To make matters worse, it is hard to control the GeO 2 growing process. Researchers have studied some alternatives. But germanium passivation took a big step forward in , when a team led by Professor Shinichi Takagi of the University of Tokyo demonstrated a way to control the growth of germanium insulator. The researchers first grew a nanometer-thick layer of another high-k insulator, aluminum oxide, on the germanium channel.
Once this layer was grown, the ensemble was placed in an oxygen-filled chamber. A fraction of that oxygen passed through the aluminum oxide layer to the underlying germanium, mixing with the germanium to form a thin layer of oxide a pairing of germanium and oxygen but technically not GeO 2.
In addition to helping control the growth process, the aluminum oxide acts as a protective cap for this weaker, less stable layer. Bridge to Higher Performance: Chipmakers may one day turn to nanowire channels, like these germanium structures. Nanowires can be surrounded by a gate on all sides for added control.
S everal years ago, inspired by this finding, and facing the difficulties involved in creating pFETs with III-V channels, my group at Purdue began investigating ways to build germanium-channel transistors.
We began by using germanium-on-insulator wafers, developed by the French wafer manufacturer Soitec. These wafers are standard silicon wafers topped with an electrically insulating layer underneath a nm-thick layer of germanium.
With these wafers, we can build transistors in which all the standard silicon parts—the source, channel, and drain regions—are made of germanium. This is not necessarily the way a chipmaker would opt to make transistors, but it was an easy way for us to start studying the basic properties of germanium devices.
One of the first obstacles we faced was finding a way to handle resistance between the source and drain regions of the transistor and the metal electrodes that connect them to the outside world.
This resistance arises from a natural electronic barrier called the Schottky barrier, which forms when a metal and a semiconductor come in contact with one another. Silicon transistors have been endlessly optimized to make this barrier as thin as possible, so that charge carriers have a very easy time tunneling across it.
Getting similar behavior in a germanium device, however, requires some smart engineering. That means nFETs, which rely on the movement of electrons through the device, will have a very high resistance, wasting heat and drawing only a fraction of the current needed for fast circuits.
A standard way of thinning the barrier is to add more dopant atoms to the source and drain regions. The physics are complicated, but think of it this way: More dopant atoms mean more free charges.
And with this profusion of free-ranging charge carriers, the electrical interaction between the metal electrodes and the semiconducting source and drain regions is stronger.
That stronger coupling tends to promote the tunneling of charges across the barrier. But what we can do is go where the dopant density is highest. We can accomplish this by taking advantage of the fact that state-of-the-art semiconductors are doped by using ultrahigh electric fields to push ions into the material. Some of these dopant atoms stop fairly quickly; some go pretty far in. Ultimately, you wind up with a bell-curve-like distribution: The concentration of dopant atoms is highest at some depth and then tapers as you go shallower or deeper.
If we recess the source and drain electrodes into the semiconducting material, we can put them in contact with the highest concentration of dopant atoms. That strategy dramatically reduces the contact-resistance problem. Regardless of whether chipmakers ultimately use this strategy to thin the Schottky barrier in germanium, it is a useful demonstration of what the material is capable of.
When we began our research, the best germanium nFETs produced currents of microamperes for each micrometer of width.
In , at the Symposia on VLSI Technology and Circuits, in Hawaii, we reported on germanium nFETs with a record drain current of about 10 times that amount and more or less on a par with silicon—not bad for a preliminary demonstration.
Some six months later, we reported the first circuits containing both germanium nFETs and pFETs, a prerequisite for making modern logic chips.
Since then, we have used germanium to build more advanced transistor designs, such as FinFETs—the current state of the art. These advanced transistor designs will likely be needed for germanium to be adopted in mass manufacturing, because they offer better control over the transistor channel.
A device with better channel control will let chipmakers take advantage of the small bandgap without compromising switching performance. For one thing, there is a need for additional wafer-scale experiments that can demonstrate transistors with high-quality germanium channels.
We also need to make refinements to the device design in order to boost the speed. Researchers continue to explore III-V materials, which could be used in addition to germanium or on their own. And there is a dizzying array of other potential improvements to transistors—and the way they are wired together—on the horizon.
That list includes carbon-nanotube transistors , vertically oriented switches, 3D circuits, channels made from a mix of germanium and tin, and transistors that operate by a process called quantum tunneling. We may end up adopting several of these technologies in the coming years. But adding germanium to the channel—even initially mixed in with silicon—is a solution that will allow chipmakers to keep improving transistors in the near term.
Germanium, the primordial material of the solid-state age, could be a powerful elixir for its next decade. Peide D. Ye is a professor of electrical and computer engineering at Purdue University in Indiana. Craig S. Germanium atoms have one more shell than silicon atoms, but what makes for the interesting semiconductor properties is the fact that both have four electrons in the valence shell.
As a consequence, both materials readily constitute themselves as crystal lattices. Substituted atoms alter the electrical properties. The process of adding these atoms is known as doping.
Doping may take place by passing a gas over the crystalline material, sometimes for several hours. If the dopant material is composed of atoms with five valence electrons, there will be extra free electrons and an n-type semiconductor is produced.
If the dopant material is composed of atoms with three valence electrons, there is a deficiency of free electrons and a p-type semiconductor is produced. Rather than saying there is a deficiency of free electrons, we can say the semiconductor has a surplus of holes. A hole is the absence of an electron. This may be simply a matter of semantics, but that is the customary terminology. Dopants that have five valence electrons and make n-type semiconductors are antinomy, arsenic and phosphorous.
Dopants that have three valence electrons and make p-type semiconductors are boron, aluminum and gallium. The same dopants are used for both silicon and germanium semiconductors. Silicon is the principal component of common sand, and for this reason it is less expensive than other intrinsic semiconductor materials. Stabin, Michael G. Martin, James E. Department of Energy, Instrumantation and Control.
June Nuclear and Reactor Physics: J. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed. Lamarsh, A. Baratta, Introduction to Nuclear Engineering, 3d ed. Glasstone, Sesonske. Nuclear and Particle Physics.
Physics of Nuclear Kinetics. When this is made into a single crystal, it can be used as a material for semiconductor products. When it crystalizes, the nuclei share electrons and they bond with 8 electrons around each nucleus. Pure silicon is an intrinsic semiconductor and conducts electrons and electron holes released by atoms in the crystal on heating.
This causes an increase in the electrical conductivity of silicon with increase in temperature. A number of things such as glass, lubricants, electronic components, solar panels and a variety of medical equipment are made of silicon.
The majority of sand found around the world is composed mostly of silica, which is an oxide of silicon. Silicon makes up Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates.
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