Transistors
A transistor, broadly speaking, is a solid-state switch that allows a small signal to control a large signal, such as a current flow. The first transistor was invented by John Bardeen, Walter Brattain, and William Shockley in 1945. They were awarded the Nobel Prize for physics in 1956. That transistor was a point-contact transistor that is no longer used today. Since that time, transistors of all types have become crucial for modern electronics to function.
A transistor has three terminals. Current flows between two of them while the third one controls the current. In semiconductors, there are two charge carriers. One is the electron, which carries negative charges. The other is a hole, which carries positive charges. The amount of electrons and holes is often unequal. Consequently, the carrier which has a larger amount is called a majority carrier and the other is called a minority carrier. Depending on what carriers produce the current, transistors can be classified as bipolar or unipolar transistors.
In bipolar transistors, the current is carried by both majority and minority carriers. The three terminals are the emitter, the collector, and the base. Two structures are possible, p-n-p and n-p-n. "p" indicates a type of semiconductor where the majority carriers are holes (positive), while "n" indicates a type of semiconductor where the majority carriers are electrons (negative). In the p-n-p type of transistors, a thin layer of n-type semiconductor is sandwiched between two p-type semiconductor layers. The n-type layer acts as the base. The two p-type layers serve as the emitter and the collector. The n-p-n type of transistor is similar in structure, but reverses the sequence of the layers.
In unipolar transistors, only the majority carriers carry the current. The field-effect transistor (FET) is an example of a unipolar transistor. The three terminals are the source, the drain, and the gate. The region between the source and drain is called a substrate. FETs are used widely in today's electronic industry. Depending on whether there is an insulation layer between the gate and the substrate, there are junction FETs (JFET) or insulated gate FETs (IGFET). Oxide is often used as the insulation material, so IGFET is often called MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor). The MOSFET built using silicon is the heart of today's computer chip.
When building integrated circuits based on millions of transistors, many issues need to be taken into account. One is the power consumption. The type of transistors used should not consume too much power or the integrated circuit will become too hot to use. In making integrated circuits, often what matters is the group behavior of many transistors. Some types of transistors behave well when only several are used in a circuit, but not as well when millions of them are put together. These kinds of transistors will not be used because they do not have good scalability. FETs built on silicon turn out to be very good, which is why silicon devices are widely used today.
Part of the reason computers are becoming faster and smaller today is that the underlying transistors can be made faster and smaller. The fastest CPU for personal computers available in the market today has about 1 GHz frequency. There is certainly some limit on how small the semiconductor transistors can be made. An atom has a size of about 0.1 nm, which is about one billionth of a foot. The transistor should be at least the size of several layers of atoms, or about one nm. Today's technology labs have reached thicknesses of around 50 nm. As they reach the lower limit, the physics of making transistors will change dramatically.
There are two major complications in making transistors smaller and faster. Nonlinear effects can become important. In a smaller device, electrons will accelerate to high speed and then experience a collision and return to a slower average speed. This effect is called velocity overshoot, and it is a nonlinear "hot electron" effect. Another complication is the change of fundamental physics. When the transistor gets even smaller, quantum effects become important. They can manifest in transistors of 50 nm size, which is smaller than what can be seen with the naked eye, but is still much larger than an atom. The nanoscale device is an active research area today as technology enters this region.
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