The current modulation of a two-section semiconductor laser is first reviewed analytically using a well-known, closed-form, modulation expression. A system of traveling-intensity equations is then used to investigate spatial effects in these lasers including cavity layout and the role played by cavity length. The numerical simulations verify the accuracy of the analytic expression for short cavities (low frequencies) but identify shortcomings as the cavity length (modulation frequency) is increased. One notable difference is the presence of resonant peaks in the modulation response. Although this effect has been addressed in the past, the arrangement of sections within the laser is shown to play a prominent role in these monolithic devices for what we believe to be the first time. In the course of this investigation the thirteen different ways a two-section semiconductor laser can be current modulated are identified and computationally investigated.