Lecture 14
FET
• The field effect transistor (FET) is a three-terminal device similar to the bipolar junction transistor.
• The FET, however, is a unipolar device that depends on only one type of charge carrier, either free electrons or holes.
• There are basically two types of FETs:
– Junction field effect transistor, abbreviated JFET.
– Metal-oxide-semiconductor field effect transistor, MOSFET. • Bipolar transistors are current-controlled devices.
• FETs are voltage-controlled devices.
– i.e., an input voltage controls an output current.
• The input impedance is extremely high (up to mega ohms).
• FETs require very little power from the driving source.
JFETs and Their Characteristics
• Figure a: n -channel JFET.
• Four leads: the drain, source, and two gates.
• Between source and drain is channel.
• Connects separate lead to each gate, dual-gate JFET.
• Dual-gate JFETs are most commonly used in frequency mixers, in communications
electronics.
• Mostly gates are internally connected and the device acts like a single-gate JFET.
• Figure b: p -channel JFET.
• Fig. a , the majority current carriers in the channel are free electrons.
Schematic Symbols
• Fig. a is the schematic symbol for the
n -channel JFET
JFET Operation
• Figure illustrates the current flow in an
n -channel JFET with the p -type gates
left disconnected.
• Amount of current flow depends upon two factors:
– Value of the drain-source voltage, VDS – Drain-source resistance r DS .
• r DS is dependent on
– Doping level
– Cross- sectional area
– Length of the doped semiconductor • Electrons flow through the channel
from source to drain.
Gate Action
• Gate helps to control the amount of current flow.
• Fig a both gates shorted to the source.
• The drain supply voltage, V DD ,
reverse-biases both p-n junctions. This results in zero gate current.
• Fig b n -channel JFET is normally biased.
• The effect of the negative gate voltage is to expand the width of the depletion
regions, which in turn narrows the channel.
• Because the channel is narrower, the drain current, I D , is reduced. By varying the gate source voltage, designated V GS , the drain current, I D , can be
Gate Action
• If V GS is made negative enough, the depletion layers touch, which pinches off the channel.
• The result is zero drain current. The
amount of gate-source voltage required to reduce the drain current, I D , to zero is called the gate-source cutoff voltage, designated V GS(off) .
• The polarity of the biasing voltages for a
p -channel JFET is opposite from that of
Shorted Gate-Source Junction
• Fig. a n -channel JFET connected to the proper biasing voltages.
• When the gate supply voltage, V GG , is reduced to zero, gate is effectively shorted to the source and V GS equals zero volts.
• Fig. b graph of ID versus VDS.
• VDS is increased from zero, the drain
current, ID , increases proportionally.
• When the drain-source voltage,
V
DS,reaches the pinch-off voltage VP,
drain current, ID, levels off.
Shorted Gate-Source Junction
• The region below VP is called the
ohmic region because ID increases in
direct proportion to VDS .
• Above VP is the current-source region,
where ID is unaffected by changes in
VDS.
• Drain current ID, levels off above VP
because at this point the channel resistance, r DS , increases in direct
proportion to VDS .
• The maximum drain current that a
JFET can have under normal operating conditions occurs when V GS is 0 V.
Drain Curves
• Figure shows a complete set of drain curves for the JFET.
• Notice that as V GS becomes increasingly more negative, the drain current, I D , is reduced.
Transconductance Curve
• Figure shows a graph of ID
versus VGS for the JFET.
• This curve is called a
transconductance curve.
• Graph is not linear because equal changes in VGS do not
Calculating the Drain Current
Exercise
• In what part of a JFET does current flow?
• When looking at the schematic symbol of a JFET, how can you tell if it is a p -channel or n -channel JFET?
• In a JFET, which two currents are identical?
• Define VGS (off) .
• For a JFET, what is the pinch-off voltage, VP ?
• How are VP and VGS(off) related?
• What happens to the pinch-off voltage of an n –channel JFET as VGS becomes more negative?
Lecture 15
MOSFETs and Their Characteristics
• The metal-oxide-semiconductor field effect transistor has a gate, source, and drain just like the JFET.
• Like a JFET, the drain current in a MOSFET is controlled by the gate source voltage V GS .
• There are two basic types of MOSFETs:
– Enhancement-type E-MOSFET
– Depletion-type D-MOSFET.
• MOSFET is insulated from the channel.
• MOSFETs are referred to as insulated gate FETs or IGFETs.
Depletion-Type MOSFET
• Fig. a, the drain terminal is at the top of the n -material and the source
terminal is at the bottom.
• The block of p -type material forms the substrate into which the n -type material is embedded.
• n-type material along the n-channel,
a thin layer of silicon dioxide (SiO2 ) is
deposited to isolate the gate from the channel.
• The solid line connecting the source and drain terminals indicates that depletion-type MOSFETs are
Zero Gate Voltage
• Depletion-type MOSFET can operate with either positive or negative gate voltages.
• It is also conducts with the gate shorted to the source for V GS = 0 V.
• VDD is connected between drain & source
with the drain positive relative to source.
• Substrate connected to source, gate shorted to source, ID will flow in n-type
channel.
• p-type substrate is grounded, n-channel
and p–type substrate are always
reverse-biased; so zero current in the substrate.
Zero Gate Voltage
• Drain curve, the drain current
increases linearly until the pinch-off voltage, VP , is reached.
• When VGS negative, pinch-off occurs
sooner, and when V GS is made positive, pinch-off occurs later.
Enhancement & Depletion Mode
• The positive gate voltage attracts freeelectrons into the channel from substrate, thereby enhancing its conductivity.
• When the gate is made positive relative to the source, the
depletion-type MOSFET is said to be operating in the enhancement mode.
• A negative voltage applied to the gate.
• The negative gate voltage sets up an electric field that repels free electrons from the channel.
• When the gate is made negative relative to the source, the
Enhancement & Depletion Mode
• Making the gate negative enough willreduce the drain current, I D , to zero.
• Fig shows transconductance curve for the n-channel depletion-type MOSFET.
• When V GS is positive, the MOSFET operates in the enhancement mode, and the drain current increases
beyond the value of I DSS .
• When V GS is negative, the MOSFET operates in the depletion mode.
• If VGS is negative enough, the drain
current, ID, will be reduced to zero.
Depletion-Type MOSFET Applications
•
D-MOSFETs are frequently used as small signal
amplifiers and frequency mixers.
•
D-MOSFETs are quite similar to JFETs, the ac analysis
used with JFETs can also be used with D-MOSFETs.
Enhancement-Type MOSFETs
• Fig a shows the construction of an
n -channel enhancement-type
MOSFET.
• p-type substrate makes contact
with the SiO2 insulator.
• There is no channel for
conduction between the drain and source terminals.
• The drain and gate are made positive with respect to the source.
• When V GS = 0 V, there is no
channel between the source and drain and so the drain current, ID,
Enhancement-Type MOSFETs
• To produce drain current, the positive gate voltage must be increased.
• This attracts electrons along the right edge of the SiO2 insulator, as
shown in Fig.
• Minimum gate-source voltage that makes drain current flow is called the threshold voltage, VGS(th).
• When the gate voltage is less than
VGS(th), ID is zero.
• The value of VGS(th) varies from one
Enhancement-Type MOSFETs
• Broken line represents the “off ” condition that exists with zero gate voltage.
• This characteristic, enhancement-type
MOSFETs are called “normally off ” devices.
• Figure 30–20 d shows a typical set of drain curves for the n -channel
• enhancement-type MOSFET. The lowest curve is the V GS(th) curve. For more positive
• gate voltages, the drain current, I D ,
increases. The transconductance curve is
• shown in Fig. 30–20 e . Notice that I D is zero when the gate-source voltage is less
Enhancement-Type MOSFETs
• Fig. d shows a typical set of drain curves for the n-channel E-MOSFET.
• Lowest curve is the V GS(th) curve.
• For more positive gate voltages, the drain current, I D , increases.
• The transconductance curve is shown in Fig. e .
E-MOSFET Application
• E-MOSFETs have many applications in electronics.
• The most important application is in digital computer electronics.
• E-MOSFETs are used because they take up very little space on a chip (an integrated circuit) compared to the space used by an equivalent circuit with bipolar transistors.
• When packaging hundreds or even thousands of transistors onto an IC, MOSFETs are used.
