Field Effect Transistors
Introduction• Two main types of FET:
- JFET –Junction field effects transistor- MOSFET – Metal oxide semiconductor field effect transistor
- D-MOSFET ~ Depletion MOSFET- E-MOSFET ~ Enhancement MOSFET
• 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 more easily integrated on IC’s.
- FET’s are also generally more static sensitive than BJT’s.
Construction and characteristics of JFET
• N-channel device will appear as the prominent device with paragraph and section devoted to the impact of using a p-channel.
• Major part of structure is n-type material.
• Top of the n-type channel is connected through an ohmic contact to a terminal referred to as the drain (D)
• The lower end-connected through an ohmic contact to a terminal referred as source (S)
• P-type materials are connected together and to the gate (G) terminal.
• JFET has two p-n junctions under no-bias conditions.
• JFET operation can be compared to a water faucet:
• 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 the n-channel from source to drain.
Construction and characteristics of JFET
N-Channel JFET Circuit Layout
Construction and characteristics of JFET
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
VGS = 0, VDS increasing to some positive value
Three things happen when VGS = 0
and VDS is increased from 0 to amore positive voltage:
• the depletion region between p-gate and n-channel increases as electrons from n-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.
• The flow of charge is relatively uninhibited and limited solely by the resistance of the n-channel between drain and source.
• The depletion region is wider near the top of both p-type materials.
• ID will establish the voltage level through the channel.• The result: upper region of the p-type
material will be reversed biased by about 1.5V with the lower region onlyreversed biased by 0.5V (greater applied reverse bias, the wider depletion region).
VGS = 0, VDS increasing to some positive value
• IG=0A p-n junction is reverse-biased for the length of the
channel results in a gate current of zero amperes.
• As the VDS is increased from 0 to a few volts, the current will
increase as determined by Ohm’s Law.
• VDS increase and approaches a level referred to as Vp, the
depletion region will widen, causing reduction in the channel width. (p large, n small).
• Reduced part of conduction causes the resistance to increase.
• If VDS is increased to a level where it appears that the 2
depletion regions would touch (pinch-off)
VGS = 0, VDS increasing to some positive value
• Vp = pinch off voltage.• ID maintain the saturation level
defined as IDSS
• Once the VDS > VP, the JFET has the characteristics of a current source.
• As shown in figure, the current is
fixed at ID = IDSS, the voltage VDS (for
level >Vp) is determined by the
applied load.
• IDSS is derived from the fact that it is
the drain-to-source current with short circuit connection from gate to source.
• IDSS is the max drain current for a JFET
and is defined by the conditions
VGS=0V and VDS > | Vp|.
VGS = 0, VDS increasing to some positive value
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.
VGS = 0, VDS increasing to some positive value
Typical JFET operation
JFET modeling when ID=IDSS, VGS=0, VDS>VP
VGS < 0, VDS at some positive value
• VGS is the controlling voltage of
the JFET.• For n-channel devices, the
controlling voltage VGS is made
more and more negative from its VGS = 0V level.
• The effect of the applied negative VGS is to establish
depletion regions similar to those obtained with VGS=0V
but a lower level of VDS to
reach the saturation level at a lower level of VDS.
• When VGS = -Vp will be sufficiently negative to establish
saturation level that is essentially 0mA, the device has been ‘turn off’.
• The level of the VGS that results in ID = 0 mA is defined by VGS
= Vp, with Vp being a negative voltage for n-channel devices
and a positive voltage or p-channel JFETs.
• In this region, JFET can actually be employed as a variable resistor whose resistance is controlled by the applied gate to source voltage.
• A VGS becomes more and more negative; the slope of each
curve becomes more and more horizontal.
VGS < 0, VDS at some positive value
• The region to the right of the pinch-off locus of the figure is the region typically employed in linear amplifiers (amplifiers with min distortion of the applied signal) and is commonly referred to as the constant-current, saturation, or linear amplification region.
VGS < 0, VDS at some positive value
Characteristic curves for N-channel JFET
Voltage-Controlled Resistor
• The region to the left of the pinch-off point is called the ohmic region.
• The JFET can be used as a
variable resistor, where VGS
controls the drain-source
resistance (rd). As VGS
becomes more negative, the resistance (rd) increases.
2
PGS
od
)VV(1
rr
And as summary in practical…
p-Channel JFETS
p-Channel JFET acts the same as the n-channel JFET, except the polarities and currents are reversed.
P-Channel JFET Characteristics
As VGS increases more positively:
• the depletion zone increases
• ID decreases (ID < IDSS)
• eventually ID = 0A
Also note that at high levels of
VDS the JFET reaches a
breakdown situation. IDincreases uncontrollably if
VDS> VDSmax.
JFET Symbols
Transfer Characteristics
• The transfer characteristic of input-to-output is not as straight forward in a JFET as it was in 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:
2
P
GSDSSD )
V
V(1II
Transfer Curve
From this graph it is easy to determine the value of ID for a given value of VGS.
Transfer Characteristics
Plotting the Transfer Curve
Shockley’s Equation Methods.• Using IDSS and Vp (VGS(off)) values found in a specification sheet, the
Transfer Curve can be plotted using these 3 steps:
• Step 1:
• Solving for VGS = 0V:
• Step 2:
• Solving for VGS = Vp (VGS(off)):
• Step 3:
• Solving for VGS = 0V to Vp:
2
P
GSDSSD )
V
V(1II
0VVIIGS
DSSD
2
P
GSDSSD )
V
V(1II
PGSD VV0I A
2
P
GSDSSD )
V
V(1II
Shorthand method
VGS ID
0 IDSS
0.3VP IDSS/2
0.5 IDSS/4
VP 0mA
Plotting the Transfer Curve
Specification Sheet (JFETs)
This information is also available on the specification sheet.
Case Construction and Terminal Identification
MOSFETs
MOSFETs have characteristicssimilar to JFETs and additionalcharacteristics that make then very useful.
There are 2 types:1. Depletion-Type MOSFET2. Enhancement-Type
MOSFET
Depletion-Type 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 thin insulating layer of SiO2. The n-doped material lies
on a p-doped substrate that may have an additional terminal connection called SS.
• VGS is set to 0V by the direct
connection from one terminal to the other.
• VDS is applied across the
drain-to-source terminals.• The result is an attraction for
the positive potential at the drain by the free electron of the n-channel and a current similar to that established through the channel of the JFET.
• In the figure, VGS has been
set at a negative voltage (-1V)
Depletion-Type MOSFET Construction
• Negative potential at gate will tend to pressure electron towards the p-type substrate and attract holes from the p-type substrate.
• Depending on negative bias established by VGS, a level recombination between electron and hoes will occur.--- it will reduce the number of free electron in the n-channel available for conduction.
• The more negative bias, the higher the rate of recombination
• ID decrease, negative bias for VGS increase
Depletion-Type MOSFET Construction
Basic OperationA Depletion MOSFET can operate in two modes: Depletion or Enhancement mode.
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 Curve still applies:
2
P
GSDSSD )
V
V(1II
Depletion-type MOSFET in Enhancement Mode
Enhancement modeVGS > 0V, ID increases above IDSS
The formula used to plot the Transfer Curve still applies:
(note that VGS is now a positive polarity)
2
P
GSDSSD )
V
V(1II
p-Channel Depletion-Type MOSFET
The p-channel Depletion-type MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed.
Symbols
Enhancement-Type 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. The Gate (G) connects to the p-doped substrate via a thin insulating 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.
• VGS=0, VDS some value, the absence of an n-channel will result in a current of effectively 0A
• VDS some positive voltage, VGS=0V, and terminal SS is directly connected to the source, there are in fact 2 reversed-biased p-n junction between the n-doped regions and p substrate to oppose any significant flow between drain and source.
• VDS and VGS have been set at some positive voltage greater than 0V, establishing the D and G at a positive potential with respect to the source
• The positive potential at the gate will pressure the holes in the p substrate along the edge of the SiO2 layer to leave the area and enter deeper regions of the p-substrate
• The result is a depletion region near the SiO2 insulating layer void of holes
Enhancement-Type MOSFET Construction
• The electrons will in the p substrate will be attracted to the +G and accumulate in the
region near the surface of the SiO2 layer
• The SiO2 layer and its insulating qualities will
prevent the negative carriers from being absorbed at the gate terminal
• VGS increase, the concentration of electrons near the SiO2 surface increase until eventually the induced n-type region can support a measurable flow between D and S
• The level of VGS that results in the significant increase in drain current is called the threshold voltage, VT.
• VGS increase beyond the VT level the density of the carriers in the induced channel will increase and ID also increase
Enhancement-Type MOSFET Construction
• If VGS constant and increase
the level of VDS, ID will
eventually reach a saturation level as occurred for the JFET
• Applying Kirchoff’s voltage law to the terminal voltage of the MOSFETVDG = VDS- VGS
• If VGS fixed at some value, 8V, VDS increased from 2 – 5V, the VDG will drop from -6V to -3V and the gate will become less and less positive with respect to the drain
Enhancement-Type MOSFET Construction
VGS is always positiveAs VGS increases, ID increases
But if VGS is kept constant and VDS is
increased, then ID saturates (IDSS)The saturation level, VDSsat is reached.
TGSDsat VVV
The Enhancement-type MOSFET only operates in the enhancement mode.
Enhancement-Type MOSFET Construction
To determine ID given VGS:
where VT = threshold voltage or voltage at which the MOSFET turns on.k = constant found in the specification sheet. k can also be determined by using values at a specific point and the formula: VDSsat can also be calculated:
2)( TGSD VVkI
2TGS(ON)
D(on)
)V(V
Ik
TGSDsat VVV
Enhancement-Type MOSFET Construction
p-Channel Enhancement-Type MOSFETs
The p-channel Enhancement-type MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed.
Symbols
Specification Sheet
MOSFET Handling
• MOSFETs are very static sensitive. Because of the very thin SiO2 layer between the external terminals and the layers of the device, any small electrical discharge can stablish an unwanted conduction.
Protection:• Always transport in a static sensitive bag• Always wear a static strap when handling MOSFETS• Apply voltage limiting devices between the Gate and
Source, such as back-to-back Zeners to limit any transient voltage.
VMOS
VMOS – Vertical MOSFET increases the surface area of the device.
Advantage:• This allows the device to handle higher currents by
providing it more surface area to dissipate the heat.• VMOSs also have faster switching times.
CMOSCMOS – Complementary MOSFET p-channel and n-channel MOSFET on
the same substrate.Advantage:Useful in logic circuit designsHigher input impedanceFaster switching speedsLower operating power levels
Summary Table
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