Transistors allow current to flow or prohibit current and can be used as switches or amplifiers. They are made of semiconductor material like silicon and operate by using doping to introduce excess electrons or holes. The transistor was invented in 1947 at Bell Labs and consists of three layers (emitter, base, and collector) in a sandwich structure. It can be used as an amplifier by biasing it to always be on, which causes any input signal to be amplified in the output. Different types of transistors exist like JFETs, MOSFETs, and CMOS which use different structures but all act as electronic switches.

1. TOPIC
TRANSISTOR
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2.  Semiconductors: ability to change from conductor to
insulator
 Can either allow current or prohibit current to flow
 Useful as a switch, but also as an amplifier
 Essential part of many technological advances
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3.  Guglie-lmo Marconi invents radio in
1895
 Problem: For long distance travel,
signal must be amplified
 Lee De Forest improves on Fleming’s
original vacuum tube to amplify signals
 Made use of third electrode
 Too bulky for most applications
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4.  Bell Labs (1947): Bardeen,
Brattain, and Shockley
 Originally made of germanium
 Current transistors made of
doped silicon
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5.  Doping: adding small amounts of other elements to create
additional protons or electrons
 P-Type: dopants lack a fourth valence electron (Boron, Aluminum)
 N-Type: dopants have an additional (5th) valence electron
(Phosphorus, Arsenic)
 Importance: Current only flows from P to N
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6.  Diode: simple P-N junction.
 Forward Bias: allows current to flow from P to
N.
 Reverse Bias: no current allowed to flow from N
to P.
 Breakdown Voltage: sufficient N to P voltage of
a Zener Diode will allow for current to flow in
this direction.
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FORWARD
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REVERSE

7. Sandwich in a P-
type layer between
two n-type layers.
Sandwich in a N-type
layer between two p-
type layers.
Developed by
Shockley (1949)
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8. BC E
backward Forward
The base-emitter diode (forward) acts as
a switch. when v1>0.7 it lets the
electrons flow toward collector. so we
can control our output current (Ic) with
the input current (Ib) by using
transistors.
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9. Emitter
Collector
Base
Transistors work in
3 regions
Active: Always on
Ic=BIb
Saturation : Ic=Isaturation
On as a switch
Off :Ic=0
Off as a switch
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10.  Transistor
Transistor Switch
• Transistors can be used as switches.
• Transistors can either
conduct or not conduct current.
• ie , transistors can either be on or off.
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11. X
Variable
Voltage
Supply
• When VBE is less than 0.7V the transistor is off and the lamp does not light.
• When VBE is greater than 0.7V the transistor is on and the lamp lights
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12V

12. TransistorCircuit: Light-Controlledcircuit
• Because of the LDR, this circuit is dependent on light.
• This transistor circuit contains
a Light-Dependent Resistor.
• The purpose of this circuit is to turn on the LED when
the light reaches a certain intensity.
1) LED = Off.
2) Cover LDR.
3) RLDR .
4) VLDR .
5) Transistor switches on.
6) LED = On.
Input = Voltage Divider
Process = Transistor
Output = LED
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13. Transistorasanamplifier:
Transistors are often used as amplifiers to increase input signal in radios,
televisions and some other applications. The circuit may be designed to
increase the current or voltage level . The power gain is the product of
current gain and voltage gain (P=V*I).
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14. as you see, the transistor is biased to be always
on. the input signal is amplified by this circuit.
the frequency of output is the same as its
input, but the polarity of the signal is inverted
The measure of amplification is the
gain of transistor.
Example:
Input Amplitude =0.2v
Output amplitude=10v
Gain=10/0.2=50
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15. JFET MOSFET CMOS
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16. How a JFET transistor works?
When the gate is negative ,it repels the electron in
the N-channel. So there is no way for electrons to
flow from source to drain.
When the negative voltage is removed from Gate ,the
electrons can flow freely from source to drain .so the
transistor is on.
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17. How a MOSFET Transistor works?
In MosFET, the Gate is insulated from p-channel or n-channel. This prevents
gate current from flowing, reducing power usage.
When the Gate is positive voltage ,it allows electrons to flow from drain to
source .In this case transistor is on.
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18. How a CMOS transistor works?
N-channel & P-channel MOSFETs can be combined in pairs with a common gate .
When Gate (input) is high ,electrons can
flow in N-channel easily . So output
becomes low. (opposite of input)
When Gate (input) is low ,holes can flow in
P-channel easily. So output becomes high.
(opposite of input)
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19. OtherTypes ofTransistors
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20. Various Types of Transistors
 Temp FET – MOSFET’s with temperature sensor
 High Electron Mobility Transistors (HEMTs) – allows high gain at very
high frequencies
 Darlington – two transistors within the same device, gain is the product
of the two INDIVIDUALS transistors
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21. Shockley Diode / Thyristor
 Four-layer PNPN semiconductor devices
 Behaves as two transistors in series
 Once on, tends to stay on
 Once off, tends to stay off
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