Structure and Symbols

• Three terminals emitter base collector
• Types NPN PNP
• Emitter heavily doped
• Base thin and lightly doped
• Collector moderately doped larger area
• Arrow shows emitter conventional current for NPN out for PNP in
• Two pn junctions in series view

Currents and Voltages

• Define $i_E, i_B, i_C$ with $i_E = i_B + i_C$
• Node voltages $v_{BE} v_{BC} v_{CE}$
• Sign convention currents enter device terminals

Core Exponential Law

• $V_T \approx 25\,\text{mV}$ at $300\,\text{K}$
• $I_S$ set by device geometry and temperature

Current Gain Relations

• Common emitter gain $\beta_F = \frac{i_C}{i_B}$
• Common base gain $\alpha_F = \frac{i_C}{i_E} = \frac{\beta_F}{\beta_F + 1}$:

Operating Regions

• Cutoff: Open Circuit in branches BE and CE

  • Condition when evaluating forward-active model: $i_B < 0$
  • $v_{BE} < 0.7$
  • $i_B = 0$ and $i_C = 0$

• Saturation: No current amplification; constant potential difference at the junctions

  • Condition when evaluating forward-active model (assuming $i_B > 0$): $V_{CE} \le 0.2$
  • $V_{BE} = 0.7$
  • $V_{CE} = 0.2$

• Forward active: Current is amplified

  • Condition when evaluating forward-active model (assuming $i_B > 0$): $V_{CE} \gt 0.2$
  • $V_{BE} = 0.7$
  • $i_C \approx \beta_F i_B$

Common Emitter Output Behavior

• Plot $i_C$ versus $v_{CE}$ for fixed $i_B$
• Flat $i_C$ in forward active for $v_{CE}$ above a knee
• $i_B = 0$ gives cutoff curve

References

• Course slides, chapter 5: Bipolar Junction Transistors