ROHM Sensors User Manual

Sensors Sensors

Sensors

Emitters
(1) Infrared LEDs

(1) Principle

In the absence of an externally applied voltage, the P-N
junction of a diode will be at thermal equilibrium and the
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

D.

be V

940 to 950 nm, and the response time is approximately
1µs.
GaAlAs infrared chips designed for higher output are
shown in Figure 3. The emitted wavelength of the chip of
Figure 3 (a) is 940 to 950 nm and the output is approxi­mately 1.3 to 1.5 times that of the GaAs chip of Figure 2.

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 GaAs 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 V
rier falls to V

D - VF and electrons flow into the P layer and

F is applied, the potential bar-

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 :

∆E = hν

Eg

ν = c / λ thus

hc

λ =

Eg

where λ is the wavelength of the emitted light, E

1.24
Eg

103 (nm)

g is the

energy band gap (1.35 ev in the case of GaAs), ∆E is the
energy difference before and after recombination, h is
Planck’s constant, ν is the frequency of the emitted light,
and c is the speed of light. The result is a peak emission
wavelength for GaAs of 940 to 950 nm.
The structure of a GaAs infrared chip is shown in Figure

2. The P-N junction of the GaAs-doped silicon is formed
by LPE (liquid crystal growth method). The luminous effi­ciency of the chip is 8 to 16%, the emitted wavelength is

(2) Structures

The structures of infrared LEDs can be divided into the
two following types.

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Sensors Sensors

1. Cast type
The chip is mounted on a lead frame and then liquid resin
is poured into the mold and allowed to harden.

2. Molded type
This type is packaged by transfer molding.

Detectors
(1) Phototransistors

(1) Principle

As shown by the equivalent circuit in Figure 5 (a), a pho­totransistor can be thought of as a photodiode connected
to a normal silicon planar transistor.

Actual phototransistor structures are as shown in (b) of
Figure 5, and consist of a single type. The principle of op­eration is essentially amplification by an NPN transistor
of photo current generated by light irradiation.

C hFEIB

I

IB : Photo current of photodiode

FE : Transistor amplification of direct current

h
Like photodiodes, phototransistors are sensitive to a
wavelength of approximately 1100 nm.

(2) Structure

Like photodiodes, phototransistors are housed in the fol­lowing three types of packages :

1. Cast type

2. Mold type
These are explained in detail on the infrared LED page.

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