Accordingly the invention of the field effect transistor was a major hurdle and it took improvements in semiconductor refining as well as fabrication technologies and in particular the development of oxide layers on silicon to enable this electronic component to be made properly. However once the field effect transistor had been invented it became a major enabler for integrated circuits as the lower current levels and hence lower heat dissipation offered by FET technology enabled integration levels to dramatically increase.
Without the use of low power FET technology the heat levels would limit the number of devices on a chip. Accordingly these electronic components would not be what they are today without the invention of the field effect transistor and in particular the MOSFET. The technology behind the FET enables the field effect transistor to provide high levels of performance. Using an electric field to control the current in a piece of semiconductor, it has a very high input impedance when compared to the bipolar transistor.
There are significant differences between these two electronic components. The field effect transistor, FET, is a three terminal device which provides voltage gain. Having a high input impedance the electric field in the vicinity of the input terminal called the gate modifies the current flowing in what is called the channel between terminals called the source and drain.
The first ideas for a field effect device started to appear quite early on in the overall history of the transistor and semiconductors in general. The first idea for a field effect device was patented by an Austro-Hungarian physicist named Julius Edgar Lilienfeld in Lilienfeld had started working on the physics behind electrons and particles in a vacuum in and he identified field electron emission.
For this work he was based in the Physics Department of the University of Leipzig. He continued to work on electrical particles and electron physics looking into the field effects associated with them. Lilienfeld had actually filed two other patents in , but the one granted in January appears to be the most important. Lilienfeld went on to make other contributions to electrical and electronic science developing the electrolytic capacitor and investigating X-rays.
By the time Lilienfeld had applied for his field effect patent, he had moved to the US in in order to pursue his patent claims and he resigned his professorship to stay in the USA permanently in He finally became a US citizen in Although Lilienfeld had described the concept for a field effect device, he was unable to make one to demonstrate the principle and although he made some major advances in thinking, Lilienfeld did not actually invent the field effect transistor as a realisable electronic component.
Like many developments, the invention of the field effect transistor involved a number of people thinking along the same lines around the same time. Another researcher named Oskar Heil, was a German physicist who spent some of his career working at the cavendish Laboratory at the University of Cambridge was well as some time at the British company, Standard Telephones and Cables, STC.
For ease of discussion, this assumes body and source are connected. This conductive channel is the "stream" through which electrons flow from source to drain. In an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch.
This is pinch-off, and the voltage to achieve it is the pinch-off voltage. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily. Conversely, in an n-channel enhancement-mode device, a positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel.
But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region free of mobile carriers called a depletion region, and the phenomenon is referred to as the threshold voltage of the FET. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion.
For either enhancement- or depletion-mode devices, at drain-to-source voltages much less than gate-to-source voltages, changing the gate voltage will alter the channel resistance, and drain current will be proportional to drain voltage referenced to source voltage.
In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear mode or ohmic mode. If drain-to-source voltage is increased, this creates a significant asymmetrical change in the shape of the channel due to a gradient of voltage potential from source to drain.
The shape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins to move away from the drain towards the source.
The FET is said to be in saturation mode ; [ 6 ] some authors refer to it as active mode , for a better analogy with bipolar transistor operating regions.
The in-between region is sometimes considered to be part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage. Even though the conductive channel formed by gate-to-source voltage no longer connects source to drain during saturation mode, carriers are not blocked from flowing. Considering again an n-channel device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions.
The electrons which comprise the channel are free to move out of the channel through the depletion region if attracted to the drain by drain-to-source voltage.
The depletion region is free of carriers and has a resistance similar to silicon. Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing resistance due to the depletion region proportionally to the applied drain-to-source voltage. This proportional change causes the drain-to-source current to remain relatively fixed independent of changes to the drain-to-source voltage and quite unlike the linear mode operation.
Thus in saturation mode, the FET behaves as a constant-current source rather than as a resistor and can be used most effectively as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel. The FET can be constructed from a number of semiconductors, silicon being by far the most common.
Most FETs are made with conventional bulk semiconductor processing techniques , using a single crystal semiconductor wafer as the active region, or channel. Among the more unusual body materials are amorphous silicon , polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors; often, OFET gate insulators and electrodes are made of organic materials, as well.
The drain and source may be doped of opposite type to the channel, in the case of depletion mode FETs, or doped of similar type to the channel as in enhancement mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate. Types of FETs are:. Thus, it is a voltage-controlled device, and shows a high degree of isolation between input and output. It is a unipolar device, depending only upon majority current flow. It is less noisy and is thus found in FM tuners and in low-noise amplifiers for VHF and satellite receivers.
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