a voltage difference V between the first contact <NUM> and the second contact <NUM> of the second neuromorphic device 10b. Hence, the first neuromorphic device 10a is allowed to communicate with the second neuromorphic device 10b via the evanescent field <NUM>, and the second neuromorphic device 10b may react in response to the signal (i.e. evanescent field <NUM>) emitted from the first neuromorphic device 10a.

<FIG> is a schematic illustration of a second neural network system <NUM>. The second neural network system <NUM> comprises two neuromorphic devices <NUM>. In the example of <FIG>, only two neuromorphic devices 10c, 10d are illustrated, however, it is understood that the second neural network system <NUM> may comprise more than two neuromorphic devices <NUM>. Further, each of the neuromorphic devices <NUM> in the example of <FIG>, comprises a resonant cavity <NUM> and a waveguide <NUM>. Each neuromorphic device <NUM> is arranged on the waveguide <NUM>. The resonant cavity <NUM> of first neuromorphic device 10c is configured such that light emitted from the light-emitting region <NUM> of the first neuromorphic device 10c is predominantly conveyed to a light output region 192c of the waveguide 190c. The second neuromorphic device 10d comprises a light input portion <NUM> configured to absorb light, and the waveguide 190d is configured to convey light from a light input region 194c of the waveguide <NUM> to the light input portion <NUM>, thereby providing the voltage difference between the first contact <NUM> and the second contact <NUM> of the second neuromorphic device 10d.

In the second neural network system <NUM>, first neuromorphic device 10c and the second neuromorphic device 10d are arranged such that light conveyed to the light output region 192c of the waveguide 190c of the first neuromorphic device 10c is conveyed to the light input region 194d of the waveguide 190d of the second neuromorphic device 10d. Hence, the first neuromorphic device 10c is allowed to communicate with the second neuromorphic device 10d via the waveguides 190c, 190d. In the example of <FIG>, a further waveguide <NUM> is illustrated. The further waveguide <NUM> may, as exemplified in <FIG>, be configured to convey light exiting the light output portion 192c to the light input portion 194d. It is further to be understood that the waveguide 190c of the first neuromorphic device 10c, the waveguide 190d of the second neuromorphic device 10d, and the further waveguide <NUM> may be sections of a same waveguide.

Hence, the first and second neural network systems <NUM>, <NUM> shown in <FIG> may propagate a signal, and by increasing the number of individual neuromorphic devices <NUM> (i.e. artificial neurons), the systems <NUM>, <NUM> may be allowed to replicate the behavior/functions of biological systems of neurons. In such case, a network of, e.g., waveguides <NUM> and/or positions of the neuromorphic devices <NUM> on the substrates <NUM> may be configured to promote communication between certain neuromorphic devices <NUM> or regions with neuromorphic devices <NUM>.

If desired, an input (e.g. an initial input signal Vac) to a neuromorphic device <NUM> of the first and/or the second neural network systems <NUM>, <NUM> may be noise (since a smallest input signal Vac may be close to <NUM> mV). Hence, the first and the second neural network systems <NUM>, <NUM> may be allowed to mimic biological systems.

Claim 1:
A neuromorphic device (<NUM>) comprising:
a semiconductor (<NUM>) nanostructure comprising:
an emitter region (<NUM>), comprising:
a central emitter layer (<NUM>) sandwiched between two outer emitter layers (414a, 414b), and
a contact emitter layer (<NUM>) arranged in contact with one of the outer emitter layers (414a), and
wherein the central emitter layer (<NUM>) has a relatively higher degree of doping than the outer emitter layers (414a, 414b), and wherein the contact emitter layer (<NUM>) has a relatively higher degree of doping than the central emitter layer (<NUM>);
a collector region (<NUM>), comprising:
a central collector layer (<NUM>) sandwiched between two outer collector layers (424a, 424b), and
a contact collector layer (<NUM>) arranged in contact with one of the outer collector layers (424a), and
wherein the central collector layer (<NUM>) has a relatively higher degree of doping than the outer collector layers (424a, 424b), and wherein the contact collector layer (<NUM>) has a relatively higher degree of doping than the central collector layer (<NUM>),
a double-barrier quantum well region (<NUM>) having at least one resonance energy level, and
a light-emitting region (<NUM>),
wherein the double-barrier quantum well region (<NUM>) and the light-emitting region (<NUM>) are arranged in between the emitter region (<NUM>) and the collector region (<NUM>);
a first contact (<NUM>) arranged in electrical contact with the emitter region (<NUM>); and
a second contact (<NUM>) arranged in electrical contact with the collector region (<NUM>); and
wherein the device is configured such that charge carriers of the emitter region (<NUM>) are caused to tunnel through the double-barrier quantum well region (<NUM>) at a resonant energy level of the double-barrier quantum well region (<NUM>) in response to a voltage difference between the first contact (<NUM>) and the second contact (<NUM>) being within a predetermined voltage difference range;
wherein a flow of charge carriers through the double-barrier quantum well region (<NUM>) and the light-emitting region (<NUM>) is formed in response to the charge carriers tunneling through the double-barrier quantum well region (<NUM>); and
wherein the light-emitting region (<NUM>) is configured to emit light in response to the flow of charge carriers through the light-emitting region (<NUM>).