Electronics

Transistors

Dr. Mona M. Soliman

Dr. Walid M. Ibrahim
IT Department
Vacuum tubes
• Purpose
▫ Used as signal amplifiers and switches
▫ Advantages
 High power and frequency operation
 Operation at higher voltages
 Less vulnerable to electromagnetic pulses
▫ Disadvantages
 Very large and fragile
 Energy inefficient
 Expensive
Invention
• Evolution of electronics
▫ In need of a device that was small, robust, reliable,
energy efficient and cheap to manufacture
• 1947
▫ John Bardeen, Walter Brattain and William Schockly
invented transistor
• Transistor Effect
▫ “when electrical contacts
were applied to a crystal
of germanium, the output
power was larger than
the input.”
Transistors

BJT (PNP) Electrical

Different types and sizes Diagram

FET and BJT Transistor

Modern Electronics
First Transistor
Transistors Purpose
▫ To amplify
▫ switch electronic signals on or off (high or low)

Microprocessor
Motor Controllers

Cell Phones

Modern Electronics
General Applications
Doping
• Process of introducing impure elements
(dopants) into semiconductor wafers to form
regions of differing electrical conductivity

Negatively charged Semiconductor Positively charged semiconductor

 Doping Effects

• P-type semiconductors
▫ Created positive charges, where electrons have been removed, in
lattice structure
• N-type semiconductors
▫ Added unbound electrons create negative charge in lattice structure

• Resulting material
▫ P-N junction
P-N junction
Forward Biasing
Reverse Biasing
 BJT Doping

• Collector (C): Larger in size and moderately doped

• Base (B): Very thin layer and lightly doped.

• Emitter (E): Smaller than (C)and heavily doped.

BJT Introduction
• P-N junction
▫ Controls current flow via external voltage
• Two P-N junctions (bipolar junction transistor, BJT)
▫ Controls current flow and amplifies the current flow
 BJT Introduction
• Bipolar Junction Transistors (BJT) consists of three
“sandwiched” semiconductor layers

• The three layers are connected to collector (C),

emitter (E), and base (B) pins

• Current supplied to the base controls the amount of

current that flows through the collector and emitter
 BJT Introduction
• Transistors are equivalent to a dam with a variable
gate that controls the amount of water flow

• A small amount of energy is required to operate the

gate.

• Amplification is achieved by a small amount of

energy can be used to control the flow of a large
amount of current.
 In the hydrodynamic analogy,
base
• The emitter correspond to the river above the dam

• The collector correspond to the river below the dam. emitter

• The base terminal corresponds to the control input

that varies the flow through the dam.
collector
BJT Schematic
NPN
• NPN
▫ BC reverse bias
▫ BE forward bias

• PNP
▫ BC forward bias
PNP
▫ BE reverse bias
BJT Characteristic Curves
Transfer Characteristic
• Characteristic curves can be drawn to show other useful parameters
of the transistor
• The slope of ICE / IBE is called the Transfer Characteristic (β)
BJT Characteristic Curves
Input Characteristic
• The Input Characteristic is the base emitter current IBE against
base emitter voltage VBE
• IBE/VBE shows the input Conductance of the transistor.
• The increase in slope of when the VBE is above 1 volt shows that the
input conductance is rising
• There is a large increase in current for a very small increase in VBE.
BJT Characteristic Curves
Output Characteristic
• collector current (IC) is nearly independent of the collector-emitter
voltage (VCE), and instead depends on the base current (IB)

IB4

IB3

IB2

IB1
 Basic Transistor Operation
 To operate the transistor properly, the two pn junction must be correctly
biased with external dc voltages.
 The figure shown the proper bias arrangement for both npn and pnp
transistor for active operation as an amplifier.
Basic Transistor Operation (cont.)

Illustration of BJT action:

C

E
 Basic Transistor Operation (cont.)
Transistor Currents:
 The directions of the currents in npn transistor and pnp transistor are
shown in the figure.
 The emitter current (IE) is the sum of the collector current (IC) and the
base current (IB)

I E  I B  IC (1)

 IB << IE and IC
 The capital letter – dc value
 Transistor Characteristic & Parameters
DC Beta (  DC ) and DC Aplha ( DC )
 The ratio of the dc collector current (IC) to the dc base current (IB) is the
dc beta
(  DC ) = dc current gain of transistor
 Range value : 20<  DC <200
 Usually designed as an equivalent hybrid (h) parameter, hFE on
transistor data sheet – hFE   DC
IC
 DC 
IB
 The ratio of the dc collector current (IC) to the dc emitter current (IE) is the
dc alpha (  DC ) – less used parameter in transistor circuits
 Range value-> 0.95<  DC <0.99 or greater , but << 1 (Ic< IE )
IC 
 DC  
IE  1
Transistor Characteristic & Parameters (cont.)
Current and Voltage Analysis:
 The current and voltage can be identified as following:
 Current: Voltage:
dc base current, IB dc voltage at base with respect to emitter, VBE
dc emitter current, I E dc voltage at collector with respect to base, VCB
dc collector current, I C dc voltage at collector with respect to emitter, VCE

forward-biased the
base-emitter junction reverse-biased the
base-collector junction

Transistor current & voltage

 Transistor Characteristic & Parameters (cont.)

Case A assume VBE < 0.7, in this case VBE is reversed:

Rc=1 k Ω
 No current will flow (same as regular diode).
 IB = IC = IE = 0.
 VCE = VCC = 5 V.
 In this case transistor is working in cut-off mode.
= 5V
 Transistor Characteristic & Parameters (cont.)

Case B assume VBE = 0.7, in this case VBE is forward bias.

Rc=1 k Ω
 Assume IB = 10 µA and β = 100.
 IC = β IB = 100 * 10 µA = 1 mA.
 VCE = VCC - ICRC = 5 – 1 = 4V.

= 5V
 In this case transistor is working in liner mode.
 This is valid when 0 < IB < 50 µA.

Case C assume VBE = 0.7 and IB ≥ 50 µA

 VCE in this case can’t be greater than VCC thus transistor is working as short circuit.
 In this case transistor is working in saturation mode.
 IC(sat) = Imax = VCC/ RC = 5/ 1kΩ = 5mA.
Transistor Characteristic & Parameters (cont.)
Transistor Operating Regions: leakage current is
neglected
1.Cutoff region:
• Both transistor junctions are reverse biased
• All terminal current are approximately equal to zero

2.Active region:
• The BE junction is forward biased and the BC junction is reverse biased
• All terminal currents have some measurable value
• The magnitude of IC depends on the values of  and IB
• VCE is approximately 0.7V and VCE falls in ranges VBE<VCE<VCC

3.Saturation:
• Both transistor junctions are forward biased.
• IC reaches its maximum values- determine by the VCC and RC
• IC is independent of the values of  and IB
• VBE is approximately 0.7V and VCE < VBE (almost 0)
 BJT Operating Regions
Operating Parameters Mode
Region

Collector-Base and base Base-Emitter

are reverse biased
Cut Off Switch OFF

Collector-Base is reverse-biased
Linear Base-Emitter is forward-biased
Amplification
(Active)

Collector-Base and base Base-Emitter

are forward biased
Saturated Switch ON
 BJT Operating Regions
Operating Parameters Mode
Region

VBE < Vcut-in (0.7)

VCE > Vsupply (VCC)
Cut Off IB ≈ IC ≈ IE ≈ 0 Switch OFF
In this case transistor act as
open circuit.
VBE ≥ Vcut-in
Vcut-in (0.7)< VCE < Vsupply (VCC)
Linear Amplification
IC = β*IB
IE =IB + IC
VBE ≥ Vcut-in (0.7)
VCE < Vcut-in (0.7)
𝑉
Saturated IC is max. 𝐼𝑐 (𝑠𝑎𝑡) = 𝑐𝑐 Switch ON
𝑅𝐶
In this case transistor act as
short circuit.
 Transistor Characteristic & Parameters (cont.)
Collector Characteristic Curve:
 Using a circuit as shown in below, we can generate a set of
collector characteristic curve that show how the collector current,
Ic varies with the VCE voltage for specified values of base current,
IB.

variable voltage
Transistor Characteristic & Parameters (cont.)
Collector Characteristic Curve:
 Assume that VBB is set to produce a certain value of IB and VCC is zero.
 At this condition, BE junction and BC junction are forward biased because the
base is approximately 0.7V while the emitter and the collector are zero.
 The IB is through the BE junction because of the low impedance path to ground,
therefore IC is zero.
 When both junctions are forward biased – transistor operate in saturation region.

variable voltage
Transistor Characteristic & Parameters (cont.)
Collector Characteristic Curve:
 As VCC is increase gradually, IC increase – indicated by point A to B.
 IC increase as VCC is increased because VCE remains less than 0.7V due to the
forward biased BC junction.
 When VCE exceeds 0.7V, the BC becomes reverse biased and the transistor
goes into the active or linear region of its operation.
 In this time, IC levels off and remains constant for given value of IB and VCE
continues to increase.

variable voltage
Transistor Characteristic & Parameters (cont.)
Collector Characteristic Curve:
 Actually, IC increase very slightly as VCE increase due to widening of the
BC depletion region
 This result in fewer holes for recombination in the base region which
effectively caused a slight increase in I C   DC I B
 When VCE reached a sufficiently high voltage, the reverse biased BC
junction goes into breakdown.

variable voltage
Transistor Characteristic & Parameters (cont.)
Collector Characteristic Curve:
 The collector current increase rapidly – as indicated at the right point C
 The transistor cannot operate in the breakdown region.
 When IB=0, the transistor is in the cutoff region although there is a very
small collector leakage current as indicated

variable voltage
Transistor Characteristic & Parameters (cont.)
DC Load Line:
 Cutoff and saturation can be illustrated in relation to
the collector characteristic curves by the use of a load line.
 DC load line drawn on the connecting
cutoff and saturation point.
 The bottom of load line is ideal
cutoff where IC=0 & VCE=VCC.
 The top of load line is saturation
where IC=IC(sat) & VCE =VCE(sat)
 In between cutoff and saturation
is the active region of transistor’s
operation.
Transistor Characteristic & Parameters (cont.)
More About beta,  DC , hFE :
-Important parameter for BJT
-Varies both IC & temperature
-Keeping the junction temperature
constant, IC cause  DC
-Further increase in IC beyond this
max. point cause  DC to decrease

Maximum Transistor Ratings:

-Specified on manufacturer’s data sheet
-Given for VCE,VBE,VBC,IC & power dissipation
-The product of VCE and IC must not exceed the max. power dissipation
-Both VCE and IC cannot be max. at the same time.
Transistor Data Sheet
Transistor as an Amplifier
• Amplification of a small ac voltage by
placing the ac signal source in the base
circuit.
• Vin is superimposed on the DC bias voltage
VBB by connecting them in series with base
resistor RB.

• Small changes in the base current circuit

causes large changes in collector current
circuit.
I C   DC I B
Transistor Characteristic & Parameters
Current and Voltage Analysis:
 When the BE junction is forward-biased, like a forward biased
diode and the voltage drop is
VBE  0.7V (3)
 Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the
voltage across RB is: VR  VBB  VBE …….(1)
B

 Also, by Ohm’s law: VRB  I B RB ……..(2)

 From (1) ->(2) :
VBB  VBE  I B RB

 Therefore, the dc base current is:

VBB  VBE
IB  (4)
RB
Transistor Characteristic & Parameters (cont.)
Current and Voltage Analysis:
 The voltage at the collector with respect to the grounded emitter is
VCE  VCC  VRC
 Since the drop across RC is: VRC  I C RC
 The dc voltage at the collector w.r.t the emitter:
VCE  VCC  I C RC (5)

where I C   DC I B
 The voltage at the collector w.r.t the base:

VCB  VCE  VBE (6)

Bipolar Transistor Biasing
Bipolar Transistor Biasing
Bipolar Transistor Biasing
Example
• Determine Operation Q-point in figure below and find the
maximum peak value of base current for linear operation (Active
Region). Assume βDC=200.
Solution
• Operation Q-point is defined by values of IC and VCE.
𝑉𝐵𝐵 − 𝑉𝐵𝐸 10−0.7
𝐼𝐵 = = = 198𝜇𝐴
𝑅𝐵 47 𝑘 Ω

𝐼𝐶 = 𝛽𝐷𝐶 𝐼𝐵 = 200 198 𝜇 = 39.6 𝑚𝐴

𝑉𝐶𝐸 = 𝑉𝐶𝐶 − 𝐼𝐶 𝑅𝐶 = 20𝑉 − (39.6𝑚𝐴)(330 Ω) = 6.93V

• Q-point is at IC=39.6mA and VCE=6.93V. Since IC(cutoff)=0, we need to know IC(sat) to

determine variation in IC can occur and still in linear operation.
𝑉 20
𝐼𝑐 (𝑠𝑎𝑡) = 𝑐𝑐 = = 60.6 𝑚𝐴
𝑅𝐶 330

• Before saturation is reached, IC can increase an amount equal to:

IC(sat) – ICQ = 60.6mA – 39.6mA = 21mA.
BJT Applications
BJT Amplifier
BJT Applications
BJT Amplifier
Transistor as a switch
A transistor when used as a switch is simply being biased so
that it is in
1. cutoff (switched off) 2. saturation (switched on)

VCC  VCE ( sat )

VCE(cutoff )  VCC IC ( sat ) 
RC
IC ( sat )
IB (min) 
DC
BJT Applications
BJT Switch
• Offer lower cost and substantial reliability over conventional
mechanical relays.
• Transistor operates purely in a saturated or cutoff state (on/off)
• This can prove very useful for digital applications (small current
controls a larger current)
Transistor in logic gates
Not Gate
• A=A
=5v

0v
AND Gate
• Y=A.B
=5v

0v
NAND Gate
• Y=A.B

=5v

0v
OR Gate
• Y=A+B
=5v

0v
NOR Gate
• Y=A+B
=5v

0v
XOR Gate
• Y=A+B
Transistor Array examples
Questions?