FIELD EFFECT TRANSISTOR (FET)

Introduction:-

A field-effect transistor (FET) is a type of transister commonly used for weak-signal amplification (for example, for amplifying wireless signals).The device can amplify analog or digital signals.It can also switch DC or function as an oscillator.Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips.A singleIC can contain many thousands of FETs, along with other components such as resistors, capacitors, and diodes.

circuit symbol:-

in real....

In the FET, current flows along a semiconductor path called the channel. At one end of the channel, there is an electrode called the source. At the other end of the channel, there is an electrode called the drain. The physical diameter of the channel is fixed, but its effective electrical diameter can be varied by the application of a voltage to a control electrode called the gate.The conductivity of the FET depends, at any given instant in time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in the current from the source to the drain. This is how the FET amplifies signals.

Channel..

The junction FET has a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is made of the opposite semiconductor type. In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons.In a JFET, the junction is the boundary between the channel and the gate.Normally, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate.However, under some conditions there is a small current through the junction during part of the input signal cycle.

Classification:-

Field-effect transistors exist in two major classifications.These are known as the junction FET (JFET) andthe metal-oxide- semiconductor FET (MOSFET).

Advantages & disadvantantages:-

The FET has some advantages and some disadvantages relative to the bipolar transister.Field-effect transistors are preferred for weak-signal work, for example in wireless communications and broadcast receivers.They are also preferred in circuits and systems requiring high impedance.The FET is not, in general, used for high-power amplification, such as is required in large wireless communications and broadcast transmitters.

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SILICON CONTROLLED RECTIFIER (SCR)

INTRODUCTION:-

A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state

device that controls current.

The name "silicon controlled rectifier" or SCR is Generel electronics's trade name for a type of thyrister.

The SCR was developed by a team of power engineers led by Gordon Hall and commercialised by

Frank W. "Bill" Gutzwiller in 1957.

SCR...

SYMBOLS:-

OPERATION:-

An SCR is a type of rectifier, controlled by a logic gate signal. It is a four-layer, three-terminal device.

A p-type layer acts as an anode and an

n-type layer as a cathode; the p-type layer closer to the n-type (cathode) acts as a gate. It is

unidirectional in nature.

CONSTRUCTION OF SCR:-

It consists of a four layers pellet of P and N type semiconductor materials. Silicon is used as the

intrinsic semiconductor to which the proper

impurities are added. The junctions are either diffused or alloyed. The Planar construction is used

for low power SCR's, here all the junctions

are diffused.

The Mesa type construction is used for high power SCR's. In this case junction J2 is obtained by

diffusion method and then the outer two

layers

are alloyed to it because the PNPN pellet is required to handle large currents. It is properly braced

with tungsten or molybdenum plates to

provide

greater mechanical strength. One of these plates is hard soldered to a copper stud, which is

threaded for attachment of heat sink.

The doping

of PNPN will depend on the application of SCR.

MODES OF OPERATION:-

In the normal "off" state, the device restricts current to the leakage current. When the

gate-to-cathode voltage exceeds a certain threshold,

the device turns "on" and conducts current. The device will remain in the "on" state even after gate

current is removed so long as current

through the device remains above the holding coupling. Once current falls below the holding

current for an appropriate period of time,

the device will switch "off". If the gate is pulsed and the current through the device is below the

holding current, the device will remain in the

"off" state.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge

into the gate to trigger the device into the "on"

state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage

rise across the device, perhaps by using

asnubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the

partially-triggered SCR may dissipate more power

than is usual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated break down

voltage (also called as break ver voltage),

but again, this does not rapidly switch the entire device into conduction and so may be harmful

so this mode of operation is also usually

avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown

voltage, so the exact trigger point will

vary from device to device.

SCRs are made with voltage ratings of up to 7,500 V, and with current ratings up to 3,000 RMS

amperes per device. Some of the larger ones

can take over 50 kA in single-pulse operation. SCRs are used in power switching, phase control,

chopper, battery charger, and inverter

circuits. Industrially they are applied to produce variable DC voltages for moters (from a few to

several thousand HP) from AC line voltage.

They control the bulk of the dimmers used in stage lighted, and can also be used in some electric

vehicles to modulate the working

voltage in a jacabson circuit. Another common application is phase control circuits used with

inductive loads. SCRs can also be found in

welding power suplies where they are used to maintain a constant output current or voltage.

Large silicon-controlled rectifier assemblies

with many individual devices connected in series are used in high voltage DC converter stations.

Two SCRs in "inverse parallel" are often used in place of a TRIAC for switching inductive loads

on AC circuits.

Because each SCR only conducts for half of the power cycle and is reverse-biased for the

other half-cycle, turn-off of the SCRs is assured.

By comparison, the TRIAC is capable of conducting current in both directions and assuring that

it switches "off" during the brief

zero-crossing of current can be difficult.

Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a

parasitic SCR. This SCR is undesired;

if it is triggered by accident, the IC can go into latch up and potentially be destroyed.

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JFET

INTRODUCTION:-

The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field effect transister. It can be used as an electrically-controlled switch or as a voltage-controlled resistance . electric charge flowflows through a semiconducting channel between "source" and "drain" terminals. By applying a bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.

Structure:-

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end form the source and drain. The gate (control) terminal has doping opposite to that of the channel, which it surrounds, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic.

Function:-

JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current depends also on the electric field between source and drain.

Symbols:-

The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable (which is not true of all JFETs).

Officially, the style of the symbol should show the component inside a circle (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices.

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MOSFET

Introduction:-

The metal�oxide�semiconductor field-effect transistor (MOSFET, MOS-FET, orMOS FET) is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925. The MOSFET includes a channel ofn-type or p-type semiconductor material (see article on semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistorwas at one time much more common.

Diagramatic representation...

Circuit symbol:-

MOSFET operation:-

A traditional metal�oxide�semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO₂) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is commonly used). As the silicon dioxide is a dielectric material, its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a semiconductor.

When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with N_A the density of acceptors, p the density of holes; p = N_A in neutral bulk), a positive voltage, V_GB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If V_GB is high enough, a high concentration of negative charge carriers forms in an inversion layer located in a thin layer next to the interface between the semiconductor and the insulator. Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region. Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage.

This structure with P-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regions.

Modes of operation

The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. In the following discussion, a simplified algebraic model is used that is accurate only for old technology. Modern MOSFET characteristics require computer models that have rather more complex behavior. For example, see Liu and the device modeling list at Designers-guide.org.

For an enhancement-mode, n-channel MOSFET, the three operational modes are:

Cutoff, subthreshold, or weak-inversion mode When V_GS <>_th: where V_th is the threshold voltage of the device. According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain, resulting in a subthreshold current that is an exponential function of gate�source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage.In weak inversion the current varies exponentially with gate-to-source bias V_GS as given approximately by:

, where I_D0 = current at V_GS = V_th and the slope factor n is given by n = 1 + C_D/ C_OX, with C_D = capacitance of the depletion layer and C_OX = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once V_DS > > V_T, but as channel length is reduced drain-induced barrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on). Frequently, threshold voltage V_th for this mode is defined as the gate voltage at which a selected value of current I_D0 occurs, for example, I_D0 = 1 �A, which may not be the same V_th-value used in the equations for the following modes. Some micropower analog circuits are designed to take advantage of subthreshold conduction. By working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible transconductance-to-current ratio, namely: g_m / I_D = 1 / (nV_T), almost that of a bipolar transistor. The subthreshold I-V relation depends exponentially upon threshold voltage, introducing a strong dependence on any manufacturing variation that affects threshold voltage; for example: variations in oxide thickness, junction depth, or body doping that change the degree of drain-induced barrier lowering. The resulting sensitivity to fabricational variations complicates optimization of circuits operating in the subthreshold mode.

Triode mode or linear region (also known as the ohmic mode)

When V_GS > V_th and V_DS < ( V_GS - V_th )

The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:

where �_n is the charge-carrier effective mobility, W is the gate width,L is the gate length and C_ox is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.

Saturation or active mode

When V_GS > V_th and V_DS > ( V_GS - V_th )The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate�source voltage, and modeled very approximately as:

The additional factor involving ?, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is:

, where the combination V_ov = V_GS - V_th is called theoverdrive voltage. Another key design parameter is the MOSFET output resistance r_O given by:

. If ? is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.As the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in V_GS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage.

SOURCE: www.electronicseveryday.blogspot.com

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