Bipolar Junction Transistor

• 3 semiconductor layers
  • NPN or PNP
• used as switch or amplifier
• different from just back to back diodes because there is interaction between the layers
• $V_e$ is voltage at emitter, $V_b$ is voltage at the base, $V_c$ is the voltage at the collector

Layers §

• emitter
  • heavily doped (like conductor)
• base
  • lightly doped (like insulator)
  • very thin
• collector
  • moderately doped (like semiconductor)

NPN §

Diagram §

Circuit Symbol §

• arrow indicates the direction of current

Regions of Operation §

Active Region §

• $V_c>V_b>V_e$
• emitter is in forward bias
• collector is in reverse bias
• used for amplification

Cutoff Region §

• $V_c,V_e>V_b$
• emitter is in reverse bias
• collector is in reverse bias
• used for switching

Saturation Region §

• $V_b>V_e,V_c$
• emitter is in forward bias
• collector is in forward bias
• used for switching

Configurations §

• different configurations are used for different applications
• 3 types
  • common emitter
    • current + voltage gain
  • common base
    • voltage gain but no current gain
  • common collector
    • current gain but no voltage gain
  • “common” indicates that both of the other non-common component are connected to the common component
    • example: common emitter means that both the base and collector are connected to the emitter

Transistor Working Demonstration §

• let’s model a a NPN, common emitter BJT transistor working in the active region
• input is applied between the base and emitter
• output is measured between the collector and emitter.

• $V_{bb}$ pushes electrons from the emitter to the base
• electrons in base have 2 options:
  • flowing through $R_B$ and the positive terminal of $V_{bb}$
  • flowing through $R_C$ and the positive terminal of $V_{cc}$
• most electrons flow to collector b/c
  • collector is lightly doped
  • base layer is very thin

In short, we just reiterated the Active Region section:

• current flows from emitter to base (emitter is forward biased)
• current flows from emitter to collector (reverse biased) (note that in the diagram in the Active Region section, current flows from base to collector because it’s a common base config)

Diagram of current:

Now we perform KCL:

$$\begin{array}{rl}{I}_b+I_c& =I_e\\ I_c& \approx I_e\\ ⟹I_c& =\alpha I_e\\ I_b+\alpha I_e& =I_e\\ ⟹I_b& =I_e(1-\alpha )\\ ⟹I_c& =\frac{\alpha }{1-\alpha }{I}_b\\ I_c& =\beta I_b\\ ⟹I_e& =(\beta +1)I_b\end{array}$$

Since $I_b$ is the input and $I_c$ is the output, you can control the output ($I_c$) by just changing the base current.

PNP §

Diagram §

Circuit Symbol §

• arrow indicates direction of current