1. Field Effect Transistors By Er. S.GHOSH

2. JFETs and Their Characteristics
N-Channel
P-Channel
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3. JFETs and Their Characteristics
Fig. (a) is the schematic symbol for the n-channel JFET, and Fig. (b) shows the symbol for the p-channel JFET.
The only difference is the direction of the arrow on the gate lead.
Fig. (a)
Fig. (b)
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4. JFETs and Their Characteristics
Fig. illustrates the current flow in an n-channel JFET with p-type gates disconnected.
The amount of current depends upon two factors:
The value of the drain-source voltage, VDS
The drain-source resistance, designated rDS
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5. JFETs and Their Characteristics
The gate regions in a JFET are embedded on each side of the channel to help control the amount of current flow in the channel.
Fig. (a) shows an n-channel JFET with both gates shorted to the source.
Fig. (b) shows how an n-channel JFET is normally biased.
Fig.
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6. JFETs and Their Characteristics
Fig. (a) shows an n-channel JFET connected to the proper biasing voltages.
The drain is positive and the gate is negative, creating the depletion layers.
Fig. (c) shows a complete set of drain curves for the JFET in Fig. (a).
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7. Biasing Techniques for JFETs
Many techniques can be used to bias JFETs.
In all cases, the gate-source junction is reverse-biased.
The most common biasing techniques are
Gate
Self
Voltage-divider
Current-source

8. Biasing Techniques for JFETs
Fig. (a) shows an example of gate bias.
Fig. (b) shows how an ac signal is coupled to the gate of a JFET.
If RG were omitted, as shown in (c), no ac signal would appear at the gate because VGG is at ground for ac signals.
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9. Biasing Techniques for JFETs
One of the most common ways to bias a JFET is with self-bias. (See Fig. a)
Only a single power supply is used, the drain supply voltage, VDD.
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10. Biasing Techniques for JFETs
Fig. shows a JFET with voltage-divider bias.
Since the gate-source junction has extremely high resistance, the R1 – R2 voltage divider is practically unloaded.
Voltage-divider bias is more stable than either gate or self-bias.
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11. Biasing Techniques for JFETs
Fig. shows one of the best ways to bias JFETs, called current-source bias.
The npn transistor with emitter bias acts like a current source for the JFET.
The drain current , ID, equals the collector current, IC, which is independent of the value of VGS.
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12. JFET Amplifiers JFETs are commonly used to amplify small ac signals.
One reason for using a JFET instead of a bipolar transistor is that very high input impedance, Zin, can be obtained.
A big disadvantage, however, is that the voltage gain, AV, obtainable with a JFET is much smaller.
JFET amplifier configurations are as follows:
Common-source (CS)
Common-gate (CG)
Common-drain (CD)

13. JFET Amplifiers
Fig. (a) shows a common-source amplifier.
For a common-source amplifier, the input voltage is applied to the gate and the output is taken at the drain.
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14. JFET Amplifiers On the input side, RG = Zin, which is 1 MΩ.
The ac equivalent circuit is shown in Fig. (b)
On the input side, RG = Zin, which is 1 MΩ.
This occurs because with practically zero gate current, the gate-source resistance, designated RGS, approaches infinity.
Fig. (b)
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15. JFET Amplifiers
Fig. (a) shows a common-drain amplifier, usually referred to as a source follower.
A source follower has a high input impedance, low output impedance, and a voltage gain of less than one, or unity.
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16. JFET Amplifiers Its big drawback is that Zin is quite low.
A common-gate amplifier has a moderate voltage gain.
Its big drawback is that Zin is quite low.
Fig. (a) shows a CG amplifier.
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17. MOSFETs and Their Characteristics
The metal-oxide semiconductor field effect transistor has a gate, source, and drain just like the JFET.
The drain current in a MOSFET is controlled by the gate-source voltage VGS.
There are two basic types of MOSFETS: the enhancement-type and the depletion-type.
The enhancement-type MOSFET is usually referred to as an E-MOSFET, and the depletion-type, a D-MOSFET.
The MOSFET is also referred to as an IGFET because the gate is insulated from the channel.

18. MOSFETs and Their Characteristics
Fig. (a) shows the construction of an n-channel depletion-type MOSFET, and Fig. (b) shows the schematic symbol.
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19. MOSFETs and Their Characteristics
Fig. shows the construction and schematic symbol for a p-channel, depletion-type MOSFET.
Fig. (a) shows that the channel is made of p-type semiconductor material and the substrate is made of n-type semiconductor material.
Fig. (b) shows the schematic symbol.
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20. MOSFETs and Their Characteristics
Fig. (a) shows the construction of an n-channel, enhancement-type MOSFET.
The p-type substrate makes contact with the SiO2 insulator.
Because of this, there is no channel for conduction between the drain and source terminals.
Fig.(a)
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21. MOSFET Biasing Techniques
Zero-bias can be used only with depletion-type MOSFETs.
Even though zero bias is the most commonly used technique for biasing depletion-type MOSFETs, other techniques can also be used.
Biasing techniques include
Self
Voltage-divider
Current-source
Drain-feedback bias is often used to bias E-MOSFETs

22. MOSFET Biasing Techniques
Fig. (a) shows a popular biasing technique that can be used only with depletion-type MOSFETs.
This form of bias is called zero bias because the potential difference between the gate-source region is zero.
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23. Handling MOSFETs
One disadvantage of MOSFET devices is their extreme sensitivity to electrostatic discharge (ESD) due to their insulated gate-source regions.
The SiO2 insulating layer is extremely thin and can be easily punctured by an electrostatic discharge.
The following is a list of MOSFET handling precautions
Never insert or remove MOSFETs from a circuit with the power on.

24. Handling MOSFETs MOSFET handling precautions (Continued)
Never apply input signals when the dc power supply is off.
Wear a grounding strap on your wrist when handling MOSFET devices.
When storing MOSFETs, keep the device leads in contact with conductive foam, or connect a shorting ring around the leads.

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