What property of an LED's semiconductor material determines its forward voltage drop?

Explication

Current flows in an LED only once the applied voltage is enough to lift electrons across the semiconductor's band gap, so the forward drop follows that band gap — and so does the color, which is why red LEDs run near 1.8 V and blue ones near 3 V. Bulk resistance, junction capacitance, and junction depth affect other behavior but not the voltage the material demands.