Page 111 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

Light-Emitting Diodes 105

proportional to (n p + p n ), as shown in Chapter 3. In order to main-
tain a steady-state value of the minority carrier concentration, the re-
combination rate is also proportional to (n p + p n ), and thus propor-
tional to the current.
The excess carrier density is localized near the p-n junction and
falls off exponentially away from the junction: n p (x) = n p (0)e –(x/L e ) ,
where L e = D e and = the recombination time.
Since light emission is caused by recombination, the light intensity
is proportional to the radiative recombination rate. The current in the
diode is proportional to the total recombination rate. If radiative re-
combination dominates, then it follows that the light intensity is lin-
early proportional to the current. In experiments, the light intensity
is seen to be linearly proportional to the current over some range. In
Fig. 6.2 we show some results measured in the laboratory for an inex-
pensive visible red LED. The light–current characteristic is linear up
to about 80 mA, after which the intensity appears to saturate. As the
current increases, the radiative recombination rate stays relatively

Figure 6.2. A basic characterization measurement for a LED is the light–current char-
acteristic. This measurement shows the region where the light intensity is proportional
to the forward current. The saturation observed here is a general feature of all LEDs,
and has its origin in the relative resistance of the active region of the LED and the sur-
rounding contact regions. At even higher levels of current, ohmic heating becomes im-
portant, and this causes the light intensity to decrease.

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