Lecture 8 Bipolar Junction Transistor (BJT) BJT 1-1 Wednesday 18/10/2016
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Lecture 8
Bipolar Junction Transistor (BJT)
BJT 1-1 Wednesday 18/10/2016
Outline Introduction to Bipolar Junction
Transistor (BJT) Definitions BJT principles and characteristics BJT structure BJT operation
• Operation modes
Currents in BJT
BJT 1-2
Introduction The transistor is the main building block of
electronics
It is a semiconductor device
It is a three terminal device
Two basic families of transistors Bipolar Junction Transistor (BJT)
Field Effect Transistor (FET)
Transistors are used in: Digital logic
Memory circuits
Amplifiers
Electronic switches
BJT 1-3
Bipolar Junction Transistor (BJT) There are two types of BJT
• pnp
• npn
The terminals are labeled
• E - Emitter
• B - Base
• C - Collector
npn and pnp BJT symbols
BJT 1-4
npn BJT Structure
BJT 1-5
The BJT is fabricated with three separately doped regions
The BJT has two junctions (boundaries between the n and the p regions) Junctions are similar to the pn junctions (diodes) where they
may be forward biased or reverse biased
Emitter-base junction (EBJ) and collector-base junction (CBJ)
These junctions have capacitance
BJT Operation
BJT 1-6
Depending on the biasing across each of the junctions, different modes of operation are obtained: cutoff, active, and saturation
BJT in linear amplifier circuits:
• Active – Operating range of the amplifier.
• Cutoff – The amplifier is basically off. There is voltage, but little current.
• Saturation – The amplifier is full on. There is current, but little voltage.
BJT Operation (Active Mode)
BJT 1-7
Two external voltage sources set the bias conditions for active mode
The emitter-base junction (EBJ) is forward biased The base-collector junction (BCJ) is reverse biased
Currents in BJT
BJT 1-8
BJT operation in active mode
Currents in BJT The current through EBJ is related to the B-E
voltage as
Due to the large differences in the doping concentrations of the emitter and the base regions, the electrons injected into the base region (from the emitter region) results in the emitter current IE
BJT 1-9
Currents in BJT (cont’d) The number of electrons injected into the collector
region is directly related to the electrons injected into the base region from the emitter region So, the collector current is related to the emitter
current
BJT basic principle: The voltage between two terminals controls the
current through the third terminal
BJT 1-10
Currents in BJT (cont’d) Emitter current is the sum of the collector and base currents:
The collector current and the base current
are related by
β represents the amplification factor of a transistor (β = 100 in transistors of interest)
The relationship between the emitter and the base currents is
The relationship between the emitter and the collector currents is
Where is , called collector efficiency (α is between 0.9 and 0.998 in transistors of interest)
BJT 1-11
Directions of currents and voltage polarities
BJT 1-12
BJT Configurations
BJT 1-13
The base is common to both input (emitter–base) and output (collector–base) of the transistor
Common-Base configuration
BJT Configurations
BJT 1-14
This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage (VCB) levels
Common-Base configuration
Emitter (or input)
Characteristics
BJT Configurations
BJT 1-15
This graph demonstrates the output current (IC) to an output voltage (VCB) for various levels of input current (IE)
Common-Base configuration
Collector (or output)
Characteristics
BJT 1-16
Lecture Summary Covered material Introduction to Bipolar Junction Transistor
(BJT) BJT principles BJT structure BJT operation modes BJT configurations Currents in BJT
Material to be covered next lecture
Continue BJT Configurations Biasing DC analysis
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