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Oct 25, 2014
Geethanjali College of Engineering and Technology Faiz Ahmed (11R11D7001) M.tech - ECEPage 1
ContentsTopic 1) a) b) 2)a) b)
Introduction Transistor History Moores Law Prediction Present Transistor Technologies
1st Transistor Thermionic triode (1907) 1st Semiconductor Transistor Point Contact Transistor (1947) Other new Semiconductor Technologies for Transistor Traditional Planar Transistori) Page 2
Ballistic Transistor 3-D Transistor Merits of 3-D Transistor Conclusion
iii) Carbon Nanotube FET 4) 5) 6)7)
TransistorA transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. The transistor is the fundamental building block of modern electronic devices, and is ubiquitous in modern electronic systems. Following its development in the early 1950s the transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, among other things.
Introduction The thermionic triode, a vacuum tube invented in 1907, propelled the electronics age forward, enabling amplified radio technology and longdistance telephony. The triode, however, was a fragile device that consumed a lot of power. Physicist Julius Edgar Lilienfeld filed a patent for a field-effect transistor (FET) in Canada in 1925, which was intended to be a solid-state replacement for the triode. From November 17, 1947 to December 23, 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in the United States, performed experiments and observed that when two gold point contacts were applied to a crystal of germanium, a signal was produced with the output power greater than the input. The transistor is the key active component in practically all modern electronics. Many consider it to be one of the greatest inventions of the 20th century.
AdvantagesThe key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are
Small size and minimal weight, allowing the development of miniaturized electronic devices. Highly automated manufacturing processes, resulting in low per-unit cost. Lower possible operating voltages, making transistors suitable for small, battery-powered applications. No warm-up period for cathode heaters required after power application. Lower power dissipation and generally greater energy efficiency. Higher reliability and greater physical ruggedness.
Extremely long life. Some transistorized devices have been in service for more than 50 years. Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes. Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.
Silicon transistors typically do not operate at voltages higher than about 1000 volts (SiC devices can be operated as high as 3000 volts). In contrast, vacuum tubes have been developed that can be operated at tens of thousands of volts.
High-power, high-frequency operation, such as that used in over-the-air television broadcasting, is better achieved in vacuum tubes due to improved electron mobility in a vacuum. Silicon transistors are much more vulnerable than vacuum tubes to an electromagnetic pulse generated by a high-altitude nuclear explosion. Vacuum tubes create a distortion, the so-called tube sound, that some people find to be more tolerable to the ear.
History Very-large-scale integration (VLSI) is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when
Introduction complex semiconductor and communication technologies were being developed. During the 1920s, several inventors attempted devices that were intended to control the current in solid state diodes and convert them into triodes. Success, however, had to wait until after World War II, during which the attempt to improve silicon and germanium crystals for use as radar detectors led to improvements both in fabrication and in the theoretical understanding of the quantum mechanical states of carriers in semiconductors and after which the scientists who had been diverted to radar development returned to solid state device development. With the invention of transistors at Bell labs, in 1947, the field of electronics got a new direction which shifted from power consuming vacuum tubes to solid state devices.
Introduction Another problem was the size of the circuits. A complex circuit, like a computer, was dependent on speed. If the components of the computer were too large or the wires interconnecting them too long, the electric signals couldn't travel fast enough through the circuit, thus making the computer too slow to be effective. Jack Kilby at Texas Instruments found a solution to this problem in 1958. Kilby's idea was to make all the components and the chip out of the same block (monolith) of semiconductor material. When the rest of the workers returned from vacation, Kilby presented his new idea to his superiors. He was allowed to build a test version of his circuit. In September 1958, he had his first integrated circuit ready. Although the first integrated circuit was pretty crude and had some problems, the idea was groundbreaking. ByPage 7
Introduction making all the parts out of the same block of material and adding the metal needed to connect them as a layer on top of it, there was no more need for individual discrete components. No more wires and components had to be assembled manually. The circuits could be made smaller and the manufacturing process could be automated. From here the idea of integrating all components on a single silicon wafer came into existence and which led to development in small-scale integration (SSI) in the early 1960s, medium-scale integration (MSI) in the late 1960s, and large-scale integration (LSI) and VLSI in the 1970s and 1980s with tens of thousands of transistors on a single chip (later hundreds of thousands and now millions). Developments The first semiconductor chips held two transistors each. Subsequent advances addedPage 8
Introduction more and more transistors, and, as a consequence, more individual functions or systems were integrated over time. The first integrated circuits held only a few devices, perhaps as many as ten diodes, transistors, resistors and capacitors, making it possible to fabricate one or more logic gates on a single device. Now known retrospectively as smallscale integration (SSI), improvements in technique led to devices with hundreds of logic gates, known as medium-scale integration (MSI). Further improvements led to large-scale integration (LSI), i.e. systems with at least a thousand logic gates. Current technology has moved far past this mark and today's microprocessors have many millions of gates and billions of individual transistors. As microprocessors become more complex due to technology scaling, microprocessor designers have encountered several challenges which force themPage 9
Introduction to think beyond the design plane, and look ahead to post-silicon:
Power usage/Heat dissipation As threshold voltages have ceased to scale with advancing process technology, dynamic power dissipation has not scaled proportionally. Maintaining logic complexity when scaling the design down only means that the power dissipation per area will go up. This has given rise to techniques such as dynamic voltage and frequency scaling (DVFS) to minimize overall power. Process variation As photolithography techniques tend closer to the fundamental laws of optics, achieving high accuracy in doping concentrations and etched wires is becoming more difficult and prone to errors due to variation. Designers now must simulate across multiple fabrication process corners before a chip is certified ready for production.
Stricter design rules Due to lithography and etch issues with scaling, design rules for layout have become increasingly stringent. Designers must keep ever more of these rules in mind while laying out custom circuits. The overhead for custom design is now reaching a tipping point, with many design houses opting to switch to electronic design automation (EDA) tools to automate their design process. Timing/design closure As clock frequencies tend to scale up, designers are finding it more difficult to distribute and maintain low clock skew between these high frequency clocks across the entire chip. This has led to a rising interest in multicore and multiprocessor architectures, since an overall speedup can be obtained by lowering the clock frequency and distributing processing. First-pass success As die sizes shrink (due to scaling), and wafer sizes go up (to lower
Introduction manufacturing costs), the number of dies per wafer increases, and the complexity of making suitable photomasks goes up rapidly. A mask set for a modern technology can cost several million dollars. This non-recurring expense deters the old iterative philosophy involving several "spin-cycles" to find erro