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Determination of Plancks Constant By Multiple light emitting diodesCHAN, NERI, PAGDATO, TOLENTINOPHYSICS 73.1 X4INTRODUCTIONWHAT IS A LIGHT-EMITTING DIODE (LED)?A semiconductor device that emits light when voltage is applied across it.A p-n junction diodeConsist of p-doped and n-doped regions in its simplest formConductor material is typically aluminum-gallium-arsenide (AlGaAs)

PARTS OF A LIGHT-EMITTING DIODE

OPERATION OF AN LEDA. No applied voltageThe p- and n-doped regions are misaligned creating a potential barrier that blocks the flow of charge carriersResults to no current flow

B. Applied voltageConduction and valence bands align and the potential barrier height decreasesResults to little current flow

OPERATION OF AN LEDC. Applied voltage exceeds turn-on voltage VoThe conduction and valence bands align and the potential barrier becomes low enough to allow charge carriers to move across the pn-junctionResults to a current flow

MAX PLANCKProposed that atoms absorb and emit radiation in discrete quantities.

Mathematically expressed as E = nhf

In this experiment, the Vo of the diode is related to Eband gap and photon frequency by eVo = Eband gap = hf

OBJECTIVESThis experiment aims to determine Plancks constant by plotting the turn-on voltage vs frequency of the LEDs wired in a parallel circuit; and

To investigate the mechanism of LEDs and relate it to the concepts encountered in the course.

METHODOLOGY

24V POWER SUPPLYDirect Currentconnected to a printed circuit board

1k RESISTORin series

LIGHT-EMITTING DIODES (LEDs)in parallel with each other+-

VOLTMETERalso in parallel

AMMETERin series

RECORD VOLTAGE (V) and CURRENT (mA) READINGSat most 15 different readings*Voltage was not allowed to exceed 4VPLOT CURRENT vs. VOLTAGEestimate the turn-on voltage from the best-fit lineCURRENT (mA)VOLTAGE (V)Vo*just an illustration

6 POSSIBLE COMBINATIONS3!PLOT TURN-ON VOLTAGE vs. FREQUENCYfrequency of the LED with higher VohPLANCKS CONSTANTRESULTS AND DISCUSSIONVOLTAGE DROP*amount of voltage lost in the LED when operated at the certain reference current

RED, ORANGE, YELLOW AND YELLOW-GREEN (1.8V)PURE GREEN, BLUE, WHITE, UV (3.3V)

PARALLEL CIRCUITSSame voltage throughoutDifferent current

Connecting several LEDs in parallel with one resistor is generally not a good ideaCurrent will take the path of the least resistanceHence, LED with less Vo steals the current from the higher VoCOLOR COMBINATIONSWHAT WORKED:Red and OrangeOrange and BlueOrange and VioletBlue and Violet

WHAT DIDNT:Red and BlueRed and Violet

WHY DIDNT IT WORK?DIFFERENT ELECTRIC PROPERTIESDifferent colors Different sizes and manufacturersManufactured in different batches

COMPONENTS CHANGEChange in characteristics because of aging

HEATINGDifferent characteristicsHeat increases, Vo decreases.

2829COLORFREQUENCY (X1014 HZ)SLOPEY-INTERCEPTTurn-on voltage (V), experimentalTurn-on voltage (V), theoreticalRED-ORANGE4.900.03020.05351.7715232.026150ORANGE-BLUE6.530.00860.02172.5232562.700155ORANGE-VIOLET7.430.04360.12032.7591743.072305BLUE-VIOLET7.430.05440.1532.81253.072305

Slope (eV.s)3.9994x10-15y-intercept (eV)-1.6197x10-1TheoreticalPlancks Constant (eV.s)4.135x10-15PERCENT DEVIATION (%)3.2793%SOLUTIONSeparate resistors in parallelCalculate resistance

If goal is to light all the LEDs, connect in seriesNeeds higher working voltageNot the scope of the study

VCONCLUSIONCONCLUSIONDetermination of Plancks constant is still feasible with multiple LEDs in parallel

Only a certain combination of LEDs will light

The LED with the higher theoretical turn-on voltage should be usedRECOMMENDATIONSHigher voltage of power supply

More LEDs in parallel

More accurate measuring devices (more decimal places)

Account for differences (better experimental procedure)

Explore how the Plancks constant will be determined by connecting in seriesQUESTIONS?