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High-speed, oxide-confined, polyimide-planarized 850 nm vertical-cavity surface emitting lasers (VCSELs) with different aperture sizes were fabricated and characterized. Comprehensive small signal measurements and analysis were conducted. The VCSELs exhibit intrinsic, parasitic and thermal maximum bandwidth limitations of 42.3 GHz, 21.5 GHz and 17.6 GHz, respectively. Devices with a 10 μm oxide aperture exhibited a maximum modulation bandwidth of 15.3 GHz, limited by thermal effects, and a modulation current efficiency factor (MCEF) of 9.2 GHz mA −1/2. A VCSEL equivalent circuit model which incorporates the frequency dependence of the polyimide dielectric permittivity and loss is presented. A genetic algorithm (GA) was utilized to extract the parasitic circuit elements from measured microwave reflection coefficients (S 11) over a frequency range of 50 MHz to 20 GHz for different device sizes at different bias currents. Good agreement between extracted and calculated parasitic circuit element values was obtained. Several modifications to the device's fabrication steps and structure are suggested to further improve the device's output power and modulation bandwidth.