The field effect transistor was proposed by Julius Lilienfeld in US patents in and 1,, Moreover, Shockley, Brattain, and Bardeen were investigating the field effect transistor in Though, the extreme difficulties sidetracked them into inventing the bipolar transistor instead. However, the materials processing technology was not mature enough until when John Atalla produced a working device. A field effect transistor FET is a unipolar device, conducting a current using only one kind of charge carrier.
If based on an N-type slab of semiconductor, the carriers are electrons. Conversely, a P-type based device uses only holes. At the circuit level, field effect transistor operation is simple.
A voltage applied to the gate , input element, controls the resistance of the channel , the unipolar region between the gate regions. Figure below In an N-channel device, this is a lightly doped N-type slab of silicon with terminals at the ends. The source and drain terminals are analogous to the emitter and collector, respectively, of a BJT. In an N-channel device, a heavy P-type region on both sides of the center of the slab serves as a control electrode, the gate.
The gate is analogous to the base of a BJT. Though it is possible to make bipolar transistors outside of a clean room , it is a necessity for field effect transistors. Even in such an environment, manufacture is tricky because of contamination control issues. The unipolar field effect transistor is conceptually simple, but difficult to manufacture. Most transistors today are a metal oxide semiconductor variety later section of the field effect transistor contained within integrated circuits.
However, discrete JFET devices are available.
What is a FET: Field Effect Transistor Types & Technology
The gate constitutes a diode junction to the source to drain semiconductor slab. The gate is reverse biased. If a voltage or an ohmmeter were applied between the source and drain, the N-type bar would conduct in either direction because of the doping. Neither gate nor gate bias is required for conduction.
Introduction to Field-Effect Transistors (FETs)
If a gate junction is formed as shown, conduction can be controlled by the degree of reverse bias. This is due to diffusion of holes from the P-type gate region into the N-type channel, giving the charge separation about the junction, with a non-conductive depletion region at the junction.
The depletion region extends more deeply into the channel side due to the heavy gate doping and light channel doping. This increases the resistance of the source to drain channel by narrowing the channel.
Field Effect Transistor, FET history
Increasing the reverse bias at c increases the depletion region, decreases the channel width, and increases the channel resistance. Increasing the reverse bias VGS at d will pinch-off the channel current. The channel resistance will be very high.
It is typically a few volts. In summation, the channel resistance can be controlled by the degree of reverse biasing on the gate. That is, the drain battery may be replaced by a low voltage AC source. This drain power supply, not shown in previous figures, distorts the depletion region, enlarging it on the drain side of the gate.
This is a more correct representation for common DC drain supply voltages, from a few to tens of volts. As drain voltage VDS increased,the gate depletion region expands toward the drain. This increases the length of the narrow channel, increasing its resistance a little. Figure below b shows the schematic symbol for an N-channel field effect transistor compared to the silicon cross-section at a. The gate arrow points in the same direction as a junction diode.
This current flow may be controlled by varying the gate voltage. A load in series with the battery sees an amplified version of the changing gate voltage.
Field effect transistor theory pdf printer
P-channel field effect transistors are also available. The channel is made of P-type material.
The gate is a heavily doped N-type region. All the voltage sources are reversed in the P-channel circuit Figure below as compared with the more popular N-channel device. Also note, the arrow points out of the gate of the schematic symbol b of the P-channel field effect transistor. As the positive gate bias voltage is increased, the resistance of the P-channel increases, decreasing the current flow in the drain circuit.
Discrete devices are manufactured with the cross-section shown in Figure below. The cross-section, oriented so that it corresponds to the schematic symbol, is upside down with respect to a semiconductor wafer. That is, the gate connections are on the top of the wafer.
However, the channel surrounding the gate is lightly doped to allow holes from the gate to diffuse deeply into the channel. That is the N- region. Junction field effect transistor: a Discrete device cross-section, b schematic symbol, c integrated circuit device cross-section.
All three FET terminals are available on the top of the die for the integrated circuit version so that a metalization layer not shown can interconnect multiple components. The N-channel region under the gate must be very thin so that the intrinsic region about the gate can control and pinch-off the channel. Thus, gate regions on both sides of the channel are not necessary. Junction field effect transistor static induction type : a Cross-section, b schematic symbol.
The static induction field effect transistor SIT is a short channel device with a buried gate. Figure above It is a power device, as opposed to a small signal device.
The low gate resistance and low gate to source capacitance make for a fast switching device. The SIT is capable of hundreds of amps and thousands of volts.
And, is said to be capable of an incredible frequency of 10 gHz. A schottky diode is a metal rectifying contact to a semiconductor compared with a more common ohmic contact. The channel is lightly doped N-. They are used as microwave power amplifiers to 30 gHz.
Field Effect Transistor – the basics
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Textbook Junction Field-effect Transistors. Home Textbook Vol.
FET Operation At the circuit level, field effect transistor operation is simple. Junction field effect transistor cross-section. N-channel JFET current flow from drain to source in a cross-section, b schematic symbol. SIT Junction field effect transistor static induction type : a Cross-section, b schematic symbol. Application of reverse bias to the gate varies the channel resistance by expanding the gate diode depletion region.
Junction Field-effect Transistors
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