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Red (R), Green (G) and Blue (B) Light Emitting Diodes (LEDs) were fabricated using p-n junctions of three different inorganic semiconductors having different band-gaps. The built-in voltages of red, green and blue diodes are V_R, V_G and V_B, respectively. A same donor and acceptor doping to be the same (N_A and N_D, respectively) in the p and n sides of all the three diodes. Which one of the following relationships about the built-in voltages is TRUE?

1. V_R > V_G > V_B
2. V_R < V_G < V_B
3. V_R = V_G = V_B
4. V_R > V_G < V_B

Correct answer: V_R < V_G < V_B

Solution

The built-in voltage of a diode is inversely related to the band-gap energy of the semiconductor material used; since blue light has the highest energy and red light the lowest, the built-in voltage for red LEDs is less than that for green, which in turn is less than that for blue.

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