presentation on optoelectronic synaptic devices based on beta gallium oxide

1. OPTICAL SYNAPTIC DEVICES
BASED ON β-Ga2O3 MATERIAL
SUBMITTED BY- BISHAL LASKAR
M.TECH(VLSI) , ROLL- 2202008
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING , IIIT-G

2. CONTENTS
Title Slide
What makes the brain laugh at
traditional chips?
3
Why neural networks 4
The synaptic magic 5
A light on the working of bio-
synapse
6
Different types of channels 7
Need for optoelectronic synapses 8
Literature review 9
β-Ga2o3 based synapses 10
Turning the thought into a device 11
Working of the device 12
Reference 16
2

3. WHAT MAKES THE BRAIN LAUGH AT TRADITIONAL CHIPS?
The answer is : Von Neumann
Architecture!
Top1 vs. operations, size ∝ parameters. Top-1 one-crop
accuracy versus amount of operations required for a single
forward pass. The size of the blobs is proportional to the
number of network parameters
• Huge Data transfer cost leads to its inefficacy to be fast. This
is mainly due to the routing length increase as the BUS
exists.
• The competitive GPU market led to increased speeds as the
architecture was being modified in every new flavour.
NVDIA Turing Google TPU *source-google images and

4. WHY NEURAL NETWORKS?
• Requires just 20W of power!
• Memory and computation are done
together just like in the neurons of our
brain.
• Volume= 2litres
• 100 billion neurons
• 100-1000 synapses between neurons =
100 trillion synapses in this volume.
• Mechanism followed- IONIC
CONDUCTION!!
• Neuromorphic computing only mimics the
brain’s mechanism. Does NOT emulate it!
vs
SO MUCH TO ACHIEVE!!!
The Gargantuan Riken’s “K” Supercomputer
• 705,024 processor cores , 1.4 Million GB of RAM
• (1.73 B Nerve cells and 10.4 T Synapses)=1% of Brain
• 40 minutes required to simulate 1 sec of brain’s activity
• 1day=24x60x60 sec, 6.5 years for 1 day of brain activity.
• REASON= The whole architecture is digital based.
The TRUTH BEHIND the
inefficacy of building a digital
architecture based computer.

5. THE SYNAPTIC MAGIC
5
• The synapse as shown in the figure has three parts, the pre-
synaptic neuron, the synaptic cleft or gap and the post-synaptic
neuron.
• The axon terminal releases neurotransmitters in a certain
fashion through the whole process of transmitting information
which lands on the receiving end and carries it forward.
• Typically studying and emulating the behavior of a biological
synapse will help us determine how to develop a device that
can carry information in the form of electrical stimuli.
• Typically number of neurons in a volume of approx 2L = 1011
and number of synapses between them = 1015.
• Peak power = 20W

6. 6
• Between the cell and the outerside there lies a thin membrane.
• Due to difference in the concentration of ions, there occurs an
electrochemical gradient resulting in a difference of potential.
• The resting potential difference is around 70 mV
• The transportation of ions from one side to the other takes
place through different channels. Depending upon their type,
they operate and quantify the number of ions passing through
them.
• When the concentration gradient changes across the
membrane, the potential also changes.
• Ions move along the channel by passive diffusion along their
concentration gradient.
• Some of the channels are always open but the others need a
signal to open or close as mentioned above.
A LIGHT ON THE WORKING OF BIO-SYNAPSE
Fig : Illustration showing the difference in concentration between the membrane creating a
gradient.
Fig : Illustration showing the spike of an action potential as observed in a neuron
Source :Alberts B, Johnson A, Lewis J, et al. “Molecular Biology of the Cell. 4th edition.”

7. DIFFERENT TYPES OF CHANNELS
7
• Voltage gated ion-channel : open only when the
membrane potential reaches a particular value.
• Ligand gated ion-channel : Open only when
bound by a specific molecule.
• Mechanically gated ion-channel : Open in
response to a physical force such as changes in
length or changes in pressure.
• Most of the ion channels are selectively permeable ,
i.e they allow only one or a small subset of ions to
pass through. For instance , a voltage gated ion
channel.
• When ion-channels open and a graded potential
occurs, the neuron moves quickly to reset its
membrane potential to its resting values.
• This is accomplished using sodium and potassium
ion pumps.
Fig : Different types of ion-channel and their operation.
Source :Alberts B, Johnson A, Lewis J, et al. “Molecular Biology of the Cell. 4th edition.”

8. NEED FOR OPTOELECTRONIC SYNAPSES
8
Brain based computing and development of synapses for interconnection and development of a wholesome
network is necessary because:
• Provides high parallelism.
• Fault tolerance of the system is very high.
• Low power consumption.
• Memory + Computation done together in the system.
The dire need for optoelectronic synapses is due to the following facts:
• Low crosstalk
• High interference immunity.
• Low power consumption compared to the previously designed synaptic devices.
• Solves interconnection issues.
ARTIFICIAL SYNAPSES
MEMRISTOR BASED FERROELECTRIC MEMORY BASED FET BASED

9. LITERATURE REVIEW
9
C.N
o
AUTHOR TIME-STAMP DESCRIPTION DEVICE MADE
[1] Xinrong Chen , Wei
Mi , Meng Li , Jinze
Tang , Jinshi Zhao
J. Semicond (JOS,
2023)
In this work, optoelectronic artificial synapses based
on a Pt/β-Ga2O3/Pt structure were fabricated on MgO
(111)by RF sputtering and bio-like synaptic plasticity
properties were verified.
MSM structures built
[2] Rongliang Li ,
Yonghui Lin , Yang
Li , Song Gao ,
Wenjing Yue
Elsevier ,
Vacuum(2021)
In this paper, a homojunction-based multi-functional
optoelectronic synapse (MFOS) is proposed and
testified.
It enables a series of basic electrical synaptic plasticity,
including paired-pulse facilitation/depression
(PPF/PPD) and long-term promotion/depression
(LTP/LTD).
MFOS device
[3] Ying Li and
Guozhen Shen
CRPS 2021 In this paper an overview over different optoelectronic
synapses and their merits,demerits,future outreach has
been presented.
MSM structures, FETs, nanowires
[4] Alberts B, Johnson
A, Lewis J
In this book details about the working of different
types of cells and neurons have been presented.
-------
[5] Samantha B Reese ,
Timothy Rema
Joule, Cell Press,
2019, Vol 3
The feasibility of producing p-type gallium oxide
which is a major challenge for commercializing the
material and challenges to be faced have been pointed
out.
-------
[6] Naif H. Al-Hardan ,
Muhammad Azmi
Abdul Hamid,
Materials Today
Physics, 2023;38
An overview of different ongoing works on gallium
oxide has been presented from the view point of
neuromorphic computing and emulation of bio-
synaptic behaviour
MSM structures, FETs, nanowires

10. WHY USE Ga2O3 ?
10
• Even though we know that the material we have selected is not an ideal candidate for photoresponsivity ,but Ga2O3
has the following properties which can be utilized for developing a very good photo-synapse.
• Ga2O3 is a Ultra Wide Band Semiconductor , meaning the Band gap of it being extremely high(4.54 - 5.3 eV), we
can utilize it for UV-C spectrum detection(200-280nm) ; Si = 1.1 eV , SiC = 3.26 eV , GaN = 3.43 eV
• It has a large breakdown electric field of 8MV/cm ; GaN (3MV/cm), SiC (3MV/cm), silicon (0.3MV/cm).
• It has very high energy.
• Its heat resistivity is very high.
• UV-C region is called the Solar-Blind region as due to the presence of the ozone layer which absorbs these
radiations, UV-C photodetectors do not experience any form of interference.

11. 11
• Ga2O3 ensures low background of the UV-C region as well as low interferences with other sources of the ER.
• This facilitates enhancement in the SNR contributing to low false alarm rate, leading to the reliability and
accuracy of the system.
• Fabrication wise Ga2O3 has the following key points
• It is the only UWBG semiconductor that can be grown from a melt compared to its WBG counterparts which
need to be grown on foreign substrates such as sapphire leading to the generation of misfit dislocations
and stresses in the epi-layers which adversely affect device performance.
• Comparing the cost of wafer , it is 5x times cheaper than SiC and GaN (according to [5])
• Here’s a list of some of the parameters of Ga2O3 which we need to consider:
• Breakdown electric field = 6 – 8 MV/cm
• Dielectric constant = 10 (high dielectric constant means it can store higher electric energy)
• Baliga’s figure of merit = 3444e (which is 4x times larger than SiC and GaN !!)
Fig : Comparison of different parameters between WBG and UWBG semiconductors
Source : DOI: 10.1002/aelm.201600501

12. 12
• Even though there are a lot of good points to
ponder about Ga2O3,, the issues we could face are:
• Low thermal conductivity
• p-type Ga2O3 is lacking and is a serious
limitation. Regarding Ga2O3, Liu et al.15 and
Feng et al.16 reported the fabrication of p–
type Ga2O3 nanowires doped with nitrogen
and zinc, respectively. In both cases, the
authors reported that the hole concentration
was too low for useful p–n junction
fabrication.
• ZnMgO and AlGaN are alternatives for detection of
UV-C spectrum but the fabrication process is very
complex and this leads to high defect density
ultimately leading to increase in the amount of
dark-current which is undesirable.

13. 13
• Ga2O3 thin-film structures have been deposited on MgO(111) substrate using RF-magnetron sputtering.
• Ga2O3 has attracted wide spread group of researchers due to its wide application in power electronics , solar –
blind photodetectors and other sensors.
• The device mentioned in [1] promises the following characteristics:
• Integration of detection and storage in one single cell.
• Detection of light from both UV and DUV range.
• Has emulated different biological synaptic behaviors such as:
• Short Term Memory (STM)
• Long Term Memory (LTM)
• Transition from STM to LTM
• Paired Pulse Facilitation (PPF)
• Learning experience behavior.
• The structured thus formed is of an MSM type(Metal-Semiconductor-Metal), notably of the form Pt/Ga2O3/Pt
type.
β-Ga2O3 BASED SYNAPSES

14. TURNING THE THOUGHT INTO A DEVICE
14
• We know that cone cells in the retina of
human eye helps us in detection of light of a
specific wavelength.
• These cone cells convert the information of
incident photons into electrical stimuli.
• Retina connects neuron layer through
synapses which are then used for transmitting
the electrical stimuli to the brain through the
optic nerve.
• The physically fabricated device here can be
seen having the MSM layered structure.
• The process of shining light and the whole
working will be discussed in the upcoming
slides.

15. DIFFERENT WORKING MODELS
15
The figures here show the working of two different types of Ga2O3 based PD wherein both the devices are single
photodetectors but the first one is made from normal Ga2O3 but the other one is made from amorphous Ga2O3
Source : DOI: 10.1002/aelm.201600501

16. 16
Source : DOI: 10.1002/aelm.201600501
In the work published by Xie Chao and Lu Xing Tong
we observe the fabrication β-Ga2O3 Nanowires for
DUV Photodetector and Image Sensor Application.
The results are clear as the PD doesn’t detect rays
beyond the UV-C region and the image formed is
completely dark.
First Fig DOI
010.1002/adom.201901257
In the work published by Ya-Cong Lu and his
team a High-performance solar-blind
photodetector array was constructed from Sn-
doped Ga2O3 microwires via patterned
electrodes. The arrays were linear.

17. DIFFERENT DEVICES MADE
17
Transistor-based devices not only detect light but are
also known to amplify the output signals
(photocurrent), thus making them a suitable solution
for low light intensities. Based on this advantage,
Xingqi et al reported the fabrication of a solar-blind
imaging array using amorphous (a-Ga2O3) thin-film
phototransistors. Amorphous structures can usually be
obtained at low process temperatures with high
uniformity, which makes the manufacturing process
affordable compared with crystalline Ga2O3.
The research of PD array will eventually lead to the
application of focal plane array (FPA) for solar blind
imaging systems.
Gallium oxide has potential application for FPAs, as it has
proven its performance in an array PD configuration. Its
high sensitivity, visible rejection ratio, and high operating
temperature can offer distinct advantages over AlGaN-
based FPAs.

18. WORKING OF THE DEVICE
18
• A typical current evolution process of the optoelectronic synaptic device
under 3 V read voltage and upon 280 nm wavelength light exposure with an
intensity of 1.83 μW/cm2 has been reported in the adjoining figure.
• After continuos illumination of light for 20s a current change of 39nA has
been observed.
• This shows that the device is photo-responsive.
• When light is turned OFF,
• Current values decayed gradually instead of a steady fall in values.
• The value of current attenuated to 65.7% of the original value after 30s.
• This proves that the device shows non-volatile properties.
• The reasons behind non-volatility of the device could be due to:
• Defects in the films (O2 vacancy carrier traps)
• Defect energy levels capture the carriers and prevents
recombination of photogenerated holes and electrons.

19. 19
• The device clearly shows that it exhibits STM and LTM properties:
• The schematic graphs show the photoresponsive currents and the transition from STM to LTM
induced by different factors, notably the duration for which light is incident on the device, the
frequencies of incident light, the intensity of light slowly increased and the number of light pulses
that are projected on the device.

20. 20
• The device clearly shows that it exhibits PPF(Paired Pulse Facilitation)
• It exhibits noteworthy paired-pulse facilitation (PPF) by changing the time interval (Δt) values of two continuous
light pulse pairs applied to a synaptic device.
• The intensity of the pulsed light stimuli was kept at 1.83 μW/cm2 and the light exposure time was kept at 1s
under 1V read voltage.
• The first figure shows the photoresponsive currents induced by two consecutive light pulses with a time interval
of 100 ms. Here, A1 and A2 represent the amplitudes of photoresponsive currents induced by the first and
second light pulses respectively, and the PPF index was defined as A2/A1.
• It is clearly observed that photoresponsive current induced by the second pulsed light stimuli is much more
than that induced by the first pulsed light stimuli, ascribing that the second light stimuli were applied before the
photonic current induced by the first one decayed to the initial state completely.

21. 21
• The device has very good learning capabilities and as shown previously that is has both STM and LTM
behaviour, it has been clearly demonstrated using multiple cases that the device perfectly retains
previously stored information even after its given a period of forgetting.
• Transition from STM to LTM and retaining the memory even permanently has been observed.

22. 22
A comparison between different parameters of fabricated devices , shape and size and their performance has
been listed here:

23. REFERENCES
23
1. Xinrong Chen , Wei Mi , Meng Li , Jinze Tang , Jinshi Zhao , Liwei Zhou , Xingcheng Zhang , Chongbiao
Luan – “Optoelectronic artificial synapses based on β-Ga2O3 films by RF magnetron sputtering.” DOI -
https://doi.org/10.1016/j.vacuum.2021.110422
2. Rongliang Li , Yonghui Lin , Yang Li , Song Gao , Wenjing Yue , Hao Kan , Chunwei Zhang and Guozhen
Shen, - “Amorphous gallium oxide homojunction-based optoelectronic synapse for multi-functional
signal processing.” DOI- https://doi.org/10.1088/1674-4926/44/7/074101
3. Ying Li and Guozhen Shen - “Advances in optoelectronic artificial synapses.” , DOI -
https://doi.org/10.1016/j.xcrp.2022.101037
4. Alberts B, Johnson A, Lewis J, et al. “Molecular Biology of the Cell. 4th edition.”
5. Samantha B Reese , Timothy Rema – “How Much Will Gallium Oxide Power Electronics Cost?” DOI :
https://doi.org/10.1016/j.joule.2019.01.011
6. Naif H. Al-Hardan , Muhammad Azmi Abdul Hamid, Azman Jalar, Mohd Firdaus-Raih “Unleashing the
potential of gallium oxide: A paradigm shift in optoelectronic applications for image sensing and
neuromorphic computing applications” DOI - https://doi.org/10.1016/j.mtphys.2023.101279

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THANK YOU