Introduction to the Bipolar Junction Transistor (BJT)

• Inventor: William Shockley, 1951

• Purpose: Amplifies electronic signals, such as radio and TV signals

• Impact: Led to inventions like the integrated circuit (IC)

  • Enabled the development of modern computers and electronics

• Focus: Basics of BJT operation and usage as a switch

• Definition: A semiconductor device that uses free electrons and holes

• "Bipolar" Meaning: Refers to “two polarities” of charge carriers (electrons and holes)

• Three Doped Regions:

  • Emitter: Bottom region, heavily doped

  • Base: Middle region, very thin and lightly doped

  • Collector: Top region, intermediate doping

• Types of Transistors:

  • NPN: p region between two n regions

  • PNP: n region between two p regions

Doping Levels in a BJT

• Emitter: Heavily doped for high electron flow

• Base: Lightly doped for control over electron flow

• Collector: Intermediate doping level, physically largest region

Emitter and Collector Diodes

• Two Junctions in a BJT:

  • Emitter-Base Junction (or Emitter Diode)

  • Collector-Base Junction (or Collector Diode)

• Acts like two back-to-back diodes

Before and After Diffusion in a BJT

• Diffusion Process: Free electrons in n regions spread across the junction and recombine with holes in p region

  • Results in the formation of two depletion layers

• Barrier Potential:

  • Silicon Transistors: Approx. 0.7 V at \(25^\circ \mathrm{C}\)

  • Germanium Transistors: Approx. 0.3 V at \(25^\circ \mathrm{C}\)

NPN and PNP Transistors

Key Points to Remember

• BJT Structure and Operation

• Importance of Doping Levels

• Emitter and Collector Junctions

• Formation of Depletion Layers and Barrier Potential

Understanding the Biased Transistor

• Unbiased Transistor: Acts as two back-to-back diodes with \(\approx 0.7 ~\mathrm{V}\) barrier potential each

  • Important for testing NPN transistors with a DMM (Digital Multimeter)

• Biased Transistor: With external voltage sources, currents flow through various transistor regions

Emitter Electrons in a Biased Transistor

• Purpose of the Emitter:

  • Heavily doped to emit free electrons into the base

• Purpose of the Base:

  • Lightly doped to pass electrons on to the collector

• Purpose of the Collector:

  • Collects most electrons from the base

• Biasing Voltages:

  • \(\mathbf{V_{BB}}\): Forward-biases the emitter diode

  • \(\mathbf{V_{CC}}\): Reverse-biases the collector diode

Flow of Base Electrons

• Action on Forward Bias (\(\mathbf{V_{BB}}\)):

  • When \(\mathbf{V_{BB}}>\) Barrier Potential (\(\sim\)0.7 V), emitter electrons enter the base region

• Electron Flow Options:

  1. Flow Left: Out of base through \(R_{B}\) (base resistor)

  2. Flow Right: Into the collector

• Outcome:

  • Most electrons move to the collector due to:

    • Light doping (long electron lifetime in base)

    • Thin base (short distance to collector)

Collector Electrons and Current Flow

• Collector Attraction:

  • \(\mathbf{V_{CC}}\) attracts free electrons into the collector region

• Electron Path:

  • Electrons flow through \(R_C\) (collector resistor) to the positive terminal of \(\mathbf{V_{CC}}\)

• Summary of Current Flow:

  • \(\mathbf{V_{BB}}\): Drives electrons from emitter to base

  • \(\mathbf{V_{CC}}\): Draws electrons through the collector and \(R_C\) to complete the circuit

Understanding Transistor Currents

• Current Types in a Transistor:

  • Emitter Current (\(I_E\))

  • Base Current (\(I_B\))

  • Collector Current (\(I_C\))

• Current Flow Representations:

  • Conventional Flow

  • Electron Flow

Comparison of Transistor Currents

• Largest Current: Emitter current (I_E), as it’s the source of electrons

• Collector Current (I_C): Nearly as large as I_E

• Base Current (I_B): Much smaller, often \(<1\%\) of I_C

Relation of Currents (Kirchhoff’s Current Law)

• \[I_E = I_C + I_B\]
  Current Relationship
• \[I_C \approx I_E\]
  _BApproximation

• Application: Holds true for both NPN and PNP transistors

DC Alpha (\(\alpha\))

• Definition:

  • Ratio of DC Collector Current to DC Emitter Current

\[\alpha_{dc} = \frac{I_C}{I_E}\]

• Typical Value:

  • Slightly \(<\)1 (Low-power: \(>0.99\), High-power: \(>0.95\))

DC Beta (\(\beta\))

• Definition:

  • Ratio of DC Collector Current to DC Base Current

\[\beta_{dc} = \frac{I_C}{I_B}\]

• Current Gain:

  • Controls larger I_C with small I_B

  • Low-power transistors: \(\beta = 100-300\)

  • High-power transistors: \(\beta = 20-100\)

Using Beta to Derive Currents

1. \[I_C = \beta_{dc} I_B\]
   Calculating Collector Current (I_C)
2. \[I_B = \frac{I_C}{\beta_{dc}}\]
   Calculating Base Current (I_B)

Transistor Connections Overview

• Three configurations:

  • CE (Common Emitter) – Focus of this chapter

  • CC (Common Collector) - Discussed later

  • CB (Common Base) - Discussed later

• CE Connection: Most widely used.

Common Emitter Circuit

• Configuration:

  • Emitter connected to ground.

  • Two loops:

    • Base Loop: Forward-biased by \(V_{BB}\) with \(R_B\) as the limiter.

    • Collector Loop: Reverse-biased by \(V_{CC}\) via \(R_C\).

• Key Insight: Base current controls the collector current.

Voltage Behavior in CE Circuit

• Base Loop:

  • Base current (\(I_B\)) generates voltage across \(R_B\).

  • Polarity shown

• Collector Loop:

  • Collector current (\(I_C\)) creates voltage across \(R_C\).

  • Collector must remain positive for proper operation.

Double-Subscript Notation

• Definition:

  • Same subscripts = Source voltage (e.g., \(V_{BB}\), \(V_{CC}\)).

  • Different subscripts = Voltage between two points (e.g., \(V_{BE}\), \(V_{CE}\)).

• Measurement: Positive meter probe on first subscript. Common probe on second subscript.

Examples of Double-Subscript Voltages

• Key Equations:

  • \(V_{CE} = V_C - V_E\)

  • \(V_{CB} = V_C - V_B\)

  • \(V_{BE} = V_B - V_E\)

• In Fig. (b) (grounded circuit):

  • \(V_E = 0\) simplifies equations:

    • \(V_{CE} = V_C\)

    • \(V_{CB} = V_C - V_B\)

    • \(V_{BE} = V_B\)

Single-Subscript Notation

• Node Voltages: Voltage relative to ground.

• Examples in Fig. (b):

  • \(V_B\): Base to ground.

  • \(V_C\): Collector to ground.

  • \(V_E = 0\): Emitter to ground.

Transistor Equivalent Circuit Actual Transistor:

• \(V_{BE}\): Voltage across the emitter-base junction.

• \(V_{CE}\): Voltage across the collector-emitter terminals.

• Transistor modeled as:

  • A diode at the base-emitter junction.

  • A current source at the collector side, where \(I_C = \beta_{dc} I_B\).

Ideal Approximation Features:

• The emitter diode modeled as an ideal diode: \(V_{BE} = 0\).

• \[I_B = \frac{V_{BB}}{R_B}\]
  \(R_B\)\(V_{BB}\)Quick and simple calculation of base current:

Usage:

• Useful for troubleshooting and quick, rough approximations.

Second Approximation Features:

• \(V_{BE} = 0.7 \, \text{V}\) for silicon transistors (\(V_{BE} = 0.3 \, \text{V}\) for germanium).

• Accounts for the voltage drop of the emitter diode.

• \[I_B = \frac{V_{BB} - V_{BE}}{R_B}\]
  Base current calculation:
• \[I_C = \beta_{dc} I_B\]
  Collector current:

Usage:

• Common in low-voltage circuits for improved accuracy over the ideal approximation.

Higher Approximations Bulk Resistance Effects:

• \[V_{BE} > 0.7 \, \text{V} \, (\text{e.g., up to } 1 \, \text{V})\]
  : increases At high currents,

• Collector bulk resistance may also affect \(V_{CE}\) and \(I_C\).

Other Higher-Order Effects:

• Leakage currents.

• Early effect (variation of \(\beta_{dc}\) with \(V_{CE}\)).

• Temperature dependence of parameters.

Recommendation:

• Use computer simulations for calculations beyond the second approximation due to complexity.

Comparison of Approximations

Key Takeaways

• Approximations trade-off accuracy for simplicity:

  • Use ideal for quick estimates.

  • Use second for better accuracy in low-power circuits.

  • Rely on higher approximations or simulations for high-power or precision designs.

• Emitter diode \(V_{BE}\):

  • \(0.7 \, \text{V}\) (silicon) is typical but increases with current in high-power circuits.

• Bulk resistances and other effects:

  • Negligible in most low-power circuits, significant in high-power designs.