ECE 333 Linear Electronics Chapter Bipolar Junction Transistors (BJTs) Physical structure of BJT  I-V Characteristics  circuits based on BJTs Compared with MOSFETs

Introduction The invention of BJT (page 305) (Bardeen, Shockley and Brattain @ 1948 in Brattain’s lab)

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 solid-state 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

6.1 Device Structure and Physical Operation 6.1.1 Figure 6.1 A simplified structure of the npn transistor.

Figure 6.2 A simplified structure of the pnp transistor.

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.)

Collector current Base current Emitter current VT : the thermal potential β is common-emitter current gain

Minority-Carrier Distribution

Equivalent Circuit Models

Example 6.1

6.1.3 Structure of Actual BJTs Most Si BJT are designed so that recombination of injected electrons in the base-emitter space charge region is smaller than the other components. Figure 6.7 Cross section of an npn BJT.

The high performance BJT Use different materials in emitter and base To increase βF , we need to increase ______ and decrease _______ 𝛽 𝐹 = 𝐷 𝐵 𝑊 𝐸 𝑁 𝐸 𝑛 2 𝑖𝐵 𝐷 𝐸 𝑊 𝐵 𝑁 𝐵 𝑛 2 𝑖𝐸

6.1.4 Operation in the saturation mode Different with that in MOSFETs

Figure 6.9 Modeling the operation of an npn transistor in saturation by augmenting the model of Fig. 6.5(c) with a forward- conducting diode DC. Note that the current through DC increases iB and reduces iC.

6.1.5 The pnp Transistor Figure 6.10 Current flow in a pnp transistor biased to operate in the active mode.

6.2 Current-Voltage Characteristics 6.2.1 Circuit Symbols and Conventions Figure 6.12 Circuit symbols for BJTs.

Figure 6.13 Voltage polarities and current flow in transistors operating in the active mode.

Example 6.2

6.2.2 Graphical representation of transistor characteristics 1/VT ≈ 40, the i-v curves are sharp 𝑖 𝐸 = 𝐼 𝑆 𝛼 𝑒 𝑣 𝐵𝐸 / 𝑉 𝑇 𝑖 𝐵 = 𝐼 𝑆 𝛽 𝑒 𝑣 𝐵𝐸 / 𝑉 𝑇 𝑖 𝐶 = 𝐼 𝑆 𝑒 𝑣 𝐵𝐸 / 𝑉 𝑇 Different with MOSFET

Exercise 6.15

6.2.3 Dependence of iC on the Collector Voltage – The Early Effect What is Early Effect (or base-width modulation effect)?

6.2.3 Dependence of iC on the Collector Voltage – The Early Effect The current including Early Effect Output resistance (not infinite)

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.

6.2.4 An Alternative Form of the Common-Emitter Characteristics The Common-Emitter Current Gain β

6.2.4 An Alternative Form of the Common-Emitter Characteristics The Saturation Voltage VCEsat and Saturation Resistance RCEsat In comparison with MOSFET Linear resistance βforced

6.2.4 An Alternative Form of the Common-Emitter Characteristics A simplified equivalent-circuit model of the saturated transistor

Example 6.3 For the circuit in Fig. 6.22, it is required to determine VBB that results in the transistor operating In active mode with VCE=5V At the edge of saturation 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 = …

Saturation mode of operation

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

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.

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?

Example 6.5: β>50 for the following circuit Assume active-mode

Example 6.5 (continue…) βforced = IC / IB = 1.5  saturation mode Assume Saturation-mode βforced = IC / IB = 1.5  saturation mode

Example 6.6 Cutoff mode

Example 6.7 analyze all node voltages and branch currents pnp Since no β is not given, we can assume β=100

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

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

Yes! saturated

6.4 Transistor Breakdown and Temperature Effects 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

Figure 6.32 The BJT common-base characteristics including the transistor breakdown region.

Figure 6.33 The BJT common-emitter characteristics including the breakdown region.

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.