Question E6A07:
Which of the following indicates that a silicon NPN junction transistor is biased on?  

Choices:  
A: Base-to-emitter resistance of approximately 6 ohms to 7 ohms  
B: Base-to-emitter resistance of approximately 0.6 ohms to 0.7 ohms  
C: Base-to-emitter voltage of approximately 6 volts to 7 volts  
D: Base-to-emitter voltage of approximately 0.6 volts to 0.7 volts    

Correct Answer is  D: Base-to-emitter voltage of approximately 0.6 volts to 0.7 volts  

To bias a silicon NPN transistor “on,” the base-emitter (BE) junction must be forward-biased like a silicon diode.

Why D is correct:
- A forward-biased silicon BE junction drops about $V_{BE} \approx 0.6$ to $0.7\ \text{V}$ at room temperature. Seeing this voltage indicates the junction is on and base current can flow, enabling collector current.

Why the others are incorrect:
- A: “6–7 ohms” BE resistance is not a valid indicator. The BE junction is a diode, not a fixed resistor; its resistance is nonlinear and depends on current.
- B: “0.6–0.7 ohms” is likewise not meaningful for a diode junction. The apparent resistance changes with operating point, not a fixed value.
- C: “6–7 volts” across BE is far too high for forward bias and would likely damage the device. Around 5–7 V is near the reverse-bias breakdown of the BE junction, not normal operation.

Useful formulas and rules of thumb:
- Silicon transistor on-state: $V_{BE} \approx 0.6 \text{ to } 0.7\ \text{V}$ at 25°C.
- Temperature coefficient: $\frac{dV_{BE}}{dT} \approx -2\ \text{mV}/^\circ\text{C}$ (VBE decreases as temperature rises).
- BE junction behaves like a diode: $I \approx I_S \left(e^{\tfrac{V_{BE}}{nV_T}} - 1\right)$, with $V_T \approx 25\ \text{mV}$ at room temp.
- Small-signal emitter resistance: $r_e \approx \dfrac{V_T}{I_E}$. Example: at $I_E = 1\ \text{mA}$, $r_e \approx 25\ \Omega$; at $10\ \text{mA}$, $r_e \approx 2.5\ \Omega$. This shows why a “fixed” ohmic value is not appropriate.