Transcript
When compared to the human brain, even the world's best
supercomputers are staggeringly inefficient and energy-intensive
machines. Our brains have upwards of 86 billion neurons, connected by
synapses that not only complete myriad logic circuits; they also
continuously adapt to stimuli, strengthening some connections while
weakening others.
That process is known as learning, and it enables the kind of rapid,
highly efficient computational processes that put Siri and Blue Gene to
shame. And, ironically, the human mind, for all its phenomenal
computing power, runs on roughly 20 watts of energy.
Now, as explained recently in the journal Nature Communications, materials
scientists at the Harvard School of Engineering and Applied Sciences
have created a new type of transistor that mimics the behavior of a
brain synapse. The novel device simultaneously modulates the flow of
information in a circuit and physically adapts to changing signals.
Exploiting unusual properties in modern materials, the synaptic
transistor could mark the beginning of a new kind of artificial
intelligence: one embedded not in smart algorithms, but in the very
architecture of a computer.
How does it work? The new transistors are analogous to the synapses
in our brains. Each time a neuron initiates an action and another
neuron reacts, the synapse between them increases the strength of its
connection. And the faster the neurons spike each time, the stronger
the synaptic connection.
Therefore, a system integrating millions of these tiny synaptic
transistors and neuron terminals has the potential to take parallel
computing into a new era of ultra-efficient high performance.
While calcium ions and receptors effect a change in a biological
synapse, the artificial version achieves the same plasticity with oxygen
ions. When a voltage is applied, these ions slip in and out of the
crystal lattice of an 80-nanometer thick film of "samarium nickelate,"
which acts as the synapse channel between an "axon" and a "dendrite"
terminal made of platinum.
The varying concentration of ions in the nickelate raises or lowers
its conductance - that is, its ability to carry information on an
electrical current. And, just as in a natural synapse, the strength of
the connection depends on the time delay in the electrical signal.
Structurally, the device consists of the nickelate semiconductor
sandwiched between two platinum electrodes and adjacent to a small
pocket of ionic liquid. An external circuit multiplexer converts the
time delay into a voltage that it applies to the ionic liquid, creating
an electric field that either drives ions into the nickelate or removes
them. The entire device, just a few hundred microns long, is embedded
in a silicon chip.
The synaptic transistor offers four immediate advantages over traditional silicon transistors:
- First, it is not restricted to the binary system of ones and zeros.
- Second, the synaptic transistor offers non-volatile memory, which
means that even when power is interrupted, the device remembers its
state.
- Third, the new transistor is inherently energy-efficient, because
the input energy required to drive this switching is very small.
- Fourth, it can operate at anywhere from about room temperature up to at least 160 degrees Celsius.
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