Si/SiGe resonant interband tunnel diode with f/sub r0/ 20.2 GHz and peak current density 218 kA/cm/sup 2/ for K-band mixed-signal applications

Ultralow Power Electronic Analog of a Biological Fitzhugh–Nagumo Neuron

“…In addition to tunnel diodes that achieve NDR behavior through band-to-band tunneling, resonant tunneling diodes (RTDs) utilizing the resonant tunneling phenomenon in multiple quantum well-like structures can also be used in this implementation of FN neurons. Due to the near unity transmission probability for carriers, Si/SiGe-based RTDs can achieve a much higher NDR peak current density (2.18 mA/μm 2 ) compared to tunnel diodes while maintaining similar capacitance per area. As a result, an RTD-based oscillator can potentially reach a much higher oscillation frequency. ,,, Recently, RTDs have therefore been explored for designing spiking neurons for applications in neuromorphic computing. ,, However, fabrication of RTDs is more complicated, since RTDs are heterojunction devices.…”

“…Biological brains can perform computational tasks at an ∼100,000× efficiency compared to the digital computers. − A typical biological neuron has a surface area of ∼10 μm 2 , spends ∼10 pJ energy to generate each spike, and operates at a frequency of ∼100 Hz, which translates to a power cost of ∼1 nW for biological systems. ,, The first set of efforts in emulating biological neurons dates back to 1960s following the FN model using voltage controlled NDR devices , paired with inductors to produce relaxation oscillations similar to neuronal spiking behavior. − The inductor element is the main scaling bottleneck of this circuit implementation of spiking neuron, as coil-based passive inductors are difficult to fabricate at nanoscale with the required inductance values. The emergence of current-controlled NDR devices featuring metal–insulator phase transition materials has enabled generation of relaxation oscillations using capacitors, leading to considerable progress in artificial spiking neurons. − There have been other approaches to producing NDR, such as band to band tunneling, resonant tunneling, Gunn effect, real space electron transfer in III–V heterostructures, body biasing of MOSFET, exploiting graphene’s unique dispersion relationship near its Dirac point, using trap-based recombination processes, redox behavior of molecular junctions, and multiple circuits. ,,− Recently, we have shown that a graphene–silicon photodetector can show voltage-dependent NDR behavior under optical illumination while operating in the photovoltaic regime . This photosensor coupled with an inductive circuit element generates optically driven voltage oscillations similar to those of ganglion cells in the retina, following the FN model of spiking neurons.…”

Observation of strain in pseudomorphic Si1−xGex by tracking phonon participation in Si∕SiGe resonant interband tunnel diodes via electron tunneling spectroscopy

“…A voltagecontrolled oscillator 4 requires high current for the generation of strong microwave signals. Past Si/ SiGe resonant interband tunnel diode ͑RITD͒ work has explored this broad current range by varying the tunneling spacer thickness [6][7][8][9][10][11][12] and the Ge percentage. 8 Increased Ge percentage in the RITD spacer manifests as an elevated current density commonly attributed to the lowering of the barrier from the reduced bandgap.…”

“…27 The design of the tunneling structures for this experiment is based on the authors' past work developing Si/ SiGe-based tunnel diodes for circuit applications. [6][7][8][9][10][11][12] The peak-to-valley current ratio ͑PVCR͒ and peak current density ͑PCD͒ are two major figures of merit characterizing a tunnel diode's electrical performance. Both are determined by the tunneling process in the diode structure along with secondary effects.…”

“…Indeed, similar observations were made with the enhanced mobility of Si/ SiGe modulation doped field effect transistors, where the channel resides at the Si/ SiGe interface. 36,37 The highly localized ␦-doping used in these RITDs may also provide a mechanism for momentum mixing by the distributed states in k-space, and thereby also contribute to the large recorded current densities in prior ␦-doped RITDs, 10,11 but this requires further study. Lastly, previous full band modeling of the phonon participation in RITDs using a nonequilibrium Green's function formalism with a secondneighbor sp 3 s * planar orbital basis focused solely on Si-only RITDs.…”