ECE 333 Linear Electronics Chapter Bipolar Junction Transistors
ECE 333 Linear Electronics Chapter Bipolar Junction Transistors (BJTs) Physical structure of BJT I-V Characteristics circuits based on BJTs Compared with MOSFETs 1
Introduction • The invention of BJT (page 305) (Bardeen, Shockley and Brattain @ 1948 in Brattain’s lab) 2
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 3
6. 1 Device Structure and Physical Operation • 6. 1. 1 Figure 6. 1 A simplified structure of the npn transistor. 4
Figure 6. 2 A simplified structure of the pnp transistor. 5
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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. ) 7
• Collector current • Base current • Emitter current VT : thermal potential β is common-emitter current gain 8
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• Minority-Carrier Distribution 10
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• Equivalent Circuit Models 12
• Example 6. 1 13
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6. 1. 3 Structure of Actual BJTs Figure 6. 7 Cross section of an npn BJT. 16
The high performance BJT • Use different materials in emitter and base To increase βF , we need to increase ______ and decrease _______ 17
6. 1. 4 Operation in the saturation mode • Different with that in MOSFETs 18
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 i. B and reduces i. C. 19
6. 1. 5 The pnp Transistor Figure 6. 10 Current flow in a pnp transistor biased to operate in the active mode. 20
6. 2 Current-Voltage Characteristics • 6. 2. 1 Circuit Symbols and Conventions Figure 6. 12 Circuit symbols for BJTs. 21
Figure 6. 13 Voltage polarities and current flow in transistors operating in the active mode. 22
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• Example 6. 2 24
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6. 2. 2 Graphical representation of transistor characteristics • 1/VT ≈ 40, the i-v curves are sharp Different with MOSFET 27
• Exercise 6. 15 28
6. 2. 3 Dependence of i. C on the Collector Voltage – The Early Effect • What is Early Effect (or base-width modulation effect)? 29
6. 2. 3 Dependence of i. C on the Collector Voltage – The Early Effect • The current including Early Effect • Output resistance (not infinite) 30
6. 2. 3 Dependence of i. C 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. 31
6. 2. 4 An Alternative Form of the Common. Emitter Characteristics • The Common-Emitter Current Gain β 32
6. 2. 4 An Alternative Form of the Common. Emitter Characteristics • The Saturation Voltage VCEsat and Saturation Resistance RCEsat T h. M t i w n o is r e a c p n a m sist o c In ar re d Line β force E F S O 33
6. 2. 4 An Alternative Form of the Common. Emitter Characteristics • A simplified equivalent-circuit model of the saturated transistor 34
• 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=5 V (b) At the edge of saturation (c) Deep in saturation with βforced=10 (VBE remains constant at 0. 7 V 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 = … 35
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Saturation mode of operation 37
6. 3 BJT Circuit at DC • Use simple model: |VBE|=0. 7 V for a conducting transistor and |VCE|=0. 2 V for a saturated transistor • Accurate model will increase complexity and impede insight in design • SPICE simulation in the final stage of design 38
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• A note on Units: a consistent set of units: volts (V), milliamps (m. A), and kilohms (kΩ) Example 6. 4: β=100. determine all node voltages and branch currents for the following circuit. 40
• 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? 41
Example 6. 5: β>50 for the following circuit Assume active-mode 42
Example 6. 5 (continue…) Assume Saturation-mode βforced = IC / IB = 1. 5 saturation mode 43
Example 6. 6 Cutoff mode 44
Example 6. 7 analyze all node voltages and branch currents pnp Since no β is not given, we can assume β=100 45
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 m. A VC=10 -2× 4. 73=0. 54 V saturation 46
Example 6. 9: the minimum β is 30 Either active or saturation mode: First assume active mode IB=0 VE=0. 7 IE=4. 3 m. A. However, IC is limited at 5/10 k = 0. 5 m. A saturation 47
Yes! saturated 48
6. 4 Transistor Breakdown and Temperature Effects 6. 4. 1 Transistor Breakdown • CBJ breakdown is usually not destructive, at BVCBO>=50 V • EBJ breakdown usually in an avalanche manner at BVEBO=6~8 V, destructive, with β permanently reduced 49
Figure 6. 32 The BJT common-base characteristics including the transistor breakdown region. 50
Figure 6. 33 The BJT common-emitter characteristics including the breakdown region. 51
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 m. A. 52
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