Explain how the properties of a semiconductor can prevent an LED from lighting up in a circuit.

An LED can fail to light up due to three key semiconductor properties: incorrect polarity (reverse bias), insufficient forward voltage to overcome the band gap, or excessive reverse voltage causing breakdown.

Step-by-Step Solution:
In reverse bias, electrons cannot cross p-n junction as depletion layer widens, preventing current flow
Forward voltage must exceed band gap energy (typically 1.5-3.0V) to enable electron-hole recombination
Forward current must stay within rated limits to prevent thermal damage
Quick check: LED polarity (longer lead = anode/+), voltage >1.5V, current limiting resistor present

Key Mistake to Avoid:
Students often forget that LEDs are diodes first - they must check polarity and include appropriate current-limiting resistors.

Quick Tip:
Remember "LED = Light Emitting DIODE" - always apply diode rules first before considering light emission properties.