EEE2045F_lecture_4_MOSFET_uct lecture notes.pdf

Outline
q The MOSFET
q Enhancement MOSFET (E-MOSFET)
q Depletion MOSFET (D-MOSFET)
q MOSFET Characteristics and Parameters
q MOSFET Biasing
2

MOSFET Introduction
q The MOSFET (metal oxide semiconductor field-effect transistor) is another category of
field-effect transistor.
q The MOSFET, different from the JFET – the gate of the MOSFET is insulated from the
channel by a silicon dioxide (SiO2) layer.
q The two basic types of MOSFETs are enhancement (E) and depletion (D).
q Of the two types, the enhancement MOSFET is more widely used.
q Because polycrystalline silicon is now used for the gate material instead of metal, these
devices are sometimes called IGFETs (insulated-gate FETs).
4

MOSFET Introduction
q MOSFETs are very popular transistors that can be made smaller than a BJT and occupy
less silicon area on an IC.
q MOSFETs are relatively straightforward to fabricate.
q Digital logic and memory can be implemented entirely using MOSFETs so that no
resistors or diodes are necessary.
5

7
Enhancement MOSFET (E-MOSFET)

q The E-MOSFET operates only in the enhancement mode and
has no depletion mode.
q Notice in the figure that the substrate extends completely to the
SiO2 layer.
q These are the circuit schematic symbols for the n-channel and
p-channel E-MOSFETs. The broken lines symbolise the
absence of a physical channel.
8

q For an n-channel device, a positive gate voltage above a
threshold value induces a channel by creating a thin layer
of negative charges in the substrate region adjacent to the
SiO2 layer, as shown in the figure.
q The conductivity of the channel is enhanced by increasing
the gate-to-source voltage and thus pulling more electrons
into the channel area.
q For any gate voltage below the threshold value, there
is no channel.
9

10
Depletion MOSFET (D-MOSFET)

q The D-MOSFET can be operated in either of two modes — the depletion mode or the
enhancement mode — and is sometimes called a depletion/enhancement MOSFET.
q Since the gate is insulated from the channel, either a positive or a negative gate voltage
can be applied.
q The n-channel MOSFET operates in the depletion mode when a negative gate-to-
source voltage is applied and in the enhancement mode when a positive gate-to-
source voltage is applied.
q D-MOSFET devices are generally operated in the depletion mode.
q The key difference between a D-MOSFET and an E-MOSFET is that the D-MOSFET has a
physically implanted conduction channel connecting the drain and the source. So
there is no need to “induce” a channel as in the E-MOSFET case.
11

q The greater the negative voltage on the gate, the
greater the depletion of n-channel electrons.
q At a sufficiently negative gate-to-source voltage,
VGS(off), the channel is totally depleted and the drain
current is zero.
q Like the n-channel JFET, the n-channel D-MOSFET
conducts drain current for gate-to-source voltages
between VGS(off) and zero.
q In addition, the D-MOSFET conducts for values of
VGS above zero
12

Enhancement MOSFET (D-MOSFET)
q With a positive gate voltage, more conduction
electrons are attracted into the channel, thus
increasing (enhancing) the channel conductivity.
q Schematic symbols for n-channel and p-channel
D-MOSFET
13

14
MOSFET Characteristics and Parameters

E-MOSFET Transfer Characteristics
q The E-MOSFET uses only channel enhancement.
q Therefore, an n-channel device requires a positive
VGS, and a p-channel device requires a negative
VGS.
q Shown is the general transfer characteristic curve
for the n-type E-MOSFET.
q There is no drain current when VGS = 0.
q Therefore, the E-MOSFET does not have a
significant IDSS parameter, as do the JFET and the
D-MOSFET.
q Notice also that there is ideally no drain current
until VGS reaches a certain non-zero value called
the threshold voltage, VGS(th). 16

E-MOSFET Transfer Characteristic
q The equation for the parabolic transfer characteristic curve of the E-MOSFET differs from
that of the JFET and the D-MOSFET because the curve starts at VGS(th) rather than VGS(off)
on the horizontal axis and never intersects the vertical axis.
q The equation for the E-MOSFET transfer characteristic curve is:
I! = K V"# − V"# $%
&
q The constant K depends on the particular MOSFET and can be determined from the
datasheet by taking the specified value of ID, called ID(on), at the given value of VGS and
substituting the values into the equation above.
q Even though the E-MOSFET is more restricted in its operating range than the D-
MOSFET, the E-MOSFET is more useful and common in IC applications because of
its simpler construction. The E-MOSFET also has the advantage that it can be used
in digital ICs with single low-voltage supplies.
17

Example 1
q The datasheet for an E-MOSFET gives ID(on) = 500mA (minimum) at VGS = 10V
and VGS(th) = 1V. Determine the drain current for VGS = 5V.
19

Example 1 – Solution
q First, solve for K:
K =
I! '(
V"# − V"# $%
& =
500mA
10V − 1V & =
500mA
81V& = 6.17mA/V&
q Now that we have K, we can calculate ID for VGS = 5V:
I! = K V"# − V"# $%
&
= 6.17mA/V& 5V − 1V & = 𝟗𝟖. 𝟕𝐦𝐀
20

D-MOSFET Transfer Characteristics
q The D-MOSFET can operate with either positive
or negative gate voltages.
q This is indicated on the general n-channel
MOSFET transfer characteristic curve.
q The point on the curve where VGS = 0 corresponds
to IDSS.
q The point where ID = 0 corresponds to VGS(off).
q As with the JFET, V"# ')) = −V*.
q The square-law expression for the JFET curve
also applies to the D-MOSFET curve.
22

E-MOSFET Biasing
q Because E-MOSFETs must have a VGS
greater than the threshold value, VGS(th),
zero bias cannot be used.
q The figure shows two ways to bias an E-
MOSFET (D-MOSFETs can also be
biased using these methods). An n-
channel device is used for illustration.
q In either the voltage-divider or drain-
feedback bias arrangement, the purpose
is to make the gate voltage more positive
than the source by an amount exceeding
VGS(th).
q In the drain-feedback bias circuit in (b),
there is negligible gate current and,
therefore, no voltage drop across RG.
This makes VGS = VDS.
24

Example 2
q Determine VGS and VDS for the E-MOSFET circuit.
q Assume this particular MOSFET has minimum values of
ID(on) = 200mA at VGS = 4V and VGS(th) = 2V.
26

q For the E-MOSFET, the gate-to-source voltage is:
V"# =
R&
R+ + R&
V!! =
15kΩ
115kΩ
24V = 𝟑. 𝟏𝟑𝐕
q To determine V!#, first find K using the minimum value of I! '( and the specified voltage
values:
K =
I! '(
V"# − V"#($%)
& =
200mA
4V − 2V & =
200mA
4V& = 50mA/V&
q Now we can calculate I! for V"# = 3.13V:
I! = K V"# − V"# $%
&
= 50mA/V& 3.13V − 2V & = 63.8mA
q Finally, we can calculate VDS:
V!# = V!! − I!R! = 24V − 63.8mA 200Ω = 𝟏𝟏. 𝟐𝐕
27

Example 3
q Determine the amount of drain current in the
circuit shown. The MOSFET has a VGS(th) = 3V.
29

q The meter indicates V!" = 8.5V. Since this is a drain-feedback configuration,
V#" = V!" = 8.5V.
q We can therefore calculate ID as:
I# =
V## − V#"
R#
=
15V − 8.5V
4.7kΩ
= 𝟏. 𝟑𝟖𝐦𝐀
30

D-MOSFET Biasing
q Recall that D-MOSFETs can be operated with either
positive or negative values of VGS.
q A simple bias method is to set VGS = 0 so that an AC signal
at the gate varies the gate-to-source voltage above and
below this 0V bias point.
q A MOSFET with zero bias is shown in (a).
q Since VGS = 0, ID = IDSS as indicated.
q The drain-to-source voltage is expressed as follows:
V!# = V!! − I!##R!
31

D-MOSFET Biasing
q The purpose of RG is to accommodate an AC signal input
by isolating it from ground, as shown in (b).
q Since there is no DC gate current, RG does not affect the
zero gate-to-source bias.
q But why is RG needed if the gate is at 0V DC?
q Until now we have only dealt with FET DC voltages, but
FETs are mostly AC voltage amplifiers. So, if RG were not
present (shorted to ground), then no AC signal would
develop at the gate.
q Inclusion of RG allows the AC signal to develop at the
gate.
32

Example 4
q Determine the drain-to-source voltage in the circuit below.
q The MOSFET datasheet gives VGS(off) = -8V and IDSS = 12mA.
34

q Since I# = I#"" = 12mA, the drain-to-source voltage is:
V#" = V## − I#""R# = 18V − 12mA 620Ω = 𝟏𝟎. 𝟔𝐕
35

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