Silicon have been the backbone of electronics due to its abundant and cheap availability with its ability to both prevent and allow the flow of electricity. We live in a world of electronics powered by silicon chips and transistors on silicon-based integrated circuits which are reaching their limit and a tech step-change along with new electronic material will be needed if future electronic gadgets are to keep getting smaller and faster. Presently, with growth in the modern digital age, the microscopic transistors squeezed onto silicon chips have been getting half the size each year. With the exponential growth of computing power, internet of Things (IoT), AI, robotics, self-driving cars, 5G and 6G phones all computing-intensive endeavours, the future of tech is at stake. The use of Silicon in a growing number of electronic applications that require increased speed, reduced latency and light detection is reaching the limit of its performance. The topping-out of silicon is a need because computing devices of the future will need to be both more powerful and more agile. However, it’s premature to be talking about a successor to silicon which will be completely replacing silicon.
Advantages of Silicon
- Silicon is the second most abundant element available in the crust of the Earth
- It is relatively less costly due to use of well-established processing techniques
- There is a huge market for crystalline Silicon (Si)
- It has highest efficiency
- Due to its hardness, large wafers can be handled safely without any damage
- It is thermally stable up to 1100oC
Disadvantages of Silicon
- It needs thick layer (crystalline)
- Limited substrate
- Producing of Silicon (Si) crystals is relatively expansive
- Some processing wasteful
- It has short life cycles
- Production and use of toxic substances in manufacturing is a matter of concern
Limitations of Silicon electronics
Silicon electronics despite its limitations has proved adaptable for last many decades and have been able to be fashioned into reliable, mass market devices available at minimal cost. Silicon became the material of choice for electronics due to many points in its favour like abundant availability, relatively easy to process, good physical properties and possesses a stable native oxide (SiO2) which happens to be a good insulator. The rate of progress in silicon electronics since the first silicon transistor in 1947 has been enormous, with the number of transistors on a single chip growing from a few thousand in the earliest integrated circuits to more than two billion of today. The semiconducting silicon chip propelled the revolution of electronics and computerisation that has made human more comfortable with electronic life. Silicon integrated circuits (IC) support practically everything we take for granted now in our interconnected digital world by controlling the systems we use and allowing us to access and share information at will. However, silicon electronics now faces a challenge as the latest circuits measure just 7nm wide with the size of individual silicon atoms (around 0.2nm) would be a hard physical limit (with circuits one atom wide) as its behaviour becomes unstable and difficult to control. Without the ability to shrink ICs further silicon electronics cannot continue producing the gains it has achieved so far. Thus, meeting this challenge may require rethinking how we manufacture devices, or even whether we need an alternative material to silicon itself.
Looking for other material
Combining a large number of transistors into a single chip enables an IC to process information faster and this speed enhancement depends critically on how easily electrons are able to move within the semiconductor material known as electron mobility of the material. Though, the electrons in silicon are quite mobile but they are much more so in other semiconductor materials such as gallium arsenide, indium arsenide, and indium antimonide. The useful conductive properties of semiconductors don’t just concern the movement of electrons but also the movement of electron holes. Electron holes are the gaps left behind in the lattice of electrons circling around the nucleus after electrons have been pushed out. Modern ICs make use of a technique called complementary metal-oxide semiconductor (CMOS) which uses a pair of transistors, one using electrons and the other electron holes. The electron hole mobility in silicon is very poor and is a barrier towards higher performance of transistors. For the last several years’ manufacturers have had to boost it by including germanium with the silicon. Second problem with silicon is that its performance degrades badly at high temperatures. Modern ICs with billions of transistors generate considerable heat which is being taken care off with a lot of effort to cool them. Alternative semiconducting materials such as gallium nitride (GaN) and silicon carbide (SiC) survive much better at higher temperatures and can also be run faster. Thus these materials can be strong prospective candidates to replace silicon in critical high-power applications such as amplifiers.
New materials for future
From the many materials under investigation as replacement or partners for silicon in order to improve its electronic performance, three materials have promise in the short term. Germanium was the first material used for semiconductor devices and a small amount of germanium is already added to improve silicon’s poor electron hole mobility, but using large amounts or even a move to all-germanium transistors would be better still. But re-alignment of the established silicon industry around germanium would be quite a problem for manufacturers. Another choice is the use of metal oxides and silicon dioxide within transistors for many years but with the miniaturisation of silicon dioxide layer to be so thin that it has begun to lose its insulating properties, leading to unreliable transistors. Despite a move to use rare-earth hafnium dioxide (HfO2) as a replacement insulator, the search is on for alternatives with even better insulating properties. Most interesting perhaps may be the use of so-called III-V compound semiconductors, particularly those containing indium as indium arsenide and indium antimonide. These semiconductors have electron mobility up to 50 times higher than that of silicon. Further, when combined with germanium-rich transistors, this approach could provide a major speed increase. However, silicon, germanium, oxides and the III-V materials are crystalline structures that depend on the reliability of the crystal for their properties. Therefore, it is not easy to simply throw them together with silicon and get the best of both and dealing with this problem, crystal lattice mismatch, is the major ongoing technological challenge.
Gallium Nitride (GaN)
Gallium nitride is a binary III/V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s and have the potential as a leading candidate for taking electronic performance to the next level. The compound is a very hard material with high heat capacity and thermal conductivity that has a Wurtzite crystal structure. GaN’s ability to conduct electrons is more than 1000 times efficient than silicon. GaN is being able to be manufactured at a lower cost than silicon has now been well established. GaN has many advantageous attributes over silicon as: being more power efficient, faster, and even with better recovery characteristics. Gallium nitride is a very hard with mechanically stable wide bandgap semiconductor. Its wide band gap of 3.4 eV is connected to specific changes in the electronic band structure, charge occupation and chemical bond regions. Having qualities of higher breakdown strength, faster switching speed, higher thermal conductivity and lower on-resistance, GaN based power devices significantly outperform silicon-based devices. Though, GaN may seem like a superior choice it won’t be replacing silicon in all applications for a while.
Growing of Gallium nitride crystals can be accomplished on a variety of substrates including sapphire, silicon carbide (SiC) and silicon (Si). Growing a GaN epitaxial layer on top of silicon we can accomplished with the existing silicon manufacturing infrastructure facilities which will eliminating the need for costly specialized production sites. This will make the production of GaN wafers at low cost as compared to silicon wafers. GaN is being used in the production of semiconductor power devices, RF components, light-emitting diodes (LEDs) and analog applications. It affords special properties for its applications in optoelectronic high-power and high-frequency devices. GaN transistors operation and working capability at much higher temperatures and at much higher voltages respectively as compared to gallium arsenide (GaAs) transistors they make ideal candidates for power amplifiers at microwave frequencies. In addition, GaN transistors also offer promising characteristics towards THz devices. Due to high power density and voltage breakdown limits, GaN is also emerging as a promising candidate towards 5G cellular base station applications.
High electron mobility of GaN produces transistors and integrated circuits that feature higher breakdown strength, faster switching speed, higher thermal conductivity and lower on-resistance than comparable silicon solutions. GaN has turned out to be an important building block in power electronics by solving design challenges, driving robust growth and innovation with products that are many times smaller, lighter and exhibit less energy loss. High electron mobility transistors (HEMTs) are transistors using a 2-dimensional electron gas (2DEG) created by a junction between two materials with different band gaps. Gallium nitride based HEMTs feature faster switching speed, higher thermal conductivity and lower on-resistance than comparable silicon-based solutions. Such favourable features exhibited by GaN transistors and integrated circuits allow their use in circuits to increase efficiency, shrink the size, and reduce the cost of a wide variety of power conversion systems.
Problems with GaN
While GaN devices are being widely used in the optoelectronics industry (such as LEDs), they are not yet commonly used in transistors for several reasons. One of the biggest difficulty in GaN transistors is that GaN devices are typically depletion type devices which are “ON” when the gate-source voltage is zero. This is a problem as power circuitry and logic depend both normally “on” and “off” transistors. Currently, there are several proposals to create GaN devices by including the addition of fluoride ions, an MIS-type gate stack, a combined GaN and Si device, and the use of a P-type material on-top of the AlGaN/GaN heterojunction that are “OFF” when the gate-source voltage is zero. It may also offer high cost due to higher material cost and costly processes involved in its manufacturing. It will take some time for GaN to take over the electronic market due to cost factor. Researchers are working towards lowering the cost of GaN development and manufacturing and once this is done, GaN will capture market in various domains including wireless, electronics, medical, automobile etc.
Dr. S. S. VERMA, Department of Physics, S.L.I.E.T., Longowal; Distt.-Sangrur (Punjab)