A blue LED has a forward voltage drop of 2.5 V. Estimate the bandgap energy for

Question

A blue LED has a forward voltage drop of 2.5 V. Estimate the bandgap energy for the semiconductor that makes the blue LED. What wavelength is emitted from the LED in nm? The blue LED is in series with AC source of V(t) = V0 cos(2pi ft) with V0=5 V,f=l Hz, and a resistor of 500 Ohm as seen in the figure. Make a plot by hand of the voltage across the resistor and the AC source voltage from t=0 to 2 s. Make sure your voltages have the correct amplitude. Make a plot by hand of the voltage across the LED and the AC source voltage from t=0 to 2 s. Make sure your voltages have the correct amplitude.

Explanation / Answer

Energy of Blue light E = h?

= (6.626*10-34 Js)(3*108 m/s)/(450*10-9 m)

= 4.4*10-19 J

= 2.76 eV

Hence the band gap = 2.76 eV

Wavelength ? = c/?

= 2.99*108 / 666.66 * 1012

= 450 nm

Total voltage in circuit

V(0) = 5 V

V(1) = 5 * cos(2?) = 5 V

V(2) = 5 * cos(4?) = 5 V

Voltage across LED is always 2.5 V in order to emit blue light

The remaining voltage will drop across resistor

so the voltage across resistor at

t = 0 , 1, 2 is

V = 2.5 , 2.5, 2.5 V respectively

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