Download Chapter 6 BJT lecture notes

ECE 333 Linear Electronics
Chapter Bipolar Junction Transistors (BJTs)
Physical structure of BJT  I-V Characteristics 
circuits based on BJTs
Compared with MOSFETs
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Introduction
• The invention of BJT (page 305)
(Bardeen, Shockley and Brattain @ 1948 in Brattain’s lab)
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Story behind the First BJT
Bardeen was a quantum physicist, Brattain a gifted
experimenter in materials science, and Shockley,
the leader of their team, was an expert in solidstate physics.
1947: W. Brattain and J. Bardeen (Bell Labs)
experimentally demonstrated the device
J. Pierce (Bell Labs) name the device: transfer +
resistor = transistor
1949: W. Shockley theoretically described bipolar
junction transistor
1956: Nobel Prize
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6.1 Device Structure and Physical Operation
• 6.1.1
Figure 6.1 A simplified structure of the npn transistor.
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Figure 6.2 A simplified structure of the pnp transistor.
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6.1.2 Operation of the npn Transistor
in the Active Mode
Figure 6.3 Current flow in an npn transistor biased to operate in the active mode. (Reverse current
components due to drift of thermally generated minority carriers are not shown.)
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• Collector current
• Base current
• Emitter current
VT : the thermal potential
β is common-emitter current gain
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• Minority-Carrier Distribution
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• Equivalent Circuit Models
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• Example 6.1
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6.1.3 Structure of Actual BJTs
Figure 6.7 Cross section of an npn BJT.
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The high performance BJT
• Use different materials in emitter and base
𝛽𝐹 =
𝐷𝐵 𝑊𝐸 𝑁𝐸 𝑛2 𝑖𝐵
𝐷𝐸 𝑊𝐵 𝑁𝐵 𝑛2 𝑖𝐸
To increase βF , we need to increase ______
and decrease _______
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6.1.4 Operation in the saturation mode
• Different with that in MOSFETs
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Figure 6.9 Modeling the operation of an npn transistor in
saturation by augmenting the model of Fig. 6.5(c) with a forwardconducting diode DC. Note that the current through DC increases
iB and reduces iC.
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6.1.5 The pnp Transistor
Figure 6.10 Current flow in a pnp transistor biased to operate
in the active mode.
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6.2 Current-Voltage Characteristics
• 6.2.1 Circuit Symbols and Conventions
Figure 6.12 Circuit symbols for BJTs.
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Figure 6.13 Voltage polarities and current flow in
transistors operating in the active mode.
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• Example 6.2
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6.2.2 Graphical representation of
transistor characteristics
𝑖𝐶 = 𝐼𝑆
𝑒 𝑣𝐵𝐸/𝑉𝑇
𝐼𝑆 𝑣 /𝑉
𝑖𝐸 = 𝑒 𝐵𝐸 𝑇
𝛼
𝐼𝑆 𝑣 /𝑉
𝑖𝐵 = 𝑒 𝐵𝐸 𝑇
𝛽
• 1/VT ≈ 40, the i-v curves are sharp
Different with MOSFET
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• Exercise 6.15
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6.2.3 Dependence of iC on the Collector
Voltage – The Early Effect
• What is Early Effect (or base-width modulation
effect)?
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6.2.3 Dependence of iC on the Collector
Voltage – The Early Effect
• The current including Early Effect
• Output resistance (not infinite)
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6.2.3 Dependence of iC on the Collector
Voltage – The Early Effect
• The equivalent circuit taking into account of
Early Effect
Figure 6.19 Large-signal, equivalent-circuit models of an npn BJT
operating in the active mode in the common-emitter configuration
with the output resistance ro included.
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6.2.4 An Alternative Form of the CommonEmitter Characteristics
• The Common-Emitter Current Gain β
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6.2.4 An Alternative Form of the CommonEmitter Characteristics
• The Saturation Voltage VCEsat and Saturation
Resistance RCEsat
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6.2.4 An Alternative Form of the CommonEmitter Characteristics
• A simplified equivalent-circuit model of the
saturated transistor
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• Example 6.3
For the circuit in Fig. 6.22, it is required to determine VBB that
results in the transistor operating
(a) In active mode with VCE=5V
(b) At the edge of saturation
(c) Deep in saturation with βforced=10
(VBE remains constant at 0.7V and β=50)
Solve:
(analysis: active mode VBE is
Forward biased
VCE is known  VC is known 
IC can be calculated  IB can be
Calculated  VBB = …
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Saturation mode of operation
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6.3 BJT Circuit at DC
• Use simple model: |VBE|=0.7V for a
conducting transistor
and |VCE|=0.2V for a saturated transistor
• Accurate model will increase complexity and
impede insight in design
• SPICE simulation in the final stage of design
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• A note on Units: a consistent set of units:
volts (V), milliamps (mA), and kilohms (kΩ)
Example 6.4: β=100. determine all node voltages
and branch currents for the following circuit.
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• Use a simple model: from a simple analysis we
know that the transistor is conducting, so
VBE=0.7 V. This is the first step
Double-Check: the transistor is in active mode, saturation mode or cut-off mode?
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Example 6.5: β>50 for the following circuit
Assume
active-mode
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Example 6.5 (continue…)
Assume
Saturation-mode
βforced = IC / IB = 1.5  saturation mode
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Example 6.6
Cutoff mode
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Example 6.7 analyze all node voltages and branch currents
Since no β is not given, we can assume β=100
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Example 6.8: β=100, determine all node voltages and currents
Assume
active-mode
Is this design good or bad? What if β were 10% higher?
If β = 110, IC = 4.73 mA VC=10-2×4.73=0.54V saturation
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Example 6.9: the minimum β is 30
Either active or saturation mode: First assume active mode IB=0 VB=0
VE=0.7IE=4.3 mA. However, IC is limited at 5/10k = 0.5 mA saturation
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Yes! saturated
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6.4 Transistor Breakdown and
Temperature Effects
6.4.1 Transistor Breakdown
• CBJ breakdown is usually not destructive, at
BVCBO>=50V
• EBJ breakdown usually in an avalanche
manner at BVEBO=6~8V, destructive, with β
permanently reduced
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Figure 6.32 The BJT common-base characteristics including the
transistor breakdown region.
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Figure 6.33 The BJT common-emitter characteristics
including the breakdown region.
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6.4.2 Dependence of β on IC and
Temperature
Figure 6.34 Typical
dependence of β on IC
and on temperature in
an integrated-circuit
npn silicon transistor
intended for operation
around 1 mA.
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