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Vertical-cavity surface-emitting lasers (VCSELs) are very important devices as the light source for a variety of optoelectronic applications, such as high-bit-rate light transmission, parallel optical signal processing, optical interconnections, optical recording, holographic based memories, and optical computing. Recently, the requirements of high-speed optical transceiver with VCSEL as a transmitter are increased for short distance data links and high speed optical communication. In order to cope with the change of market, the establishment of standard and development for the 10 Gbps ethernet transceiver that use in the next generation local area network are accelerated. Up to now, 850-nm VCSELs have shown the modulation bandwidths in excess of 20 GHz [ 11.However, it is difficult to obtain the high-speed modulation bandwidth of VCSEL even if the relaxation oscillation for single mode VCSEL is as high as 70 GHz. It is because the small signal modulation bandwidths of VCSELs are achieved from intrinsic modulation and extrinsic characteristics including parasitic parameters such as resistance and capacitance.In this letter, the modulation bandwidth characteristics of the oxide-confined post-type VCSELs and the effect of parasitic parameters will be reported. From the measured and calculated modulation spectra, the intrinsic modulation limits of VCSEL devices are extracted and compared with the trench-type VCSEL fabricated fiom same wafer. The high-speed modulation bandwidth of 13.4 GHz was obtained fiom the 850-nm oxide-confined VCSELs. Fig. 1 shows a schematic cross section of an oxide-confined VCSEL structure with a corresponding small-signal equivalent circuit [2]. Coplanar pads for on wafer probing were fabricated on a thick polyimide to reduce the capacitance between the pad and the substrate to approximately 0.13 pF. To analyze the effect of oxide-current aperture sizes on the modulation response and to optimize its size for high-frequency performance, VCSELs with various oxide aperture sizes were fabricated.The measurements of small signal modulation response are performed using an HP 85 10 network analyzer and 50-lrm multimode optical fiber connected at New Focus 25 GHz photodetector. Figure 2 shows the smoothed small-signal modulation responses of 8-pn VCSEL at several bias current levels. The modulation frequency is increased with increasing the bias current. The maximum 3dB-modulation frequency of 13.4 GHz was observed at 17 mA.As shown in Fig. 3, the maximum 3dB modulation frequencies of the each post-type VCSELs with the different oxide-current aperture size are increased with increasing its size. And the maximum modulation bandwidth was achieved at the VCSEL with 8-pn oxidecurrent aperture size. It is generally known the laser speed is almost determined by the device parasitic elements causing the saturation of the bandwidth before attaining the maximum frequency [3]. The parasitic capacitance is dominated by pad size on the p-DBR mirror, which is related with oxide area. That is, with the oxide-cur...

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