3 dB frequency

The 3 dB frequency of an LED can be determined by measuring the frequency at which the detector signal decreases to one-half of the low-frequency value. The frequency response of the detector needs to be taken into account in the measurement. A 3 dB frequency of about 500 MHz was determined for the LED for which the rise and fall times are shown in Fig. 24.2. Comparison of the 3 dB frequencies for different LED structures revealed that the highest 3 dB frequencies are attained with devices having low parasitic resistances and capacitances. This can be achieved by a small p-type contact area, a thick SiO2 isolation layer on the p-type side of the device, a small area of the bonding pad on the p-type side of the device, and a mesa etch limiting the p-n junction depletion capacitance at zero bias.

Fig. 24.6. “Eye diagram” of the received optical signal of an RCLED. The optical signal is measured as the photocur­rent of a Si photodiode mea­sured with a sampling oscillo­scope. The data rate of the RCLED is 622 Mbit/s. Also indicated are the three current levels for the “0” state (/Q), “1” state (/j), and decision current (/D).

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