Field–Effect Transistors

FETFET’s (Field – Effect Transistors) are much like BJT’s (Bipolar Junction Transistors).
Similarities:• Amplifiers• Switching devices• Impedance matching circuits
Differences:• FET’s are voltage controlled devices whereas BJT’s are current controlled
devices.
• FET’s also have a higher input impedance, but BJT’s have higher gains.
• FET’s are less sensitive to temperature variations and because of there
construction they are more easily integrated on IC’s.
• FET’s are also generally more static sensitive than BJT’s.

FET Types• JFET ~ Junction Field-Effect Transistor• MOSFET ~ Metal-Oxide Field-Effect Transistor- D-MOSFET ~ Depletion MOSFET- E-MOSFET ~ Enhancement MOSFETJFET Construction
There are two types of JFET’s: n-channel and p-channel.
The n-channel is more widely used.

There are three terminals: Drain (D) and Source (S) are connected to n-channel
Gate (G) is connected to the p-type material

Basic Operation of JFETJFET operation can be compared to a water spigot:

The source of water pressure – accumulated electrons at the negative pole of the applied
voltage from Drain to Source
The drain of water – electron deficiency (or holes) at the positive pole of the applied
voltage from Drain to Source.
The control of flow of water – Gate voltage that controls the width of the n-channel,
which in turn controls the flow of electrons in then-channel from source to drain.

JFET Operating Characteristics

There are three basic operating conditions for a JFET:A.

VGS = 0, VDS increasing to some positive value

B.

VGS < 0, VDS at some positive value

C.

Voltage-Controlled Resistor

A. VGS = 0, VDS increasing to some positive value

Three things happen when VGS = 0 and VDS is increased from 0 to a more positive voltage:• the depletion region between p-gate and n-channel increases as electrons fromn-channel combine with holes from p-gate.
• increasing the depletion region, decreases the size of the n-channel which
increases the resistance of the n-channel.
• But even though the n-channel resistance is increasing, the current (ID) from
Source to Drain through the n-channel is increasing. This is because VDS is increasing.

Pinch-offIf VGS = 0 and VDS is further increased to a more positive voltage, then the depletion zone
gets so large that it pinches off the n-channel. This suggests that the current in the nchannel (ID) would drop to 0A, but it does just the opposite: as V DS increases, so does ID.

Saturation

At the pinch-off point:• any further increase in VGS does not produce any increase in ID. VGS at
pinch-off is denoted as Vp.
• ID is at saturation or maximum. It is referred to as IDSS.
• The ohmic value of the channel is at maximum.

B. VGS < 0, VDS at some positive value

As VGS becomes more negative the depletion region increases.

Transfer Characteristics
The transfer characteristic of input-to-output is not as straight forward in a JFET as it wasin a BJT.
In a BJT,  indicated the relationship between IB (input) and IC (output).

In a JFET, the relationship of VGS (input) and ID (output) is a little more complicated:
VGSID
IDSS(1 )2VP[Formula 5.3]

Transfer Curve

From this graph it is easy to determine the value of ID for a given value of VGS.

Plotting the Transfer Curve
Using IDSS and Vp (VGS(off)) values found in a specification sheet, the Transfer Curve canbe plotted using these 3 steps:
VGS
ID
IDSS(1 )2
V
P

Step 1:[Formula 5.3]

Solving for VGS = 0V:

ID
IDSS
VGS

0V

[Formula 5.4]

Step 2:

VGS
ID
IDSS
(1 )2
V
P[Formula 5.3]

IDA0VGS
VP[Formula 5.5]

VGSID
IDSS(1 )2
VP[Formula 5.3]

Solving for VGS = Vp (VGS(off)):

Step 3:
Solving for VGS = 0V to Vp:

Important RelationshipMOSFETs
MOSFETs have characteristics similar to JFETs and additional characteristics that makethen very useful.
There are 2 types:• Depletion-Type MOSFET• Enhancement-Type MOSFETMOSFET Construction

The Drain (D) and Source (S) connect to the to n-doped regions. These N-doped regions
are connected via an n-channel. This n-channel is connected to the Gate (G) via a thininsulating layer of SiO2. The n-doped material lies on a p-doped substrate that may have anadditional terminal connection called SS.

Basic Operation and characteristics- Depletion MOSFETDepletion-type MOSFET in Depletion Mode
Depletion mode
The characteristics are similar to the JFET.
When VGS = 0V, ID = IDSS
When VGS < 0V, ID < IDSS
The formula used to plot the Transfer Curvestill applies:
VGSID
IDSS(1 )2
V
P

Depletion-type MOSFET in Enhancement Mode
Enhancement mode
VGS > 0V, ID increases above IDSS
The formula used to plot theTransfer Curve still applies:(note that VGS is now a positive polarity)
VGSID
IDSS(1 )2VPEnhancement-Type MOSFETThe Drain (D) and Source (S) connect to the to n-doped regions. These n-doped regions
are connected via an n-channel. The Gate (G) connects to the p-doped substrate via a thininsulating layer of SiO2. There is no channel. The n-doped material lies on a p-doped
substrate that may have an additional terminal connection called SS.

Enhancement-Type MOSFET Construction