Love-Electronics: Metal oxide semiconductor field effect transistor (MOSFET)

· CONTENTS:

· General Definition

· Representation of various MOSFET’s

· Metal Oxide Semi Conductor Formation

· MOSFET structure and channel formation

· Modes of operation

· Body effect

· Advantages of using MOSFET

· MOSFET scaling

· Disadvantages of using MOSFET

· Various types of MOSFET

· MOSFET:-

A type of metal oxide semiconductor field effect transistor (MOSFET) used to switch large amounts of current.MOSFETs use a vertical structure with source and drain terminals at opposite sides of the chip. The vertical orientation eliminates crowding at the gate and offers larger channel widths. In addition, thousands of these transistor "cells" are combined into one in order to handle the high currents and voltage required of such devices.
A Power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle large amounts of power

Example of nMOSFET:-

A cross section through an nMOSFET when the gate voltage VGS is below the threshold for making a conductive channel; there is little or no conduction between the terminals source and drain; the switch is off. When the gate is more positive, it attracts electrons, inducing an n-type conductive channel in the substrate below the oxide, which allows electrons to flow between the n-doped terminals; the switch is on.

· Circuits Symbols Representing MOSFETS’s:

A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at right angles into the same direction as the channel. Sometimes three line segments are used for enhancement mode and a solid line for depletion mode. Another line is drawn parallel to the channel for the gate.

Comparison of enhancement-mode and depletion-mode MOSFET symbols, along with JFET symbols (drawn with source and drain ordered such that higher voltages appear higher on the page than lower voltages):

				P-channel
				N-channel
JFET	MOSFET enh	MOSFET enh (no bulk)	MOSFET dep

· A Metal Oxide Semiconductor Structure:-

Metal–oxide–semiconductor structure on P-type silicon.

A traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (Si O2) 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.

· MOSFET Structure and Channel Formation:-

Cross section of an NMOS without channel formed: OFF state

Cross section of an NMOS with channel formed: ON state

A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration by a MOS capacitance between a body electrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer which in the case of a MOSFET is an oxide, such as silicon dioxide. If dielectrics other than an oxide such as silicon dioxide (often referred to as oxide) are employed the device may be referred to as a metal–insulator–semiconductor FET (MOSFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain), each connected to individual highly doped regions that are separated by the body region. These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a '+' sign after the type of doping.

If the MOSFET is an n-channel or nMOSFET, then the source and drain are 'n+' regions and the body is a 'p' region. As described above, with sufficient gate voltage, above a threshold voltage value, electrons from the source (and possibly also the drain) enter the inversion layer or n-channel at the interface between the p region and the oxide. This conducting channel extends between the source and the drain, and current is conducted through it when a voltage is applied between source and drain.

For gate voltages below the threshold value, the channel is lightly populated, and only a very small sub threshold leakage current can flow between the source and the drain.

If the MOSFET is a p-channel or pMOSFET, then the source and drain are 'p+' regions and the body is a 'n' region. When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at the surface of the n region, analogous to the n-channel case, but with opposite polarities of charges and voltages. When a voltage less negative than the threshold value (a negative voltage for p-channel) is applied between gate and source, the channel disappears and only a very small sub threshold current can flow between the source and the drain.

The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel.

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

Cutoff, sub threshold, or weak-inversion mode

When VGS < Vth:

where Vth 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 sub threshold 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 sub threshold leakage.

In weak inversion the current varies exponentially with gate-to-source bias VGS as given approximately by:[3][4]

$I_D \approx I_{D0}e^{\begin{matrix}\frac{V_{GS}-V_{th}}{nV_{T}} \end{matrix}}$ ,

Where ID0 = current at VGS = Vth and the slope factor n is given by

n = 1 + CD / COX,

With CD = capacitance of the depletion layer and COX = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once VDS > > VT, 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 Vth for this mode is defined as the gate voltage at which a selected value of current ID0 occurs, for example, ID0 = 1 μA, which may not be the same Vth-value used in the equations for the following modes.

Some micro power analog circuits are designed to take advantage of sub threshold conduction.[5][6][7] By working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible trans conductance-to-current ratio, namely: gm / ID = 1 / (nVT), almost that of a bipolar transistor.[8]

The sub threshold I–V curve 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 for leakage and performance.[9][10]

MOSFET drain current vs. drain-to-source voltage for several values of VGS − Vth; the boundary between linear (Ohmic) and saturation (active) modes is indicated by the upward curving parabola.

Cross section of a MOSFET operating in the linear (Ohmic) region; strong inversion region present even near drain

Cross section of a MOSFET operating in the saturation (active) region; channel exhibits pinch-off near drain

· BODY EFFECT:-

Ohmic contact to body to ensure no body bias; top left: subthreshold, top right:Ohmic mode, bottom left:Active mode at onset of pinch-off, bottom right: Active mode well into pinch-off - channel length modulation evident.

· ADVANTAGES OF USING MOSFET:-

The MOSFET's advantages in most digital circuits do not translate into supremacy in all analog circuits. Digital circuits switch, spending most of their time outside the switching region, while analog circuits depend on MOSFET behavior held precisely in the switching region of operation. , MOSFETs are widely used in many types of analog circuits because of certain advantages. The characteristics and performance of many analog circuits can be designed by changing the sizes (length and width) of the MOSFETs used. By comparison, in most bipolar transistors the size of the device does not significantly affect the performance. MOSFETs' ideal characteristics regarding gate current (zero) and drain-source offset voltage (zero) also make them nearly ideal switch elements, and also make switched capacitor analog circuits practical. . Also, they can be formed into capacitors and gyrator circuits which allow op-amps made from them to appear as inductors, thereby allowing all of the normal analog devices, except for diodes (which can be made smaller than a MOSFET anyway), to be built entirely out of MOSFETs. This allows for complete analog circuits to be made on a silicon chip in a much smaller space.

Some ICs combine analog and digital MOSFET circuitry on a single mixed-signal integrated circuit, making the needed board space even smaller.

· MOSFET SCLAING:-

Over the past decades, the MOSFET has continually been scaled down in size; typical MOSFET channel lengths were once several micrometers. Smaller MOSFETs are desirable for several reasons. The main reason to make transistors smaller is to pack more and more devices in a given chip area. This results in a chip with the same functionality in a smaller area, or chips with more functionality in the same area. Since fabrication costs for a semiconductor wafer are relatively fixed, the cost per integrated circuits is mainly related to the number of chips that can be produced per wafer. Hence, smaller ICs allow more chips per wafer, reducing the price per chip.

Trend of Intel CPU transistor gate length

· DISADVANTAGES OF USING MOSFET:-

1. Heat production

Large heat sinks to cool power transistors in a TRM-800 audio amplifier

The ever-increasing density of MOSFETs on an integrated circuit is creating problems of substantial localized heat generation that can impair circuit operation. Circuits operate slower at high temperatures, and have reduced reliability and shorter lifetimes. Heat sinks and other cooling methods are now required for many integrated circuits including microprocessors.

(2)MOSFETs are at risk of thermal runaway. As their on-state resistance rises with temperature, if the load is approximately a constant-current load then the power loss rises correspondingly, generating further heat. When the heat sink is not able to keep the temperature low enough, the junction temperature may rise quickly and uncontrollably, resulting in destruction of the device.