The shrinking characteristics of transistors etched on silicon always need to push the forefront of manufacturing technology. However, the discovery of atomic thin materials such as graphene and carbon nanotubes improves the prospect of replacing our manufacturing needs with the natural properties of these materials. If carbon nanotubes with the width of 1 nm can be simply used, it is unnecessary to etch the feature of 1 nm in silicon.
At present, some remarkable successes have been achieved, such as 1 nanogate made of single carbon nanotube. However, this work usually involves a difficult process, that is, how to put atomic thin materials in the right position to manufacture functional equipment. The rest of the hardware is usually made of bulky materials borrowed from more traditional transistor designs.
However, a new research paper published this week describes a record design of the smallest transistor gate length to date. This record was set by the edge of the graphene sheet, which means that the grid has only one carbon atom. Moreover, the design team ensures that the whole transistor is easy to manufacture and relatively compact by using the second atomic thin material on the key components (plus clever arrangement of parts).
Towards atomization
A standard transistor design includes two conductive electrodes: a source and a drain, separated by a semiconductor. The state of a semiconductor, that is, whether it is conductive or insulating, is determined by the third conductive electrode, which is called the gate. Although there are many criteria to measure the size of a transistor, the gate length is one of the most important criteria.
Silicon may be the most famous semiconductor, but there are also semiconductors with thin atoms in these materials, and molybdenum disulfide is the most prominent. Although molybdenum disulfide is not as thin as a single atom because of the arrangement of chemical bonds, it is still very dense. Considering its useful properties, good characteristics and easy use, researchers use molybdenum disulfide as their semiconductor material. The source electrode and the drain electrode are just simple metal strips that contact molybdenum disulfide.
In the previous 1 nano device, the gate was made of a single carbon nanotube. It is difficult to get smaller, but it is not impossible. Graphene flakes are like flat carbon nanotubes: a piece of carbon atoms connected together. Although the length and width of the sheet are much larger than that of the nanotube, its thickness is only the thickness of one carbon atom. So, if you can use the edge of graphene as the gate, you can get a very small gate length.
However, all these materials are used in countless test equipment. The secret of this new job lies in how they are arranged. Part of this arrangement is just to make the edges of graphene sheets act as grids in the right direction. However, a significant advantage of this design is that it is easy to manufacture, because it does not need to position the atomic thin material very accurately.
Clever geometry
To make this device, the researchers started with layers of silicon and silicon dioxide. Silicon is a pure structure-the transistor itself contains no silicon. Graphene sheets are coated on silicon and silicon dioxide to form a gate material. On top of this, the researchers placed a layer of aluminum. Although aluminum is a conductor, the researchers let it stay in the air for several days, during which its surface is oxidized to alumina. Therefore, the bottom of graphene sheet is silicon dioxide, and the top is aluminum oxide, both of which are insulators. This isolates the graphene edge from the rest of the transistor hardware.
To expose the edges of graphene in a useful way, researchers simply etched into the underlying silicon dioxide along the edges of aluminum. This will cut the graphene sheet, exposing a linear edge that can be used as a gate. At this point, the whole device is covered with a thin layer of hafnium oxide, which is an insulator and provides a little space between the gate and other hardware.
Above: equipment structure diagram. Black is silicon dioxide substrate, blue is graphene, red is aluminum/alumina layer, and yellow is molybdenum dioxide. The hafnium oxide layer is not shown.
Next, a molybdenum disulfide semiconductor wafer is placed on the whole (now three-dimensional) structure. Therefore, the edge of graphene (now embedded in the wall of the vertical part of the device) is close to molybdenum disulfide. The edge of graphene can now be used as a gate to control the conductivity of semiconductors. The length of the gate is also the thickness of the graphene sheet-a single carbon atom, which is 0.34 nm.
From there, the research team simply placed the source and drain on both sides of the gate. The three-dimensional layout makes this easy. The source is placed above, the drain is placed below, and there is a vertical wall in the middle. Researchers call their device a sidewall transistor because the gate is located in the middle of the sidewall. )
It's not just design.
Although many characteristics of the device are obtained by modeling, researchers have obviously made dozens of transistors. Some of them are sacrificed to image and confirm whether the material is in the expected position based on the manufacturing process. But everything else is used to prove that hardware can really work like a transistor, although it needs quite high voltage to do this. Its leakage is also low enough for low power operation.
Of course, researchers have proposed various methods to improve transistors. However, the performance of these early demonstration devices is a bit off topic and beyond its function.
What is really important is that researchers have found a way to really use the smallest atomic thin material as a part of functional transistors. By adding graphene and molybdenum sulfide to the equipment, this can be done without special precise positioning. This is partly because the graphene parts (edges) that need to be accurately located are produced by etching. Moreover, the location of molybdenum disulfide must be good enough to cover the gate and extend to the location where the source and drain can be connected.
Of course, it will take us a long time to make billions of devices based on this structure easy to locate. But this is definitely a necessary step to achieve the goal.