Floating-Gate MOS Synapse Transistors

Abstract

Our goal is to develop silicon learning systems. One impediment to achieving this goal has been the lack of a simple circuit element combining nonvolatile analog memory storage with locally computed memory updates. Existing circuits [63, 132] typically are large and complex; the nonvolatile floating-gate devices, such as EEP ROM transistors. typically are optimized for binary-valued storage [17], and do not compute their own memory updates. Although floatinggate transistors can provide nonvolatile analog storage [1, 15], because writing the memory entails the difficult process of moving electrons through Si02 , these devices have not seen wide use as memory elements in silicon learning systems.We have fabricated synapse transistors that not only possess nonvolatile analog storage, and compute locally their own memory updates, but also permit simultaneous memory reading and writing, and compute locally the product of their stored memory value and the applied input. To ensure nonvolatile storage, we employ standard floating-gate MOS technology, but we adapt the physical processes that write the memory to perform a local learning function. Although the Si02 electron transport still is difficult, and does require high voltages, because our devices integrate both memory storage and local computation within a single device, we expect them to find wide application iil silicon learning systems.We call our devices synapse transistors because, like neural synapses [11], they compute the product of their stored analog memory and the applied input. Also like neural synapses, they can learn from the input signal, without

A vlsi recurrent network of integrate-and-fire neurons connected by plastic synapses with long-term memory

“…Hence, in the past, and still now, one of the favorite solutions for the problem of long-term storage relies on floating gates (see, e.g., [4], [5] and references therein).…”

A simulation model for floating-gate MOS synapse transistors

“…(1)-(3). We add a voltage-dependent voltage source V floating_gate from ground to the floating gate, through an arbitrarily valued resistor R 1 . There is no current through R 1 , because V floating_gate tracks the floating-gate voltage itself.…”

“…Synapse transistors [1] are a new class of devices that are rapidly gaining acceptance as standard elements in MOS circuit design. Researchers have used synapse transistors in applications ranging from weight storage in a learning array [2], to trimming a digital-to-analog converter [3], to nulling input offsets in a capacitive-feedback operational amplifier [4], to unsupervised vector quantization [5].…”

On the characterization of the trapped charge in FG-CMOS inverters

“…Early UV techniques were employed to remove this charge [1][2][3], but the UV exposure in postfabrication represents a serious drawback from the manufacturing perspective. Other approaches include extra circuitry and high voltages for the use of electron tunneling and/or hot electron injection [4][5][6]. Recently, the quasifloating gate (QFG) concept has demonstrated not only to overcome the initial charge problem, but also set the FG to a given DC bias potential through a very high resistance.…”