The brightness of an LED depends upon _____; when source voltage is much _____ than the diode threshold voltag

Question

The brightness of an LED depends upon _____; when source voltage is much _____ than the diode threshold voltage, the brightness of the LED remains approximately constant.

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The correct answer is Option A: forward current, higher. The brightness of an LED is directly proportional to the forward current flowing through the device, as the photon emission process is dependent on the recombination rate of charge carriers. When the source voltage significantly exceeds the threshold voltage (knee voltage), the current becomes limited by the external series resistor, causing the LED to operate in a stable region where brightness remains constant for a fixed current.

Step-by-Step Solution & Concepts:
The brightness of an LED is directly proportional to the forward current flowing through the device, as the photon emission process is dependent on the recombination rate of charge carriers. When the source voltage significantly exceeds the threshold voltage (knee voltage), the current becomes limited by the external series resistor, causing the LED to operate in a stable region where brightness remains constant for a fixed current.

Working Principle: As the forward voltage $V_f$ across the LED exceeds the threshold voltage $V_{th}$, charge carriers are injected into the junction region. The recombination of electrons and holes releases energy in the form of photons. The intensity of light $L$ is proportional to the forward current $I_f$, expressed as $L \propto I_f$. Once $V_{source} \gg V_{th}$, the circuit current $I_f = \frac{V_{source} - V_{th}}{R_{series}}$ determines the light output.

Key Formulas: $L \propto I_f$ — Relationship between light intensity and forward current; $I_f = \frac{V_{source} - V_{th}}{R_{series}}$ — Determination of forward current in a standard LED circuit

Key Points: Brightness is directly proportional to the number of electrons recombining per unit time.; LEDs act as current-controlled devices rather than voltage-controlled devices.; A current-limiting resistor is essential to prevent thermal runaway and permanent damage.; Saturation of light intensity occurs due to non-radiative recombination at very high currents.

Advantages: High luminous efficiency; Long operational lifespan; Fast switching capability

Disadvantages: Requires constant current driver; Sensitive to temperature fluctuations

Applications: Indicator lamps; Digital displays; Optical communication links

Additional Info: Threshold voltages typically range from $1.8	ext{V}$ for red LEDs to $3.5	ext{V}$ for blue/white LEDs.. Option B is incorrect because reverse bias prevents current flow and inhibits light emission.. Option C is incorrect as junction capacitance affects switching speed, not steady-state brightness.. Option D is incorrect because ambient temperature typically degrades brightness due to increased non-radiative recombination.

Answer: A is correct — The brightness of an LED is proportional to forward current, and it remains constant when the source voltage is significantly higher than the threshold voltage, provided a current-limiting resistor is present.

Exam Tip: Always remember that LEDs are current-driven devices; treat the LED voltage drop as nearly constant ($V_f$) in circuit analysis problems.

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Option B: reverse bias, lower

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Option C: junction capacitance, equal

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Option D: ambient temperature, smaller

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