Semiconductor Devices and Circuits
Presented by:
Md. Shafiul Islam
Lecturer, Dept. of EEE,
KUET
Semiconductor Devices and Circuits
Reference
Principle of electronics
by V K Mehta
Outlines
Semiconductor and it’s property
Band diagram of semiconductor
Semiconductor doping
Semiconductor devices
Semiconductor circuits and applications
Semiconductor
Classification of materials
On the basis of electrical conductivity, materials can be classified into
three types.
Conductor
Semiconductor (not good conductor but in between conductor and
insulator)
Insulator
“Semiconductor can carry current under some condition”
Semiconductor
“A semiconductor is a substance whose resistivity lies in between
conductor and insulator”(according to electrical conductivity)
Properties of semiconductor
The resistivity of a semiconductor is less than an insulator but more
than a conductor.
Semiconductors have negative temperature co-efficient of resistance
i.e. the resistance of a semiconductor decreases with the increase in
temperature and vice-versa. For example, germanium is actually an
insulator at low temperatures but it becomes a good conductor at high
temperatures.
When a suitable metallic impurity (e.g. arsenic, gallium etc.) is added
to a semiconductor, its current conducting properties change
appreciably.
Commonly used semiconductors
Germanium (32)
Commonly used semiconductors
Silicon (14)
Properties of semiconductor
A semiconductor is a substance which has almost filled valence band
and nearly empty conduction band with a very small energy gap (1.1 eV)
separating the two.
Energy band diagram
Classification of semiconductor
Intrinsic semiconductor
Extrinsic semiconductor(this can be classified into two types)
n-type semiconductor
p- type semiconductor
Intrinsic semiconductor: An extremely pure form of semiconductor is
called intrinsic semiconductor.
Intrinsic semiconductor
Intrinsic semiconductor
An extremely pure form of semiconductor is called intrinsic
semiconductor.
Why we need extrinsic semiconductor?
Because the conduction capability of intrinsic semiconductor is very
small.
But for useful purposes in semiconductor device we need to increase
the amount of current.
That’s means we need to increase the conductivity of the material
Increase in conductivity means increase in carrier concentration.
Carrier concentration can be altered in a semiconductor by adding
impurity
“The process of impurity addition to semiconductor is called
doping”
Types of doping
Two types
I. n- type doping (n- negative type)
II. p- type doping (p- positive type)
“Extrinsic semiconductor is obtained from the process of impurity
adding to the intrinsic semiconductor to change the carrier concentration
to change the electronic property of the semiconductor material, this
process is named as doping”
n- type doping
From n-type doping we obtain n-type semiconductor
“When a small amount of pentavalent impurity is added to a pure
semiconductor, it is known as n-type semiconductor”
Impurities which produce
n-type semiconductor are
known as donor impurities
because they donate or
provide free electrons to the
semiconductor crystal.
n- type doping
The current conduction in an n-type semiconductor is predominantly by
free electrons i.e. negative charges and is called n-type or electron type
conductivity
“So, majority carrier in n- type semiconductor is electron and minority
carrier is hole”
p- type doping
From p-type doping we obtain p-type semiconductor
“When a small amount of trivalent impurity (In, Ga) is added to a
pure semiconductor, it is called p-type semiconductor.”
Such impurities
which produce p-type
semiconductor are known as
acceptor impuritiesbecause
the holes created can
accept the electrons.
p- type doping
The current conduction in an p-type semiconductor is predominantly by
free electrons i.e. negative charges and is called n-type or electron type
conductivity.
“Majority carrier in p- type semiconductor is hole and minority
carrier is electron”
Majority and minority carrier
Semiconductor devices
(crystal diode, transistors)
semiconductor device is nothing but a simple p-n
junction or many p-n junctions
p-n junction
When a pure semiconductor’s one side is doped with n-type doping and
another side in doped with p-type doping a junction is formed between
the n-type and p-type semiconductor is called p-n junction
“p-n junction is important since the control element for semiconductor
devices.”
n-type semiconductor – majority carrier(electron), minority carrier
(Hole)
p- type semiconductor- majority carrier(hole), minority carrier
(electron)
The density of electrons and holes are high in n side and p side
respectively
p-n junction
The density of electrons and holes are high in n side and p side
respectively.
So electrons are diffused from n side and move towards p side and
holes are diffused from p side and move towards n side.
These electrons and holes recombine near the junction
Since the density of electrons is decreased at n side, so positive
charges appears near n side at the junction.
And similarly density of holes is decreased at p side, so negative
charges appears near the p side at the junction.
This region of positive and negative charges at the junction is called
depletion region. (depleted– emptied, no free charge carrier in this
region)
p-n junction
p-n junction
At equilibrium the depletion region acts as a barrier for further
diffusion of electron and holes, stops the movement of electrons and
holes.
The positive and negative charges at this region set up electric filed.
Direction of electric field is from positive charge to negative charge
The potential difference in the depletion region is called, Built in
potential or barrier potential.
The value of built in potential, 𝑣𝑜 in a Si p-n junction is 0.7 v
And the value of built in potential, 𝑣𝑜 in a Ge p-n junction is 0.3 v
p-n junction
In electronics, the term bias refers to the use of d.c voltage to establish
certain operating conditions.
Two types of biasing
Forward bias
Reverse bias
Forward Biasing
When external d.c. voltage applied to the junction is in such a direction
that it cancels the potential barrier, thus permitting current flow, it is
called forward biasing.
Reverse Biasing
When the external d.c. voltage applied to the junction is in such a
direction that potential barrier is increased, it is called reverse biasing.
“In forward biasing the junction resistance decreases and in reverse
biasing resistance increases”
Breakdown voltage: It is the minimum reverse voltage at which pn
junction breaks down with sudden rise in reverse current.
Peak inverse voltage: It is the maximum reverse voltage that can be
applied to the p-n junction without damage to the junction.
PIV<Breakdown voltage
Crystal diode
A pn junction is known as a semi-conductor or *crystal diode.
Anode (+)
cathode (-)
V-I characteristics of a p-n junction diode (crystal diode)
V-I characteristics of a p-n junction diode (crystal diode)
Crystal diode Rectifier
Rectification means converting AC signal into DC signal
Crystal diode converts AC signal into pulsating DC signal
Pulsating DC– Filtering– Constant DC
Tow types of rectifier circuits
a) Half wave rectifier
b) Full wave rectifier
Half wave Rectifier(Clipper circuit)
In half-wave rectification, the rectifier conducts current only during the
positive half-cycles of input ac supply.
The negative half-cycles of ac supply are suppressed i.e. during negative
half-cycles, no current is conducted and hence no voltage appears across
the load. Therefore, current always flows in one direction (i.e. dc)
through the load though after every half-cycle.
“That’s why diode is a unidirectional device”
Half wave Rectifier(Clipper circuit)
Disadvantages of Half wave Rectifier
The main disadvantages of a half-wave rectifier are :
(i) The pulsating current in the load contains alternating component
whose basic frequency is equal to the supply frequency (𝑓𝑜𝑢𝑡 = 𝑓𝑖𝑛 ).
Therefore, an elaborate filtering is required to produce steady direct
current.
(ii) The a.c. supply delivers power only half the time. Therefore, the
output is low.
Efficiency of Half wave Rectifier
The ratio of d.c. power output to the applied input a.c. power is known as rectifier
efficiency.
𝐷𝐶 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟
𝜂=
𝑖𝑛𝑝𝑢𝑡 𝑎𝑐 𝑝𝑜𝑤𝑒𝑟
Diode conducts and supplies power to the load only during positive half cycle of the
input ac supply.
Output dc power is pulsating dc. So for output dc power we need to find the average
load current.
Efficiency of Half wave Rectifier
Let voltage at the secondary side of the transformer, v = 𝑣𝑚 sin 𝜃
Resistances of the diode and the load are𝑟𝑓 𝑎𝑛𝑑 𝑅𝐿
So the average current,
1 𝜋𝑣
𝐼𝑎𝑣 =
𝑑𝜃
2𝜋 0 𝑅
1 𝜋 𝑣𝑚 sin 𝜃
=
𝑑𝜃
2𝜋 0 𝑅𝐿 + 𝑟𝑓
𝜋
𝑣𝑚
=
sin 𝜃 𝑑𝜃
2𝜋(𝑅𝐿 + 𝑟𝑓 ) 0
𝑣𝑚
=
∗2
2𝜋(𝑅𝐿 + 𝑟𝑓 )
𝑣𝑚
𝐼𝑚
=
=
𝜋(𝑅𝐿 + 𝑟𝑓 )
𝜋
(ℎ𝑒𝑟𝑒, 𝐼𝑚 =
𝑣𝑚
𝑅𝐿 + 𝑟𝑓
)
Efficiency of Half wave Rectifier
2
So output dc power, 𝑃𝑑𝑐 = 𝐼𝑎𝑣 ∗ 𝑅𝐿 =
2
𝐼𝑚
𝜋
𝐼𝑚
2
2
Input ac power, 𝑃𝑎𝑐 = 𝐼𝑟𝑚𝑠 ∗ (𝑅𝐿 +𝑟𝑓 ) =
ℎ𝑒𝑟𝑒, 𝐼𝑟𝑚𝑠 𝑓𝑜𝑟 ℎ𝑎𝑙𝑓 𝑐𝑦𝑐𝑙𝑒 =
𝑃𝑑𝑐
𝜂=
=
𝑃𝑎𝑐
𝐼𝑚
2
𝐼𝑚
𝜋
1 1
×
2 2𝜋
2𝜋
0
∗ 𝑅𝐿
2
∗ (𝑅𝐿 +𝑟𝑓 )
𝑣𝑚 𝑠𝑖𝑛𝜃
𝑅𝐿 + 𝑟𝑓
2
𝑑𝜃
=
𝐼𝑚
2
2
∗ 𝑅𝐿
0.406𝑅𝐿
.406
=
=
= .406 × 100% = 40.6%
2
𝑟
𝑓
(𝑅𝐿 +𝑟𝑓 ) 1 +
∗ (𝑅𝐿 +𝑟𝑓 )
𝑅𝐿
𝑟𝑓
𝑟𝑓 ≪ 𝑅𝐿 , 𝑠𝑜, 1 +
≈1
𝑅𝐿
Max. rectifier efficiency = 40.6%
This shows that in half-wave rectification, a maximum of 40.6% of a.c. power is
converted into d.c. power.
Full wave Rectifier
The following two circuits are commonly used for full-wave
rectification:
(i) Centre-tap full-wave rectifier
(ii) Full-wave bridge rectifier
Full wave bridge Rectifier
Working:
During positive half cycle diode
D1 and D3 are forward biased.
D2 and D4 are reverse biased
So current flows through
D1, 𝑅𝐿 , D3 and return back to the secondary
winding of the transformer output voltage
is obtained across the load, 𝑅𝐿
Full wave bridge Rectifier
Working:
During negative half cycle diode
D2 and D4 are forward biased.
D1 and D3 are reverse biased
So current flows through
D2, 𝑅𝐿 , D4 and return back to the secondary
winding of the transformer and output voltage
is obtained across the load, 𝑅𝐿 .
So we obtain power at load for both cycle of
the input ac
Full wave bridge Rectifier
Working:
During negative half cycle diode
D2 and D4 are forward biased.
D1 and D3 are reverse biased
So current flows through
D2, 𝑅𝐿 , D4 and return back to the secondary
winding of the transformer and output voltage
is obtained across the load, 𝑅𝐿 .
So we obtain power at load for both cycle of
Input ac supply
Load Current path for positive and negative cycle of the
input ac
Here, 𝑓𝑜𝑢𝑡 = 2𝑓𝑖𝑛
Peak Inverse Voltage
The peak inverse voltage is the maximum voltage that can be applied
across the diode during reverse biased condition. It must be less than the
breakdown voltage of the diode.
Here, the PIV of the diodes is 𝑉𝑚 volt.
disadvantages Full wave bridge Rectifier
Disadvantages:
i) It requires four diodes
ii) As during each half-cycle of a.c. input two diodes that conduct are
in series, therefore, voltage drop in the internal resistance of the
rectifying unit will be twice as great as in the center tap circuit. This
is objectionable when secondary voltage is small.
Efficiency of Full wave Rectifier
𝐷𝐶 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟
𝜂=
𝑖𝑛𝑝𝑢𝑡 𝑎𝑐 𝑝𝑜𝑤𝑒𝑟
Diodes conduct and supplies power to the loads for full cycle of the input ac supply.
Output dc power is pulsating dc. So for output dc power we need to find the average
load current.
Efficiency of Full wave Rectifier
Let voltage at the secondary side of the transformer, v = 𝑣𝑚 sin 𝜃
Resistances of the diode and the load are𝑟𝑓 𝑎𝑛𝑑 𝑅𝐿
So the average current,
1 𝜋𝑣
𝐼𝑎𝑣 =
𝑑𝜃
𝜋 0 𝑅
𝜋
1
𝑣𝑚 sin 𝜃
=
𝑑𝜃
𝜋 0 𝑅𝐿 + 𝑟𝑓
𝜋
𝑣𝑚
=
sin 𝜃 𝑑𝜃
𝜋(𝑅𝐿 + 𝑟𝑓 ) 0
𝑣𝑚
=
∗2
𝜋(𝑅𝐿 + 𝑟𝑓 )
2𝑣𝑚
2𝐼𝑚
=
=
𝜋(𝑅𝐿 + 𝑟𝑓 )
𝜋
(ℎ𝑒𝑟𝑒, 𝐼𝑚 =
𝑣𝑚
𝑅𝐿 + 𝑟𝑓
)
Efficiency of full wave Rectifier
2
So output dc power, 𝑃𝑑𝑐 = 𝐼𝑎𝑣 ∗ 𝑅𝐿 =
Input ac power, 𝑃𝑎𝑐 = 𝐼𝑟𝑚𝑠 2 ∗ (𝑅𝐿 +𝑟𝑓 )
ℎ𝑒𝑟𝑒, 𝐼𝑟𝑚𝑠 𝑓𝑜𝑟 𝑓𝑢𝑙𝑙 𝑐𝑦𝑐𝑙𝑒 =
2𝐼𝑚
𝜋
𝑃𝑑𝑐
𝜂=
=
𝑃𝑎𝑐
𝐼𝑚
2
1
2𝜋
2𝐼𝑚 2
∗ 𝑅𝐿
𝜋
𝐼𝑚 2
=
∗ (𝑅𝐿 +𝑟𝑓 )
2
2𝜋
0
𝑣𝑚 𝑠𝑖𝑛𝜃
𝑅𝐿 + 𝑟𝑓
2
𝑑𝜃
=
𝐼𝑚
2
2
∗ 𝑅𝐿
0.812𝑅𝐿
.812
=
=
= .812 × 100% = 81.2%
2
𝑟
𝑓
(𝑅𝐿 +𝑟𝑓 ) 1 +
∗ (𝑅𝐿 +𝑟𝑓 )
𝑅𝐿
𝑟𝑓
𝑟𝑓 ≪ 𝑅𝐿 , 𝑠𝑜, 1 +
≈1
𝑅𝐿
Max. rectifier efficiency =81.2%
This is double the efficiency due to half-wave rectifier. Therefore, a full-wave
rectifier is twice as effective as a half-wave rectifier.
Transistor
Transfer+ resistor
“It transfers a current from a low resistance to
high resistance”
Transistor
A transistor consists of two p-n junctions formed by sandwiching either p-type or ntype semiconductor between a pair of opposite types. Accordingly ;
there are two types of transistors, namely;
I. n-p-n transistor
II. p-n-p transistor
A transistor has two p-n junctions. And three terminals, named Emitter, base and
collector. And the middle section is a very thin layer.
Transistor
A transistor has two p-n junctions. Three terminals. Among them BE is always
forward biased and BC junction is reversed biased. So, One junction is forward
biased and the other is reverse biased. The forward biased junction has a low
resistance path whereas a reverse biased junction has a high resistance path. The
weak signal is introduced in the low resistance circuit and output is taken from the
high resistance circuit. Therefore, a transistor transfers a signal from a low
resistance to high resistance.
Transistor
Emitter: The section on one side that supplies charge carriers (electrons or holes) is
called the emitter. The emitter is always forward biased w.r.t. base so that it can supply a
large number of majority carriers.
Collector: The section on the other side that collects the charges is called the collector. The
collector is always reverse biased.
Base: The middle section which forms two p-n junctions between the emitter and collector
is called the base. The BE junction is forward biased, allowing low resistance for the emitter
circuit. The BC junction is reverse biased and provides high resistance in the collector
circuit.
Transistor
Base is thinner than emitter
Collector is wider than both emitter and base
Emitter is highly doped, base is lightly doped and collector is moderately doped
Base is thinner and lightly dope to reduce the recombination of the carrier that is
injected from emitter to base junction.
Emitter is highly doped so that it can inject a large amount of majority carrier
Collector is wider than both emitter and base. During transistor operation, much
heat is produced at the collector junction. The collector is made larger to dissipate
the heat
Transistor
•
•
•
•
•
•
To operate transistor, it has two ports.
Input and output ports.
Each port has two terminals
So for two port we need 4 terminals
But in transistor, we have only three terminals
So one terminal is considered in both port (one terminal is common in both input
and output)
So there can be three configuration.
Common Base configuration(input port BE, output port BC)
Common emitter configuration (input port BE, output port CE)
Common collector configuration (input port CE, output port CB)
Working principle of n-p-n Transistor(CB)
𝐼𝐸 (100%) = 𝐼𝐶 (95%) + 𝐼𝐵 (5%)
Symbols of n-p-n and p-n-p transistors
Why it is called bipolar junction transistors?
The current in this transistors, flows due to the flow of both electrons and holes.
That’s why it is called bipolar junction transistors.
Transistor is called current controlled device.
𝐼𝐸 (100%) = 𝐼𝐶 (95%) + 𝐼𝐵 (5%)
Output current is controlled by controlling the input current, that’s why it is called
current controlled device.
Transistor Circuit as an Amplifier
A transistor raises the strength of a weak signal and thus acts as an amplifier.
The weak signal is applied between emitter-base junction and output is taken
across the load 𝑅𝐶 connected in the collector circuit.
In order to achieve faithful amplification, the input circuit should always remain
forward biased.
As the input circuit has low resistance, therefore, a small change in signal voltage
causes an appreciable change in emitter current. This causes almost the same
change in collector current due to transistor action. The collector current flowing
through a high load resistance RC produces a large voltage across it. Thus, a weak
signal applied in the input circuit appears in the amplified form in the collector
circuit. It is in this way that a transistor acts as an amplifier.
Transistor Circuit as an Amplifier
The reason is as follows.
The collector-base junction is reversed biased and has a very high
resistance of the order of mega ohms. Thus collector-base voltage has
little effect on the collector current. This means that a large resistance
𝑅𝐶 can be inserted in series with collector without disturbing the
collector current relation to the emitter current viz.𝐼𝐶 = 𝛼𝐼𝐸 + 𝐼𝐶𝐵𝑂
Therefore, collector current variations caused by a small base-emitter
voltage fluctuations result in voltage changes in RC that are quite
high—often hundreds of times larger than the emitter-base voltage.
Transistor circuit as an amplifier
Emitter Current amplification factor(α) in common base
configuration
Common base means base is common to both input and output circuit
So input current is 𝐼𝐸 and output current, 𝐼𝐶 .
Emitter Current amplification factor(α) in common base
configuration
Common base means base is common to both input and output circuit
So input current is 𝐼𝐸 and output current, 𝐼𝐶 .
The ratio of change in collector current to the change in emitter current at
constant collector-base voltage 𝑽𝑪𝑩 is known as current amplification factor.
Then amplification factor 𝛼 =
∆𝐼𝐶
∆𝐼𝐸
(𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶 )
So 𝛼 is less than unity. Usually 0.9 to 0.99
Total collector current consists of :
i)Part of emitter current which reaches the collector terminal i.e. ***α𝐼𝐸
ii) The leakage current 𝐼𝑙𝑒𝑘𝑎𝑔𝑒 This current is due to the movement of minority
carriers across base-collector junction on account of it being reverse biased. This is
generally much smaller than α𝐼𝐸
Emitter Current amplification factor(α) in common base
configuration
So total collector current, 𝐼𝐶 = 𝛼𝐼𝐸 + 𝐼𝑙𝑒𝑎𝑘𝑎𝑔𝑒
if 𝐼𝐸 = 0 (i.e., emitter circuit is open), a small leakage current still flows in the
collector circuit. This 𝐼𝑙𝑒𝑎𝑘𝑎𝑔𝑒 is abbreviated as 𝐼𝐶𝐵𝑂
, meaning collector-base current with emitter open.
𝐼𝐶 = 𝛼𝐼𝐸 + 𝐼𝐶𝐵𝑂
𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
𝐼𝐶 = 𝛼(𝐼𝐵 +𝐼𝐶 ) + 𝐼𝐶𝐵𝑂
𝐼𝐶 1 − 𝛼 = 𝛼𝐼𝐵 + 𝐼𝐶𝐵𝑂
𝛼
𝐼𝐶𝐵𝑂
𝐼𝐶 =
𝐼𝐵 +
1−𝛼
1−𝛼
Input and output characteristics of common base configuration
Base Current amplification factor(𝜷) in common emitter configuration
Common emitter means emitter is common to both input and output circuit
So input current is 𝐼𝐵 and output current, 𝐼𝐶 .
Base Current amplification factor(𝛽) in common emitter
configuration
The ratio of change in collector current (∆𝐼𝐶 ) to the change in base current (∆𝐼𝐵 ) is known
as emitter current amplification factor.
∆𝐼𝐶
𝛽=
∆𝐼𝐵
Again, α =
∆𝐼𝐶
∆𝐼𝐸
∆𝐼𝐶 ∆𝐼𝐸
𝛽=
×
∆𝐼𝐸 ∆𝐼𝐵
=𝛼×
=𝛼
∆𝐼𝐶
∆𝐼𝐵
∆𝐼𝐶 +∆𝐼𝐵
∆𝐼𝐵
+ 1 = 𝛼(𝛽 + 1)
𝛽 = 𝛼𝛽 + 𝛼
𝛽 1−𝛼 =𝛼
𝛼
𝛽=
1−𝛼
Base Current amplification factor(𝛽) in common emitter
configuration
𝛼
𝛽=
1−𝛼
If 𝛼 tends to 1 then 𝛽 tends to infinity.
So current amplification factor is very high in common emitter configuration.
That’s why it is used in about 90 to 95 percent of all transistor applications.
Base Current amplification factor(𝛽) in common emitter
configuration
In common emitter circuit, 𝐼𝐵 is the input current and 𝐼𝐶 is the output current.
We know that, 𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
And 𝐼𝐶 = 𝛼𝐼𝐸 + 𝐼𝐶𝐵𝑂
𝐼𝐶 = 𝛼(𝐼𝐵 + 𝐼𝐶 ) + 𝐼𝐶𝐵𝑂
𝐼𝐶 = 𝛼𝐼𝐵 + 𝛼𝐼𝐶 + 𝐼𝐶𝐵𝑂
(1 − 𝛼)𝐼𝐶 = 𝛼𝐼𝐵 + 𝐼𝐶𝐵𝑂
𝛼
𝐼𝐶𝐵𝑂
𝐼𝐶 =
𝐼𝐵 +
1−𝛼
1−𝛼
𝐼𝐶𝐵𝑂
𝐼𝐶 = 𝛽𝐼𝐵 +
1−𝛼
𝐼𝐶 = 𝛽𝐼𝐵 + 𝐼𝐶𝐸𝑂
Where, 𝐼𝐶𝐸𝑂 is the collector emitter current when base is open
Base Current amplification factor(𝛽) in common emitter
configuration
base Current amplification factor(𝜸) in common collector
configuration
The ratio of change in emitter current (∆𝐼𝐸 ) to the change in base current (∆𝐼𝐵 ) is
known as emitter current amplification factor.
∆𝐼𝐸
𝛾=
∆𝐼𝐵
Again, α =
∆𝐼𝐶
∆𝐼𝐸
𝑎𝑛𝑑 𝛽 =
∆𝐼𝐶
∆𝐼𝐵
∆𝐼𝐸 ∆𝐼𝐶
𝛾=
×
∆𝐼𝐶 ∆𝐼𝐵
1
= ×𝛽
𝛼
𝛼
1
=
×
1−𝛼
𝛼
1
𝛾=
1−𝛼
Comparison of transistor configuration
Transistor switching action
Switch has two state.
On state and off state.
To operate transistor as a switch transistor is needed to operate in cut-off region
(switch off) and saturation region (switch on).
Transistor switching action
In cut-off region, both BE and BC junctions must be reversed biased.
In saturation region, both of junctions must be forward biased.
Transistor switching action
In cut-off region, both BE and BC junctions must be reversed biased.
In saturation region, both of junctions must be forward biased.
For switch off, 𝑉𝐵𝐸 ≤ 0 𝑎𝑛𝑑 𝑉𝐵𝐶 ≤ 0
And for on, 𝑉𝐵𝐸 > 0 𝑎𝑛𝑑 𝑉𝐵𝐶 > 0
During cut off 𝐼𝐵 = 0 and output current 𝐼𝐶 = 𝛽𝐼𝐵 so output current is also 0.
That means no current flows through collector emitter terminal. So CE junction is
opened and 𝑉𝐶𝐸 = 𝑉𝐶𝐶
During saturation both BE and BC junctions are forward biased. So CE junction is
also forward biased and ideally output voltage, 𝑉𝐶𝐸 = 0𝑉and output current
becomes maximum and 𝐼𝐶 = 𝐼𝐶𝑠𝑎𝑡 .
So in cut-off and saturation region transistor operate as switch off and switch on
,respectively.
Field Effect transistors (FET)
Why FET?
Because,
BJT has low input impedance and considerable noise level.
But FET has very much higher input impedance and noise level is smaller than BJT.
BJT is Bipolar junction transistor, here current is due to flow of both electrons and
holes.
But in FET current flows only either flow of electron or hole. That’s why it is
unipolar transistor .
In BJT output current is controlled by input current, that’s why it is called current
controlled device.
In FET output current is controlled by input voltage (electric field), that’s why it is
called voltage controlled device.
Types of Field Effect transistors (FET)
Two basic types of FET.
i) Junction field effect transistors (JFET)
ii) Metal oxide semiconductor field effect transistors (MOSFET)
Junction Field Effect transistors (JFET)
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.
Two types, a) n- channel JFET b) p-channel JFET.
Junction Field Effect transistors (JFET)
Gate must be reversed with respect to source.
(i) The input circuit (i.e. gate to source) of a JFET is reverse biased. This means
that the device has high input impedance.
(ii) The drain is so biased w.r.t. source that drain current flows from the drain to
source.
(iii) In all JFETs, source current 𝐼𝑠 is equal to the drain current 𝐼𝑠 i.e. 𝐼𝑠 = 𝐼𝐷
Operation of n-channel Junction Field Effect transistors (JFET)
𝑉𝑔𝑠 = 0V, And 𝑉𝑑𝑠 > 0𝑉
Pinch-off condition
𝑉𝑔𝑠 < 0𝑉 and 𝑉𝑑𝑠 > 0𝑉
Junction Field Effect transistors (JFET)
The two p-n junctions at the sides form two depletion layers.
The current conduction by charge carriers 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
The greater the reverse voltage𝑉𝐺𝑆 , 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 voltage𝑉𝐺𝑆 .In other words, the magnitude of drain current can be changed
by altering𝑉𝐺𝑆
Symbols of n-channel and p-channel JFET
Difference between JFET and BJT
See article 19.6 Vk Mehta
Salient features of JFET, 19.9
Shorted gate drain current (𝑰𝑫𝒔𝒔 ) and Pinch-off voltage(𝑽𝒑 )
It is the drain current with source short-circuited to gate (i.e. 𝑉𝐺𝑆 = 0) and
drain voltage (𝑉𝐷𝑆 =𝑉𝑃 ) equal to pinch off voltage. It is sometimes called
zero-bias current.
Pinch off voltage: It is the minimum drain-source voltage at which the
drain current essentially becomes constant.
Metal oxide semiconductor Field Effect transistors (MOSFET)
In JFET gate is always reversed biased w.r.t source. So we can only decrease the
channel width (this is depletion mode). For n channel with negative gate voltage and
for p channel with positive gate voltage.
But there is another type of FET where we can enhance the width of the channel
This is called MOSFET.
A field effect transistor (FET) that can be operated in the enhancement-mode is
called a MOSFET
Two types
i) Depletion type MOSFET (D-MOS) (can be operated in both depletion and
enhancement mode)
ii) Enhancement type MOSFET (E-MOS) (only in enhancement mode)
n-channel depletion type MOSFET
Operation of n-channel D-MOSFET (Depletion mode)
i)𝑉𝑔𝑠 = 0𝑉, 𝑉𝑑𝑠 > 0𝑉
ii) 𝑉𝑔𝑠 < 0𝑉, 𝑉𝑑𝑠 > 0𝑉
Operation of n-channel D-MOSFET (Enhancement mode)
Thank you
for your kind attention