We present a comparative study of lateral and L-shaped PN junction Si optical phase shifters for Mach–Zehnder modulators in Si photonics based on TCAD simulation. First, we introduce an easy fabrication method for L-shaped PN junctions by inverting dominant dopant type in Si. Then, we present the quantitative comparison of Si optical phase shifters. The L-shaped PN junction device shows the larger modulation efficiency of VpiL and the lower optical phase shifter loss compared to the lateral one. The VpiL for the vertical one is 0.89 Vcm, nearly half that of the lateral one, 1.76 Vcm at the same optical phase shifter loss, 10.5 dB cm−1. Finally, taking advantage of single-drive configuration and optimizing input characteristic impedance, the large-signal simulation with 1 mW input power and a 500 μm phase shifter shows the dynamic optical modulation amplitude of 0.22 mW for the vertical one, which could be a promising solution for a compact device footprint without a traveling wave electrode.