doping profile – профиль распределения примеси
a field effect transistor – полевой транзистор
carrier - носитель
dopant atoms – атом примеси
lesser-doped region – небольшая область легирования
insulator - изолятор
to deplete – уменьшать, сокращать
edge effects – краевые эффекты
conductivity – электропроводимость
crystalline structure – кристаллическая структура
to be capable of – быть способным к ч-л.
dimension – измерение, мера, масштаб
inability –недееспособность, несостоятельность
obstacle – препятствие, преграда
large-scale usage – широкомасштабное использование
Nanowire electrical devices
By controlling the doping profile along the length of the NW, active devices can be integrated into a NW. Afield effect transistor (FET) can be created if a nanowire has a small section that contains fewer carriers than the rest of the wire. Lowering the concentration of the dopant atoms in the growing atmosphere for a period of time can make this lesser-doped region. If another wire is placed over the top of this region, with an insulator separating the two wires, a FET is created. To control the current, a charge is place on the top wire to deplete the carriers in the FET regions of the lower wires. The rest of the wire is not affected because its concentration of carriers is high enough that it is not depleted.
NW FETs have a few advantages over CNT FETs. A great advantage is the ability to control the doping, and therefore the semiconducting properties of the NW during construction. Due to their size, nanowires show unusual electrical properties. Unlike CNTs, which exhibit ballistic conduction, nanowire conduction is influenced by edge effects. The tube structure of carbon nanotubes dictates that all atoms are fully bonded to other atoms (in a defect-free structure). However, NWs are a solid wire, and therefore atoms on the edge are not completely bonded. While the core of the NW is metallic, and thus conducting, the atoms on the outside of the wire lower the conductivity of the wire because they often contain defects in the crystalline structure. As the nanowire shrinks, the atoms on the surface of the wire represent more and more of the overall structure. The edge effects become more prominent, worsening the overall conduction of the NW.
At first glance, NWs and CNTs seem to be very similar. Both are capable of forming active devices and interconnect wires with dimensions of a few nanometers. However, there are some differences that make NWs more promising than CNTs. While CNTs are physically strong, and their metallic form has excellent conduction properties, the inability to grow CNTs with desired properties is a major obstacle to their large-scale usage.
UNIT 7.
Topical Vocabulary
a set of – набор ч-л.
a programmable switch – программируемый переключатель
to make up connections – устанавливать соединение, связь
to sandwich – располагать в промежутках между слоями
crossing micro-scale wires - скрещенные провода микроскопических размеров
to remain stable – оставаться стабильным, в устойчивом состоянии
Text 1. Molecular Devices
Even though NWs and CNTs can be used as active devices as well as wires in nano-electronics, there is also a set of molecules that could be used as the active devices. These molecules behave as diodes or programmable switches that can make up the programmable connections between wires. Chemists have designed these carbon-based molecules to have electrical properties similar to their solid-state counterparts. Molecular devices have one huge advantage over solid-state devices: their size. Thousands of molecules can be sandwiched between two crossing micro-scale wires to create an active device that takes up very little area.
In addition to being very small, molecular devices tend to be non-volatile: the configuration of the molecules remains stable in the absence of electrical stimulation. In the presence of electrical stimulation, programmable molecular device can be turned “on” and “off”, which can be used to perform logic.
Text 2.
Topical Vocabulary
logic devices – логические устройства
a positive voltage bias – положительное напряжение смещения
a molecular resonant tunneling diode – молекулярно-резонансный туннельный диод
negative differential resistance – отрицательное дифференциальное (внутреннее) сопротивление
a negative slope – отрицательный наклон
valley voltage – напряжение впадины
peak voltage – максимальное действующее напряжение
Molecular diodes
Diodes are devices that generally act as a one-way valve, allowing current to flow in only one direction. Diodes are generally not used as logic devices because they are static devices that consume lots of power. Static devices cannot be turned “on” and “off”; they simply conduct under a positive voltage bias and do not conduct otherwise. If a diode could be turned “off” so it does not conduct even with a positive voltage bias, it would have greater use.
One such diode is a molecular resonant tunneling diode (RTD). Molecular RTDs exhibit negative differential resistance (NDR) that can be turned on and off. Devices that exhibit NDR have a region of their I-V curve that has a negative slope known as the NDR region (see Figure 10). A negative slope indicates that the current reduces as the voltage increases. The I-V curve has two important voltage points: the peak and the valley. The peak voltage is the point of the highest current value, and the valley voltage is the point where the current is the lowest when the voltage is above the peak voltage.
Text 3.
Topical Vocabulary
rotaxane - ротаксан
catenane - катенан
covalent bonds – ковалентный, атомные связи
interlocking rings – сцепленные кольца
to trap – устанавливать
a rod – прут
hysteresis – гистерезис (неоднозначная зависимость скорости счета от напряжения)
oxidation – окисление, оксидирование
reduction – восстановление, преобразование
to rotate – вращать, поворачивать
back and forth – вперед-назад
encounter – встреча, столкновение (частиц)
Molecular switches
In addition to molecular diodes, there are also molecules that behave like simple switches. The most widely-known molecular switches are from a group of molecules called rotaxanes and catenanes. Rotaxanes and catenanes are molecules that are made up of two or more components that are mechanically linked. This means that the components can move in relation to one another without breaking covalent bonds.
Catenanes are made up of two or more interlocking rings. Rotaxanes consist of at least one ring (called a macrocycle) that is trapped on a rod that has two bulky ends, which prevents the ring from “sliding” off.
Rotaxane is common chemistry nomenclature for the number of components in the molecule and catenanes are molecules that have been fabricated and shown to exhibit hysteretic I-V characteristics with two stable states. Hysteresis in this case means that the device turns on and off at different voltages. The molecules are switched “on” and “off” with high voltage, and operated with lesser voltages. For example, catenane is switched on with 2 volts, off with -2 volts, and is read with ~0.1 volts. These molecules are mechanically switched “on” and “off” when one component is moved in relation to the other by either oxidation (removal of electrons) or reduction (addition of electrons). For catenane, one ring rotates through the other, and for rotaxane the ring slides back and forth on the rod. These molecules are essentially variable resistors that can be switched between two resistance values.
Since these molecules conduct current in both directions, this may limit the applications which molecular switches can be used in. These switches will have to be incorporated with other devices to create logic. Molecular switches are probably better suited for memory devices where only one transistor is encounter per memory read.