We present an experimental equipment for studying the charge carrier distribution in the interior of bipolar 4H-SiC high power devices by means of laser absorption measurements. Since the light absorption coefficient in a semiconductor depends on the electron and hole concentration, the attenuation of a laser beam transmitted through a sample is an integral function of the local charge carrier density. In order to detect the tiny changes in the light intensity caused by the plasma-optical effect, a highly sensitive measurement set-up has been developed. Its crucial components are a low-noise blue laser and a high-speed and broad-band photo-diode amplifier circuit. Sample preparation is sophisticated and requires special care. We investigated charge carrier profiles in 4H-SiC pin-diodes in the high-injection regime at current densities between 175 A/cm² and 350 A/cm². The measured charge carrier profiles are in good agreement with computer simulations.

Two different optical measurement techniques have been combined in one single experimental platform to provide detailed insight into the interior of 4H-SiC bipolar devices with respect to their coupled electronic and thermal behavior: First, free carrier absorption (FCA) measurements yield time-resolved electron and hole densities profiles during turn-on and under stationary conditions; and second, light deflection measurements provide information about the gradients of the electron and hole densities as well as that of the temperature gradient. The full measurement process is also simulated on the computer as “virtual experiment” on the basis of high-fidelity physical device models. Investigations on high-blocking 4H-SiC bipolar diodes exemplify the optical probing methodology and the numerical simulation.