emosfet slides

PRESENTED BY :

SHEIKH ABDUL WAHABWAHAB ALI

SYED BILAL HAIDERSYED MUHAMMAD ZEEJAH HASHMI

PRESENTED TO: MISS FARWAH AHMED

ENHANCED MOSFET (EMOSFET)

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MOSFET

MOSFET stands for (Metal Oxide Semiconductor Field Effect Transistor)

It is a semiconductor device which is widely used in switching and amplification in electronic devices.

The MOSFET is a four terminal device having

Source (S), Gate (G), Body(B), Drain (D).

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CONTD…

The MOSFET works electronically by varying the width of a channel along with charge carriers flow (electrons or holes). The charge carriers enters through source and exit via drain.

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DEPLETION MOSFETENHANCEMENT MOSFET

TYPES OF MOSFET

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GRAPHICAL REPRESENTATION

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ENHANCEMENT MODE MOSFET

Enhancement-mode MOSFETs are the common switching elements in most MOS. These devices are off at zero gate–source voltage, and can be turned on by pulling the gate voltage either higher than the source voltage.

When the gate voltage is zero then the current between source and gate is zero, that’s why normally E-MOSFET is normally off when gate voltage is zero.

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1.The drain and the source are connected to n-doped regions

2.These n-doped regions are not connected to any external source

3.The gate connected to the p-doped substrate through SiO2 thin layer

4. Figure shows the structure of N-channel E-MOSFET. At this stage there is no source voltage applied on it so there will be no movement of free electrons from source to drain and “VGS =0” here.

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We can get the current only through positive gate to source (VGS) voltage, when the gate to source is positive it attracts electrons into P-region and then recombine with the holes. When the gate voltage is positive enough, so electrons begin to flow from source to drain.

This effect creates a thin n-type inversion layer through this layer free electrons can easily flow from source to drain.

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Conditions…

The minimum VGS that just creates that n-type inversion layer is called the “Threshold Voltage”.

When VGS < VGS(th)

then the drain current will be equal to zero.

When VGS > VGS(th)

then n-type inversion layer connects source to the drain and drain current begins to flow.

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DRAIN CURVES

Figure show the drain curves for E-MOSFET for very low drain to source voltage.

The lowest curve shows when VGS < VGS(t) then the drain current is almost zero and the drain current line touches x-axis.

As soon as VGS starts raising and at the stage when VGS > VGS(t) then drain current starts on increasing as shown in the graph.

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TRANSCONDUCTANCE CURVE…

Click icon to add pictureThere is no current until VGS =VGS (t)

The drain current then rapidly increases until it reaches the saturation current ID (sat) beyond this point the device is biased in the Ohmic region therefore ID

(sat) remains constant , even though VGS increases to ensure hard saturation, a gate voltage of VGS(on) is uesd well above VGS(t) as shown in graph

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BIASING IN OHMIC REGION

Click icon to add pictureWhen E-MOSFET is biased in the ohmic region there is a Qtest point in that region at VGS= VGS(on) curve the manufacturer measures ID(on) and VDS(on) at this Qtest point.

Manufacturer calculates the value of RDS (on) by using the ohm’s law, the formula is

RDS(on) = VDS (on) / IDS (on)”

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The figure shows the load line between saturation current

ID(sat) and the cut off voltage VDD

When VGS = 0 then the “Q” point is at the lower end of the dc load line and when the “Q” point is below the Q(test) point then the device is biased in the ohmic region in other word the E-MOSFET is biased in the ohmic region when

“ID(sat) < ID(on) and “VGS = VGS(on)”

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JAZAK ALLAH…. !!!