Skip to content Skip to navigation

Connexions

You are here: Home » Content » AE_Lecture8_MOSFET & JFET_PartA

Navigation

Recently Viewed

This feature requires Javascript to be enabled.
 

AE_Lecture8_MOSFET & JFET_PartA

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Lecture8_PartA describes the structure and static characteristics of MOSFET.

AE_Lecture8_PartA Static characteristics of FIELD EFFECT TRANSISTORS

A Field Effect Transistor is like a pentode. It is an analogue of pentode. Both are Voltage Controlled Current Sources.

FET is of two types JFET and MOSFET.

JFET is n channel FET known as nJFET and p channel FET known as pJFET.

Similarly MOS is n channel NMOS and p channel PMOS.

MOS can be enhancement mode which is normally-off or depletion mode normally-on device.

So NMOS can be (E)NMOS and (D)NMOS.

PMOS can be (E)PMOS and (D)PMOS.

The following is the circuit diagram of an Amplifier using N-channel JFET:

Figure 1
Figure 1 (Picture 1.png)

Measurement of characteristics of N-channel JFET

Figure 2
Figure 2 (Picture 30.png)

Figure 2 gives the dc output characteristics nJFET. Note that Gate Voltage is always kept negative so that the gate current is always zero. We indicate the Gate-Drain breakdown also. As gate voltage becomes more negative , Avalanche Breakdown of Gate to Drain Junction takes place earlier.When (VDS – VGS) exceeds VP (Vpinchoff), the channel gets pinched off towards the Drain End and current becomes constant. This is saturation region. The dotted parabola seperates the Ohmic region or Triode Region from Saturation Region or Pentode Region. Here the pinch off voltage is -5V.

Figure 3
Figure 3 (Picture 31.png)

Figure 3 gives the output characteristics of (E)NMOS with a Threshold Voltage of 1V. Therefore when VDS increases and becomes equal to (VGS - VTh) the channel pinches off near the drain end and current saturates.. As VDS assumes larger values the pinched off region becomes wider, (VGS - VTh) drops across the channel and excess part of VDS drops across the pinched off region. Since there is a constant voltage dropping across the channel and since the channel is of constant length hence IDS is constant. But in real life there is a shortening of channel length leading to a slope of the saturated current. This channel length modulation is responsible for a finite output impedance rds of the voltage controlled current source modeling (E)NMOS under incremental condition.

Figure 4
Figure 4 (Picture 32.png)

In Figure 4 we give the output characteristics of (D)NMOS. Depletion type device is normally-on device hence it gives a characteristic for positive gate voltage, zero gate voltage and negative voltage. In (D)NMOS when gate voltage becomes equal to Pinch-off voltage then (D)NMOS turns off. Here pinch off voltage is -4V.

Figure 5
Figure 5 (Picture 33.png)

Figure 6
Figure 6 (Picture 34.png)

Figure 7
Figure 7 (Picture 35.png)

Comparing the transfer characteristics of nJFET, (E)NMOS and (D)NMOS as given in Figure 5, 6 and 7 we can say that (D)NMOS is most flexible device from design point of view as it accepts both positive and negative voltages as the gate voltage.

NMOS Transistor (Pentode and Triode Region)

Figure 8
Figure 8 (Picture 36.png)
Figure 9
Figure 9 (Picture 37.png)

TRIODE REGION:

Figure 10
Figure 10 (graphics1.png)
______(1)

Kn=Kn'multiplied_by

Figure 11
Figure 11 (graphics2.png)

Kn'= µnCox″ ...........Kn’ ia a transconductance parameter which is fixed for a given technology and cannot be changed by the circuit designer. Whereas Kn = Kn’×(W/L) = this is also a transconductance parameter but it includes the aspect ration (W/L) of the given MOSFET. The aspect ratio gives the geometry of the device and this is under control of a circuit designer . This second transconductance parameter can be controlled when he is generating the masks for a given circuit or system during ASIC(Application Specefic IC) or SOC(System –on- Chip) design. W is the width of the gate and L is the length of the gate or the length of the channel.

Figure 12
Figure 12 (graphics3.png)

Figure 13
Figure 13 (graphics4.png)
Figure 14
Figure 14 (graphics5.png)

Triode Region exists as long as

Figure 15
Figure 15 (graphics6.png)
.

Channel should not pinch off on the drain side.

PENTODE REGION

At

Figure 16
Figure 16 (graphics7.png)
___________(2)

channel pinches off at drain end. From here onwards current becomes constant or current saturates.

Substituting (2) in (1),

Figure 17
Figure 17 (graphics8.png)
Figure 18
Figure 18 (graphics9.png)
Figure 19
Figure 19 (graphics10.png)
Figure 20
Figure 20 (graphics11.png)

Figure 21
Figure 21 (graphics12.png)
Quadratic Law

Figure 22
Figure 22 (graphics13.png)

Where:

Figure 23
Figure 23 (graphics14.png)

Figure 24
Figure 24 (graphics15.png)

Pinch Off locus is parabolic

Figure 25
Figure 25 (graphics16.png)

For any VGS , at V*DS = VGS - VTN the saturation sets in as the channel gets pinched off at that point. Beyond that point: VDS = V*DS + ΔV and V*DS drops across the channel and ΔV drops across the depleted region towards the drain end as shown in Figure 11. Since the voltage across the channel is constant at V*DS and since the channel is approximately constant ( it shortens slightly as Drain to Source Voltage increases) for practical purposes we assume that the drain current is constant and hence it has saturated. But in practice due to channel length shortening, there is slight increase in drain current with drain to source voltage. Hence output characteristics does have a slope. At higher currents the slope is higher.

Figure 26
Figure 26 (Picture 38.png)
VTN=1V

Kn=25 µA/V2

Figure 27
Figure 27 (graphics17.png)
Figure 28
Figure 28 (Picture 2.png)

N channel JET has identical formulation except threshold voltage is replaced by pinch off voltage.

Content actions

Download module as:

PDF | EPUB (?)

What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

Downloading to a reading device

For detailed instructions on how to download this content's EPUB to your specific device, click the "(?)" link.

| More downloads ...

Add module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks