Extrinsic electrical, thermal, and optical issues rather than intrinsic factors currently constrain the maximum bandwidth
of directly modulated vertical cavity surface emitting lasers (VCSELs). Intrinsic limits based on resonance frequency,
damping, and K-factor analysis are summarized. Previous reports are used to compare parasitic circuit values and
electrical 3dB bandwidths and thermal resistances. A correlation between multimode operation and junction heating
with bandwidth saturation is presented. The extrinsic factors motivate modified bottom-emitting structures with no
electrical pads, small mesas, copper plated heatsinks, and uniform current injection. Selected results on high speed
quantum well and quantum dot VCSELs at 850 nm, 980 nm, and 1070 nm are reviewed including small-signal 3dB
frequencies up to 21.5 GHz and bit rates up to 30 Gb/s.