Microsystems for Bioelectronics

Note that for dimensional scaling with barrier height/operational energy held constant, the switching

time will remain constant, i.e. devices using heavier particles should not be inferior to electron-based

devices.

Several recent demonstrations indeed suggest the possibility of physical realizations for a sub-5 nm

binary switch. The atomic-scale switch reported in [5, 23], for example, opens or closes an electrical

circuit by the controlled reconfiguration of silver atoms within an atomic-scale junction. Such ‘atom ic

relays’ operate at room temperature and the only moveable parts of the switch are the contacting

atoms, which open and close a nm-scale gap. Experimentally, a critical device size (gap) of 1 nm was

reported [23]. The atomic relay operates at a relatively low voltage of 0.6 V. The experimentally

measured switching time was 1

m

s, though the authors projected the switching time for optimized

devices will be in the range of 1 ns [23].

Moving atoms/ions also plays a key role in the mechanism for the operation of a recently reported

‘memristor’, utilizing TiO

2

thin films, where the switching occurs due to ionic motion of oxygen

vacancies [8, 24, 25]. Memristor-type devices may have a potential for extreme scaling. Also, some

researchers believe that memristors could offer a new fundamental circuit element, which could allow

for realization of complex functions with lower device count. The latter is very important for nano-

morphic cell applications, where volume is one of the prima ry concerns. The model of memristive

behavior has recently been proposed as a possible mechanism in the adaptive behavior of unicellular

organisms (amoebas) [25], and it was suggested that memristors could be used to build biology-

inspired electronic circuits [25].

As a final remark, ions in liquid electrolytes play an important role in biological information

processors, such as the brain. For example, in the human brain, the distribution of calcium ions in

dendrites may represent another crucial variable for processing and storing information [9]. Calcium

ions enter the dendrites through voltage-gated channels in a membrane, and this leads to rapid local

modulations of calcium concentration within dendritic tree [9]. Based on the brain analogy, the binary

state could be realized by a single ion that can be moved to one of two defined positions, separated by

a membrane (the barrier) with voltage-controlled conduc tance. These or similar structures might be

used to make an atom-based binary switch scalable to ~1 nm or below.

3.5 SUMMARY

In this chapter bounds for energy and complexity of a nanomorphic implementation of logic control

unit were developed based on fundamental physics and assuming ideal conditions/best case scenarios.

The idea of an energy barrier was developed and used as a unifying concept for the analysis of the

physical scaling limits for the binary switch (e.g. field effect transi stor) and for estimates of switching

energies and times. Also the interconnect lines were included using a model of the probability of

transmitted electron location that was ultimately used to quantify energy losses. Results of the deri-

vations in this chapter are summarized in Table 3.7. It was shown that the micro-scale processor could

have reasonable complexity and perform extensive information processing with the available micron-

scale energy supply discussed in Chapter 2.

Aggressive FET scaling is mandatory for implementation of micron-scale systems, in order to

achieve a full functionality of the logic unit. Transition from planar (2D) layout to a 3D configuration

could result in a significant energy reduction. A summary o f projected characteristic device critical

84 CHAPTER 3 Nanomorphic electronics