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Electric fields drive the degradation of wide-bandgap semiconductor devices. However, directly mapping electric field inside an active device region remains challenging. An electric-field-induced second harmonic generation (EFISHG) technique has been developed to map the electric field in the active region of gallium nitride (GaN) based devices at a sub-micron resolution. To illustrate the capabilities of the approach, quantitative electric field measurements have been performed in different GaN based devices. In GaN based high-electron-mobility transistors (HEMTs), the impact of carbon impurity in the epitaxial buffer layer of the device has been examined. Carbon is a p-dopant in GaN and small changes in its concentration can dramatically change the bulk Fermi level, sometimes resulting in a floating buffer that is "short-circuited" to the device channel via dislocations. The measurements show that, despite similar device terminal characteristics, very different electric field distributions can occur in devices with different carbon concentration. It is also shown that dislocation related leakage paths can lead to inhomogeneity in the electric field. Device design and manufacturing of vertical devices requires edge termination to manage peak electric fields, but validation of its effectiveness is presently rather indirect. The lateral electric field distribution of GaN-on-GaN p-n diodes with partially compensated ion-implanted edge termination (ET) has been characterized. The distributed electric field demonstrates the effectiveness of the ET structure. However, its effectiveness is strongly dependent on the acceptor charge distribution in the ET layer. A generally lower amount of acceptor charge can be inferred from the measured electric field distribution resulting from excessive ion implantation energy or dose during ET fabrication, causing lower than optimal breakdown voltage. Localized field crowding can be observed, when the remaining acceptors uncompensated by the implant in the PC layer are nonuniformly distributed around the periphery of the devices. The vertical electric field distribution of GaN p-n diodes with ion-implanted two-step bevel ET has been characterized. A series of fabrication process parameters affects the ultimate effectiveness of this complicated edge termination structure including net Mg acceptor concentration in p+ GaN, residual density of donor-like damages induced by dry etching and ion implantation dose to compensate the etching damages. With direct electric field characterization, the effect of those device internal characteristics can be clearly inferred and feedback can be given for improved device development and manufacturing. A scheme of structure design and fabrication process optimization has been proposed based on electric field measurement. The quantification of electric field by EFISHG technique relies on the absence of interference between fundamental SHG and EFISHG waves in GaN with a backside measurement geometry. This allows the total SHG signal to be proportional to the square of applied electric field and electric field can be quantified simply. An optical model is presented taking into consideration wave propagation, ultrashort pulsed laser and phase mismatch. A clear illustration has been given showing that how the backside measurement eliminates the interference.