Field Effect Transistors, Physics tutorial


Field Effect Transistor (FET) is the unipolar device that conducts current using only one type of charge carrier. If it is produced on the N type semiconductor slab it utilizes electrons as carriers while conversely, the P type based device utilizes only holes as carriers. Practical semi-conducting devices like Junction Gate Field Effect Transistor, were only developed much later after transistor effect was seen and described in 1947.

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), that superseded Junction Field Effect Transistor and had more reflective effect on electronic development, was first suggested in year 1960

Field effect transistor operation:

Visualize it as the device that controls flow of electrons from source to drain by affecting size and shape of conductive channel created and influenced by the voltage, or lack of voltage applied across gate and source terminals - with conductive channel acting as medium by which electrons flow from source to drain.

In n-channel depletion-mode Field Effect transistor device, the negative gate-to-source voltage causes depletion region to increase the width and intrude on channel from sides; this narrow channel creating greater impedance to electron flow through channel. If depletion region enlarges to entirely close the channel, resistance of channel from source to drain becomes very big, and Field Effect Transistor is efficiently turned off - just like the switch. If you were to apply the positive gate-to-source voltage, you would be increasing channel size and this permits more electrons to flow easily across channel; that presents lower impedance to electron flow. In n-channel enhancement-mode Field Effect transistor, a positive gate-to-source voltage is essential to generate, and to increase conductive channel as channel doesn't exist naturally inside transistor. Positive voltage attracts free-floating electrons inside body towards gate, forming the conductive channel. Enough electrons should be attracted near gate to counter ions utilized in doping semiconductor of Field Effect Transistor and this creates region free of mobile carriers that must be remembered - as depletion region. Phenomenon is referred to as threshold voltage of FET.

Any additional gate-to-source voltage will attract even more electrons towards gate that are able to make conductive channel from source to drain in procedure known as inversion.

If drain-to-source voltages is much less than gate-to-source voltages, changing gate voltage will modify channel resistance, and drain current will be proportional to drain voltage when referenced to source voltage. In this mode Field Effect Transistor functions like the variable resistor and operation of device can be said to be in linear mode or ohmic mode.

If drain-to-source voltage is increased, the important asymmetrical change in shape of channel is made because of gradient of voltage potential from source to drain. Shape of inversion region becomes contracted and expressed as pinched-off near drain end of channel. If drain-to-source voltage is increased further, pinch-off point of channel starts to move away from drain towards source.  Field Effect Transistor is said to be in saturation mode; or active mode.

The saturation mode, or region between ohmic and saturation, is utilized when amplification is needed and region between is at times considered to be part of ohmic or linear region, even where drain current is not roughly linear with drain voltage. Although conductive channel generated by gate-to-source voltage no longer connects source to drain during saturation mode, charge carriers aren't blocked from flowing. If consider again the n-channel device, depletion regions exists in p-type body, surrounding conductive channel and drain and source regions. Electrons that include channel electrons are free to move out of channel through depletion region if attracted to drain by drain-to-source voltage. Depletion region is free of carriers and has the resistance like silicon. Any increase of drain-to-source voltage will increase distance from drain to pinch-off point, increasing resistance because of depletion region proportionally to applied drain-to-source voltage. This proportional change causes drain-to-source current to remain relatively fixed independent of changes to drain-to-source voltage and quite unlike linear mode operation. Therefore in saturation mode, FET behaves as the constant-current source rather than as resistor and can be utilized most effectively as voltage amplifier. In this case, gate-to source voltage decides level of constant current through channel.

Terminals of the field effect transistor:

There are three terminals that always find on all Field Effect Transistors they are given below:

  • Gate terminal
  • Drain terminal
  • Source terminal

They bear analogous relationship of base, collector, and emitter with terminals of Bipolar Junction Transistors. With exception of Junction Field Effect Transistor, all Field Effect Transistors has fourth terminal that is severally referred to as body, base, bulk, or substrate; and that serves to bias Field Effect Transistor in operation. This fourth terminal must not be trivialized in circuit designs as its presence is significant in physical layout of integrated circuits. Size of gate is distance between source and drain.

Width is extension of Field Effect Transistor perpendicular to cross section and typically the width is much larger than length of gate. For gate length of 1 μm suppose upper operating frequency limit of approx 5 GHz while 0.2 μm gate length sets the upper frequency limit in region of 30 GHz. Like in Bipolar Junction Transistors, names of terminals refer to their functions therefore assume gate terminal as though it were controlling opening and closing of the physical gate by which electrons passes. This gate allows electrons to flow through or blocks their passage by creating or eliminating the channel between source and drain.

Electrons flow from the source terminal towards the drain terminal if influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually the body terminal is connected to the highest or lowest voltage within the circuit, depending on type. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit; however there are several uses of the Field Effect Transistor which do not have such a configuration, such as in the cases of transmission gates and cascode circuits.

Kinds of field effect transistors:

There exist only two families of Field Effect Transistors -Junction Field Effect Transistors usually expressed as JFET on one hand, and Insulated Gate Field Effect Transistors or IGFET for short. There is also very common transistor known as MOSFET that stands for Metal-Oxide-Semiconductor Field Effect Transistor. MOSFET and IGFET are same thing and name MOSFET only expresses original construction of IGFET from layers of metal that formed gate, an insulating oxide layer and semiconductor material.

Further classification divides Field Effect Transistors in depletion mode and enhancement-mode subject to channel being turned on or off with zero gate-to-source voltage. For enhancement mode Field Effect Transistors, channel is non conductive at zero bias voltage and gate potential is utilized to enhance conduction. Depletion mode Field Effect Transistors channel are conductive at zero bias voltage and gate potential of opposite polarity will deplete channel, decreasing conduction.

Most Junction Field Effect Transistors are depletion-mode devices as diode junctions would forward bias and conduct if they were enhancement mode devices.

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