Integration & Applications

The previous three chapters treated the LED, laser diode, and photodiode as individual devices – in practice, however, they almost never work in isolation but as links in a transmission chain: transmitter (LED or laser diode) → transmission medium (optical fiber or free space) → receiver (photodiode). This chain clarifies why material choice in the previous chapters was so tightly coupled: an InGaAsP DFB laser at 1.55 µm only yields a working system if an InGaAs photodiode with matching responsivity sits at the other end, in exactly this wavelength window – transmitter and receiver aren't a random material pairing but a coordinated system component.

In system design, the efficiencies and losses of each stage add up: the transmitter's wall-plug efficiency, the fiber coupling efficiency (typically the lossiest step for edge emitters, one reason for the growing use of VCSELs with their round, easily coupled beam profile), fiber attenuation over the link length, and finally the photodiode's receiver sensitivity. The power budget (link budget) – transmit power minus all losses – must ultimately exceed the photodiode's minimum detectable power; if the margin isn't sufficient, intermediate amplifiers (optical fiber amplifiers) or more sensitive APD receivers are used.

In display technology, LEDs serve in several, technically quite different roles. In conventional LCD screens, a row of white InGaN LEDs (see Chapter 2) merely serves as backlighting – the actual image is formed by downstream liquid crystal cells that transmit or block the LED light pixel by pixel. In OLED displays, by contrast, each pixel is itself a light emitter: organic semiconductor layers emit light directly, substantially improving contrast (true black by fully switching off individual pixels) and viewing-angle stability – though at the cost of maximum brightness and long-term stability compared to inorganic semiconductors.

µLED displays pursue a third approach: instead of organic emitters, tiny inorganic InGaN/AlGaInP LED chips (edge length often below 50 µm) are used directly as individually addressable pixels – the same semiconductor technology from Chapter 2, just massively miniaturized and transferred in extremely high volumes onto a carrier substrate ("mass transfer," one of the biggest manufacturing bottlenecks of this technology). µLEDs thereby combine the contrast advantages of self-emitting pixels with the brightness, lifetime, and material stability of inorganic III-V semiconductors, but due to manufacturing complexity are currently still limited to premium applications and large display diagonals.