DEPARTMENT OF COMPUTER

ENGINEERING

COEN 309 COMPUTER

ELECTRONICS 1
(2-UNITS)
Semester 1
2018/2019
1

H. Bello salau (PhD)

bellosalau@abu.edu.ng
 FIELD EFFECT TRANSISTORS
 Thus far we have focused on BJT where both holes and electrons play part in the
conduction process.

 The ordinary or bipolar transistor has two principal disadvantages. First, it has a low
input impedance because of forward biased emitter junction. Secondly, it has
considerable noise level. The field effect transistor (FET) has, by virtue of its
construction and biasing, large input impedance which may be more than 100
megaohms. The FET is generally much less noisy than the ordinary or bipolar
transistor.

Types of Field Effect Transistors

 A bipolar junction transistor (BJT) is a current controlled device i.e., output
characteristics of the device are controlled by base current and not by base voltage.
However, in a field effect transistor (FET), the output characteristics are controlled by
input voltage (i.e., electric field) and not by input current. There are two basic types of
field effect transistors: (i) Junction field effect transistor (JFET) (ii) Metal 2oxide
semiconductor field effect transistor (MOSFET).
 JUNCTION FIELD EFFECT TRANSISTOR
 A junction field effect transistor is a three terminal semiconductor device in
which current conduction is by one type of carrier i.e., electrons or holes, which is
controlled by means of an electric field between the gate electrode and conducting
channel of the device

 Note: The JFET has high input impedance and low noise level.

 Constructional details: A JFET consists of a p-type or n-type silicon bar containing

two pn junctions at the sides as shown in the Fig.

 The bar forms the conducting channel for the charge carriers. If the bar is of n-
type, it is called n-channel JFET as shown in Fig. (i) in next slide and if the bar is
of p-type, it is called a p-channel JFET as shown in Fig. (ii) in next slide. The two
pn junctions forming diodes are connected internally and a common terminal
called gate is taken out.

 Other terminals are source and drain taken out from the bar as shown. Thus
3 a
JFET has essentially three terminals viz., gate (G), source (S) and drain (D).
 JUNCTION FIELD EFFECT TRANSISTOR

 JFET polarities: Fig. (i) shows n-channel JFET polarities whereas Fig. (ii) shows
the p-channel JFET polarities. Note that in each case, the voltage between the gate
and source is such that the gate is reverse biased. This is the normal way of JFET
connection. The drain and source terminals are interchangeable.

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 JUNCTION FIELD EFFECT TRANSISTOR
 Note:
 The input circuit (i.e. gate to source) of a JFET is reverse biased. This means that the device has
high input impedance.

 The drain is so biased w.r.t. source that drain current ID flows from the source to drain.

 In all JFETs, source current IS is equal to the drain current i.e. IS = ID.

Working Principle of JFET

 Principle: The two pn junctions at the sides form two depletion layers. The
current conduction by charge carriers (i.e. free electrons in this case) is through the
channel between the two depletion layers and out of the drain. The width and
hence resistance of this channel can be controlled by changing the input voltage
VGS.

 Note: The resistance of the channel depends upon its area of X-section. The
greater the X-sectional area of this channel, the lower will be its resistance and
5
the
greater will be the current flow through it.
 JUNCTION FIELD EFFECT TRANSISTOR
Working Principle of JFET
 The greater the reverse voltage VGS, the wider will be the depletion layers and
narrower will be the conducting channel. The narrower channel means greater
resistance and hence source to drain current decreases. Reverse will happen
should VGS decrease.

 Thus JFET operates on the principle that width and hence resistance of the
conducting channel can be varied by changing the reverse voltage VGS. In other
words, the magnitude of drain current (ID) can be changed by altering VGS.

The working of JFET is as under :

i. When a voltage VDS is applied between drain and source terminals and voltage
on the gate is zero [Fig. (i) next slide], the two pn junctions at the sides of the bar
establish depletion layers. The electrons will flow from source to drain through a
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channel between the depletion layers. The size of these layers determines the
width of the channel and hence the current conduction through the bar.
 JUNCTION FIELD EFFECT TRANSISTOR
Working Principle of JFET
The working of JFET is as under :

ii. When a reverse voltage VGS is applied between the gate and source [See Fig. (ii)],
the width of the depletion layers is increased. This reduces the width of conducting
channel, thereby increasing the resistance of n-type bar. Consequently, the current
from source to drain is decreased. On the other hand, if the reverse voltage on the
gate is decreased, the width of the depletion layers also decreases. This increases the
width of the conducting channel and hence source to drain current.

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 JUNCTION FIELD EFFECT TRANSISTOR
Working Principle of JFET
 It is clear from the above discussion that current from source to drain can be
controlled by the application of potential (i.e. electric field) on the gate. For this
reason, the device is called field effect transistor.

 It may be noted that a p-channel JFET operates in the same manner as an n -

channel JFET except that channel current carriers will be the holes instead of
electrons and the polarities of VGS and VDS are reversed.

Note: If the reverse voltage VGS on the gate is continuously increased, a state is
reached when the two depletion layers touch each other and the channel is cut off.
Under such conditions, the channel becomes a non-conductor.

Schematic Symbol of JFET

The schematic symbol of JFET is shown in next slide. The vertical line in the symbol
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may be thought as channel and source (S) and drain (D) connected to this line. If the
channel is n-type, the arrow on the gate points towards the channel as shown in
 JUNCTION FIELD EFFECT TRANSISTOR
Schematic Symbol of JFET
The schematic symbol of JFET is shown in next slide. The vertical line in the symbol
may be thought as channel and source (S) and drain (D) connected to this line. If the
channel is n-type, the arrow on the gate points towards the channel as shown in Fig.
(i). However, for p-type channel, the arrow on the gate points from channel to gate
See Fig. ii.

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 IMPORTANCE OF JFET
 A JFET acts like a voltage controlled device i.e. input voltage (VGS) controls the
output current. This is different from ordinary transistor (or bipolar transistor)
where input current controls the output current. Thus JFET is a semiconductor
device acting like a vacuum tube (The gate, source and drain of a JFET correspond
to grid, cathode and anode of a vacuum tube).

 The need for JFET arose because as modern electronic equipment became
increasingly transistorised, it became apparent that there were many functions in
which bipolar transistors were unable to replace vacuum tubes. Owing to their
extremely high input impedance.

 JFET devices are more like vacuum tubes than are the bipolar transistors and
hence are able to take over many vacuum-tube functions. Thus, because of JFET,
electronic equipment is closer today to being completely solid state. The JFET
devices have not only taken over the functions of vacuum tubes but they now also
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threaten to depose the bipolar transistors as the most widely used semiconductor
devices.
 IMPORTANCE OF JFET
 As an amplifier, the JFET has higher input impedance than that of a conventional
transistor, generates less noise and has greater resistance to nuclear radiations.

Difference Between JFET and Bipolar Transistor

 There is only one type of carrier in JFET, holes in p-type channel and electrons in
n-type channel. For this reason, it is also called a unipolar transistor. However, in
an ordinary transistor, both holes and electrons play part in conduction.
Therefore, an ordinary transistor is sometimes called a bipolar transistor.

 As the input circuit (i.e., gate to source) of a JFET is reverse biased, therefore, the
device has high input impedance. However, the input circuit of an ordinary
transistor is forward biased and hence has low input impedance.

 The primary functional difference between the JFET and the BJT is that no current
(actually, a very, very small current) enters the gate of JFET (i.e. IG = 0A).
However, typical BJT base current might be a few μA while JFET gate current
11 a
thousand times smaller [See Fig. in next slide].
 DIFFERENCE BETWEEN JFET AND BIPOLAR TRANSISTOR

 A bipolar transistor uses a current into its base to control a large current between
collector and emitter whereas a JFET uses voltage on the ‘gate’ ( = base) terminal
to control the current between drain (= collector) and source ( = emitter). Thus a
bipolar transistor gain is characterised by current gain whereas the JFET gain is
characterised as a transconductance i.e., the ratio of change in output current
(drain current) to the input (gate) voltage.

 In JFET, there are no junctions as in an ordinary transistor. The conduction is

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through an n- type or p-type semi-conductor material. For this reason, noise level
in JFET is very small.
 JFET AS AN AMPLIFIER
 The weak signal is applied between gate and source and amplified output is
obtained in the drain-source circuit (see the Fig.). For the proper operation of
JFET, the gate must be negative w.r.t. source i.e., input circuit should always be
reverse biased. This is achieved either by inserting a battery VGG in the gate
circuit or by a circuit known as biasing circuit. In the present case, we are
providing biasing by the battery VGG. A small change in the reverse bias on the
gate produces a large change in drain current. This fact makes JFET capable of
raising the strength of a weak signal.

 During the positive half of signal, the reverse bias on the gate decreases. This
increases the channel width and hence the drain current. During the negative
half-cycle of the signal, the reverse voltage on the gate increases. Consequently,

 the drain current decreases. The result is that a small change in voltage at the gate
produces a large change in drain current. These large variations in drain current
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produce large output across the load RL. In this way, JFET acts as an amplifier
 JFET AS AN AMPLIFIER

Definition of Some Important Terms

 Shorted-gate drain current (IDSS): It is the drain current with source short-circuited
to gate (i.e. VGS = 0) and drain voltage (VDS) equal to pinch off voltage. It is
sometimes called zero-bias current.

 Pinch off Voltage (VP): It is the minimum drain-source voltage at which the drain
current essentially becomes constant.

 Gate-source cut off voltage VGS (off): It is the gate-source voltage where the
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channel is completely cut off and the drain current becomes zero.
 EXPRESSION FOR DRAIN CURRENT (ID)
 The mathematical expression for drain current ID is shown below based on the
mathematical analysis of the transfer characteristic of JFET.

 Example: The Fig. shows the transfer characteristic curve of a JFET. Write the
equation for drain current.

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 EXPRESSION FOR DRAIN CURRENT (ID)
 Example: A JFET has the following parameters: IDSS = 32 mA ; VGS (off) = – 8V ;
VGS = – 4.5 V. Find the value of drain current

 A JFET has a drain current of 5 mA. If IDSS = 10 mA and VGS (off) = – 6 V, find
the value of (i) VGS and (ii) VP.

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 ADVANTAGES OF JFET
A JFET is a voltage controlled, constant current device (similar to a vacuum pentode)
in which variations in input voltage control the output current. It combines the many
advantages of both bipolar transistor and vacuum pentode. Advantages of JFET are:

 It has a very high input impedance (of the order of 100 MΩ). This permits high
degree of isolation between the input and output circuits.

 The operation of a JFET depends upon the bulk material current carriers that do
not cross junctions. Therefore, the inherent noise of tubes (due to high-
temperature operation) and those of transistors (due to junction transitions) are
not present in a JFET.

 A JFET has a negative temperature co-efficient of resistance. This avoids the risk
of thermal runaway.

 A JFET has a very high power gain. This eliminates the necessity of using driver
stages. 17

 A JFET has a smaller size, longer life and high efficiency.

 METAL OXIDE SEMICONDUCTOR FET (MOSFET)
 The main drawback of JFET is that its gate must be reverse biased for proper
operation of the device i.e. it can only have negative gate operation for n-channel
and positive gate operation for p-channel.

 This means that we can only decrease the width of the channel (i.e. decrease the
conductivity of the channel) from its zero-bias size. This type of operation is
referred to as depletion-mode operation.

 Therefore, a JFET can only be operated in the depletion-mode. However, there is a

field effect transistor (FET) that can be operated to enhance (or increase) the width
of the channel (with consequent increase in conductivity of the channel) i.e. it can
have enhancement-mode operation. Such a FET is called MOSFET.

 A field effect transistor (FET) that can be operated in the enhancement-mode is

called a MOSFET.

 A MOSFET is an important semiconductor device and can be used in any 18

of the
circuits covered for JFET. However, a MOSFET has several advantages over JFET
including high input impedance and low cost of production.
 TYPES OF MOSFET
 There are two basic types of MOSFETs viz:

 Depletion-type MOSFET or D-MOSFET:. The D-MOSFET can be operated in both

the depletion-mode and the enhancement-mode. For this reason, a D-MOSFET is
sometimes called depletion/enhancement MOSFET.

 Enhancement-type MOSFET or E-MOSFET: The E-MOSFET can be operated only

in enhancement-mode.

Next Lecture: Single and Multistage Amplifier

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