ROHM Sensors Data Manual/Specification Sheet

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Emitters 940 to 950 nm, and the response time is approximately 1µs.

(1) Infrared LEDs

Ga Al As infrared chips designed for higher output are

(1) Principle

shown in Figure 3. The emitted wavelength of the chip of

In the absence of an externally applied voltage, the P-N

Figure 3 (a) is 940 to 950 nm and the output is approxi-

junction of a diode will be at thermal equilibrium and the

mately 1.3 to 1.5 times that of the Ga As chip of Figure 2.

Fermi levels of the P layer and N layer will be equal (Fig. 1 (a)). In this case, the height of the potential barrier will be VD.

The chip of Figure 3 (b) is a so-called N-side up chip with an emission wavelength of 880 nm. It produces a high output, approximately 1.5 times that of the Ga As chip of Figure 2. The response times of both chips of Figure 3 are approximately 1µs, the same as the chip of Figure 2.

When an external voltage VF is applied, the potential bar- rier falls to VD - VF and electrons flow into the P layer and holes into the N layer. As carriers (holes and electrons) flow, they recombine, and at that time the difference in energy before and after recombination is released as light. The wavelength of the emitted light is given by the following equation Eg ∆E = hν ν = c λ thus hc 1.24 λ =
103 (nm) Eg Eg where λ is the wavelength of the emitted light, Eg is the energy band gap (1.35 ev in the case of Ga As), ∆E is the

energy difference before and after recombination, h is (2) Structures Planck’s constant, ν is the frequency of the emitted light, The structures of infrared LEDs can be divided into the and c is the speed of light. The result is a peak emission two following types.

wavelength for Ga As of 940 to 950 nm. The structure of a Ga As infrared chip is shown in Figure 2. The P-N junction of the Ga As-doped silicon is formed by LPE (liquid crystal growth method). The luminous effi- ciency of the chip is 8 to 16%, the emitted wavelength is