LIGHT-EMITTING DIODES (LED)

The increasing use of digital displays in calculators, watches, and all forms of instrumentation

has contributed to the current extensive interest in structures that will

emit light when properly biased. The two types in common use today to perform this

function are the light-emitting diode (LED) and the liquid-crystal display (LCD). Since

the LED falls within the family of p-n junction devices and will appear in some of the networks in the next few chapters, it will be introduced in this chapter. 

As the name implies, the light-emitting diode (LED) is a diode that will give off

visible light when it is energized. In any forward-biased p-n junction there is, within

the structure and primarily close to the junction, a recombination of holes and electrons.

This recombination requires that the energy possessed by the unbound free electron

be transferred to another state. In all semiconductor p-n junctions some of this

energy will be given off as heat and some in the form of photons. In silicon and germanium

the greater percentage is given up in the form of heat and the emitted light

is insignificant. In other materials, such as gallium arsenide phosphide (GaAsP) or

gallium phosphide (GaP), the number of photons of light energy emitted is sufficient

to create a very visible light source.

 The process of giving off light by applying an electrical source of energy is

called electroluminescence.

As shown in Fig. 1 with its graphic symbol, the conducting surface connected

to the p-material is much smaller, to permit the emergence of the maximum number

of photons of light energy. Note in the figure that the recombination of the injected

carriers due to the forward-biased junction results in emitted light at the site of recombination.
There may, of course, be some absorption of the packages of photon energy
in the structure itself, but a very large percentage are able to leave, as shown in the figure.

Figure 1 (a) Process of

electroluminescence in the LED;

(b) graphic symbol.

The appearance and characteristics of a subminiature high-efficiency solid-state
lamp manufactured by Hewlett-Packard appears in Fig. 1.55. Note in Fig. 1.55b that
the peak forward current is 60 mA, with 20 mA the typical average forward current.
The test conditions listed in Fig. 1.55c, however, are for a forward current of 10 mA.
The level of VD under forward-bias conditions is listed as VF and extends from 2.2
to 3 V. In other words, one can expect a typical operating current of about 10 mA at
2.5 V for good light emission.

Two quantities yet undefined appear under the heading Electrical/Optical Characteristics
at TA  25°C. They are the axial luminous intensity (IV) and the luminous
efficacy (v). Light intensity is measured in candela. One candela emits a light flux
of 4 lumens and establishes an illumination of 1 footcandle on a 1-ft2 area 1 ft from
the light source. Even though this description may not provide a clear understanding
of the candela as a unit of measure, its level can certainly be compared between similar
devices. The term efficacy is, by definition, a measure of the ability of a device
to produce a desired effect. For the LED this is the ratio of the number of lumens
generated per applied watt of electrical energy. The relative efficiency is defined by the luminous intensity per unit current, as shown in Fig. 1.55g. The relative intensity
of each color versus wavelength appears in Fig. 1.55d.
Since the LED is a p-n junction device, it will have a forward-biased characteristic
(Fig. 1.55e) similar to the diode response curves. Note the almost linear increase in relative
luminous intensity with forward current (Fig. 1.55f). Figure 1.55h reveals that the
longer the pulse duration at a particular frequency, the lower the permitted peak current
(after you pass the break value of tp). Figure 1.55i simply reveals that the intensity is
greater at 0° (or head on) and the least at 90° (when you view the device from the side).

Figure 1.55 Hewlett-Packard subminiature high-efficiency red solid-state lamp: (a) appearance;

(b) absolute maximum ratings; (c) electrical/optical characteristics; (d) relative intensity versus wavelength;

(e) forward current versus forward voltage; (f) relative luminous intensity versus forward current;

(g) relative efficiency versus peak current; (h) maximum peak current versus pulse duration;

(i) relative luminous intensity versus angular displacement. (Courtesy Hewlett-Packard Corporation.)

Figure 1.55 Continued.

LED displays are available today in many different sizes and shapes. The lightemitting

region is available in lengths from 0.1 to 1 in. Numbers can be created by

segments such as shown in Fig. 1.56. By applying a forward bias to the proper p-type

material segment, any number from 0 to 9 can be displayed.

Figure 1.56 Litronix segment display.

There are also two-lead LED lamps that contain two LEDs, so that a reversal in

biasing will change the color from green to red, or vice versa. LEDs are presently

available in red, green, yellow, orange, and white, and white with blue soon to be

commercially available. In general, LEDs operate at voltage levels from 1.7 to 3.3 V,

which makes them completely compatible with solid-state circuits. They have a fast

response time (nanoseconds) and offer good contrast ratios for visibility. The power

requirement is typically from 10 to 150 mW with a lifetime of 100,000 hours. Their

semiconductor construction adds a significant ruggedness factor.