The eye diagram allows one to estimate the overall performance of an optical communication system. The eye diagram is the receiver signal of a randomly generated digital signal. An eye diagram of an LED operating at 622 Mbit/s is shown in Fig. 24.6. A data rate of 622 Mbit/s is used in the well-known “synchronous optical network” (SONET) standard. The figure indicates the level of the “1” state and of the “0” state and of the decision level, i. e. the boundary between what is interpreted by the receiver as a “0” and “1”. The figure also reveals the “eye”. An “open eye” such as the one shown in the figure, allows for a low bit-error rate. The “eye” shown in Fig. 24.6 is wide open, indicating that low bit-error-rate data transmission is possible at that frequency using LEDs. The “on” and “off’ pulse-generator voltages of the diode were 1.4 and
1.1 V, respectively. A pulse-shaping RC circuit with R = 20 Q and C = 100 pF was used for the measurement. Minimizing parasitic elements (e. g. bond pad capacitance) and employment of an RC pulse shaping circuit should make transmission rates of 1 Gbit/s possible.
As the data rate is increased, the eye will close, i. e. the photocurrent of the “0” and “1” level cannot be clearly distinguished. This results in an increase in the bit-error rate.
24.5 Carrier lifetime and 3 dB frequency Shortening the minority carrier lifetime through either very high doping of the active region or deliberate introduction of deep traps will increase the maximum modulation frequency. Deep traps have a two-fold effect. Firstly, they reduce the minority carrier lifetime, thereby increasing the 3 dB frequency. Secondly, they reduce the emission intensity and increase the heat generated inside the LED. Very high concentrations of a shallow dopant, on the other hand, will shorten the carrier lifetime and often, but not necessarily, hurt the device efficiency (Ikeda et al., 1977).
We therefore analyze the effect of lifetime reduction on emission intensity and 3 dB frequency. According to Eq. (24.8), the 3 dB frequency of an LED depends on the radiative lifetime according to f dB = 31/2 / (2п t), where t-1 = Tr-1 + Tnr-1. The internal device efficiency is given by nint = Tnr / (тг + Tnr). In the limit of small non-radiative lifetimes, f3 dB к Tnr-1 and Пій к Tnr. Thus, although the modulation bandwidth can be increased by the deliberate introduction of deep traps, the power-bandwidth product cannot. The relation between 3 dB frequency, output power, and radiative and non-radiative lifetime is shown in Fig. 24.7. The 3 dB frequencies and intensity levels are calculated according to the equations stated above. At frequencies much higher than the 3 dB frequency, a linear decrease of the optical intensity on the log-log scale can be assumed (Wood, 1994).
If, however, one were to succeed in decreasing the minority carrier lifetime without affecting the efficiency, the modulation speed of an LED could be increased. Although modulation rates as high as 1.7 GHz have been demonstrated for devices having a highly doped active region, the increase in bandwidth has been accompanied by a decrease in device efficiency (Chen et al., 1999).
Chen et al. (1999) proposed very high Be doping (NA = 2 x 1019 to 7 x 1019 cm-3) of a GaAs active region to shorten the lifetime without significantly degrading the internal quantum
efficiency. For a test device that was doped at 2 x 1019 cm-3, the authors found a cutoff frequency of 440 MHz and an internal quantum efficiency of 25-30%. The authors found a cutoff frequency of 1.7 GHz and an internal quantum efficiency of 10% for a device that was doped at 7 x 1019 cm-3.