If you strip away the heat sinks and optics to ask what is an LED made of, the answer lies in a highly engineered stack of compound semiconductors. At its core, a modern white LED consists of an Indium Gallium Nitride (InGaN) epitaxial die mounted on a sapphire or silicon carbide substrate, coated with a YAG (Yttrium Aluminum Garnet) phosphor layer, and encapsulated in a silicone resin. But for electrical designers and installers, knowing the physical makeup of an LED is not just academic trivia. The specific thermal and electrical properties of these materials directly dictate your constant-current driver selection, inrush current calculations, and dimmer compatibility.
The Semiconductor Stack and Circuit Impact
Unlike incandescent filaments that act as simple resistive loads, the materials inside an LED create a highly non-linear voltage-current (V-I) curve. A minor voltage fluctuation across the InGaN die causes a massive spike in current, which is why LEDs require constant-current (CC) drivers rather than simple voltage transformers. Furthermore, the physical substrate used to grow the semiconductor crystal introduces a severe thermal bottleneck that impacts circuit protection.
| Component Layer | Primary Material | Electrical / Thermal Property | Circuit Design Impact |
|---|---|---|---|
| Epitaxial Die | InGaN / GaN | Non-linear V-I curve; ~2.8V to 3.4V forward voltage | Mandates constant-current (CC) drivers to prevent thermal runaway and die destruction. |
| Substrate | Sapphire (Al2O3) | Poor thermal conductivity (~35 W/m·K) | Traps heat at the junction; requires aggressive thermal derating in enclosed fixtures. |
| Phosphor Layer | YAG:Ce (Yttrium Aluminum Garnet) | Stokes shift energy loss (~20% heat generation) | Shifts blue light to white; degrades faster at high junction temps, causing color shift. |
| Encapsulant | Silicone Resin | High optical clarity; UV resistant | Protects bond wires; yellowing over time indicates driver over-current or thermal failure. |
The sapphire substrate is the most critical material to understand from a thermal perspective. While sapphire provides the correct crystalline lattice structure to grow the GaN die, its thermal conductivity is roughly 10 times worse than copper. This means heat generated at the semiconductor junction struggles to escape into the PCB, making the LED highly susceptible to thermal runaway if the driver pushes too much current.
Lumens, Watts, and Circuit Impact Math
When sizing branch circuits for LED lighting, you cannot rely on old incandescent rules of thumb. Modern LEDs achieve vastly higher efficacy, but the internal switching power supplies (drivers) introduce complex power factor and inrush characteristics that dictate breaker and wire sizing.
| Target Lumens | Incandescent Wattage | Modern LED Wattage (2026 Efficacy) | Efficacy (lm/W) | Apparent Power at 0.7 PF (VA) |
|---|---|---|---|---|
| 800 lm | 60W | 7.5W | 106 lm/W | 10.7 VA |
| 1100 lm | 75W | 10W | 110 lm/W | 14.3 VA |
| 1600 lm | 100W | 14W | 114 lm/W | 20.0 VA |
| 2600 lm | 150W | 22W | 118 lm/W | 31.4 VA |
Note: Efficacy figures reflect high-quality commercial COB and SMD packages. Cheap consumer bulbs often drop to 80-90 lm/W due to poor thermal management.
Inrush Current and Power Factor (PF) Calculations
Let us run the circuit math for a kitchen remodel using ten 14W LED downlights (1600 lumens each) on a standard 120V AC nominal branch circuit.
- Steady-State Current: 10 fixtures × 14W = 140W. At 120V, this is just 1.16 Amps. A 15A breaker seems vastly oversized.
- Power Factor (PF) Impact: Low-cost LED drivers often have a PF of 0.6 to 0.7. Apparent power (VA) = Real Power (W) / PF. 140W / 0.7 = 200VA. The circuit must carry 200VA / 120V = 1.66 Amps. Wire sizing must always be based on VA, not just Watts.
- Inrush Current: LED drivers use large electrolytic capacitors on the DC bus. When you flip the switch, these capacitors look like a dead short for the first 100 microseconds. A single 14W fixture can pull 80A to 120A of inrush current. Ten fixtures switched simultaneously can spike to >1000A.
Dimmer Compatibility, Minimum Loads, and Flicker
Because LEDs draw so little steady-state current, they break the physics of traditional phase-cut dimmers. Understanding which dimmer and driver to use depends entirely on your fixture count and the minimum load requirements of the switch.
Which Dimmer/Driver for This Fixture Count?
- 1 to 4 Fixtures (Low Load): Use a trailing-edge (ELV) electronic dimmer. Trailing-edge dimmers use MOSFETs or IGBTs that do not require a minimum holding current to stay latched. Pair these with dimmable constant-current drivers that support ELV phase-cut.
- 5 to 15 Fixtures (Medium Load): A high-quality leading-edge (Triac) dimmer rated specifically for LED loads (like the Lutron Diva DVCL-153P) will work, provided you meet the minimum load requirement.
- 16+ Fixtures or Commercial Spaces: Abandon phase-cut dimming entirely. Use a 0-10V or DALI constant-current driver. This separates the low-voltage control signal from the mains power, eliminating inrush damage and flicker entirely.
Why Flicker Happens and the Fix
Flicker in LED circuits usually happens because of a mismatch between the dimmer's minimum load requirement and the LED's actual draw. A standard Triac-based leading-edge dimmer requires a minimum current (often 15W to 25W) to keep the internal semiconductor latched 'on' during the AC cycle. If you connect only two 7W LEDs (14W total), the current drops below the holding threshold halfway through the sine wave. The Triac shuts off prematurely, the driver's capacitor recharges, the voltage spikes, and the Triac fires erratically. This manifests as a visible strobe or flicker.
The Fix: If you are locked into a phase-cut dimmer and a low fixture count, install a bypass resistor (such as the Lutron LUT-MLC) across the line and load terminals at the first fixture. This bleeds a few milliamps of dummy current to keep the dimmer latched. For a permanent, code-compliant fix in new construction, upgrade to 0-10V dimming.
Heat, Enclosures, and Thermal Derating
Returning to the physical materials: because the sapphire substrate traps heat at the junction, the LED's lifespan is entirely dependent on the junction temperature (Tj). Most commercial InGaN dies are rated for a maximum Tj of 85°C or 105°C. For every 10°C increase in junction temperature above the nominal design point, the LED's lumen output degrades twice as fast (the Arrhenius equation in practice).
This material reality creates strict enclosure constraints. If you install an LED retrofit bulb into an IC-rated (Insulation Contact) airtight recessed can, the ambient temperature inside the enclosure can easily reach 50°C to 60°C. The driver's electrolytic capacitors will dry out, and the sapphire substrate will fail to shed heat.
Derating Math: If a fixture is rated for 1000 lumens at 25°C ambient, installing it in an enclosed, insulated housing requires you to derate the driver current. A common rule of thumb for enclosed fixtures is to reduce the driver output current by 20% to 30%. If your driver is adjustable via DIP switches or a potentiometer, drop the output from 700mA to 500mA. You will lose roughly 15% of your total lumens, but you will extend the L70 lifespan of the phosphor and die from 25,000 hours to over 50,000 hours.
Ultimately, understanding what an LED is made of bridges the gap between component-level physics and jobsite electrical practice. The GaN die demands constant current, the sapphire substrate demands thermal management, and the low overall wattage demands precise dimmer matching. Respect the materials, and the circuit will perform flawlessly.
