Low-frequency and high-power transistors are widely used in power electronics, industrial control, and other fields. A core challenge lies in optimizing the internal electric field distribution to prevent breakdown. Breakdown is typically caused by electric field concentration leading to localized overheating or avalanche effects, especially under high-voltage and high-current conditions. Through structural optimization, material improvement, and process control, the electric field can be effectively dispersed and local peak values reduced, thereby improving the device's breakdown voltage and reliability.
The root cause of electric field concentration lies in abrupt changes in the transistor's internal geometry or material inhomogeneities. For example, a small radius of curvature in PN junctions such as the collector and emitter junctions can lead to dense electric field lines, forming localized high field strengths. To address this issue, field ring technology can be used, which involves introducing a floating field ring around the main junction. The field ring alters the electric field distribution near the main junction, increasing the radius of curvature of the curved junction and suppressing surface electric field concentration. As the depletion region of the main junction expands, the potential gradient of the field ring guides the electric field lines to a more uniform distribution, thereby reducing localized peak electric fields and increasing the breakdown voltage.
Material selection is equally crucial for optimizing the electric field distribution. As a critical region of transistors, the epitaxial layer's doping concentration and thickness directly affect the electric field distribution. For example, in high-voltage devices, using a low-doped epitaxial layer can expand the depletion region width and disperse the electric field intensity. Simultaneously, multilayer composite epitaxial structures, such as N-N-N or P-N-P structures, can further optimize the electric field distribution. This structure introduces additional electric field peaks, reducing the main electric field peak and simultaneously expanding the longitudinal depletion region width, thereby improving the device's longitudinal breakdown voltage.
Base region design is another crucial aspect of optimizing the electric field distribution. Appropriate selection of base region width and doping concentration can balance current amplification and breakdown voltage. A wider base region can reduce the base region transport coefficient but enhance the uniformity of the electric field distribution and reduce local hot spots. Furthermore, forming a thick base region through deep diffusion processes can increase the emitter perimeter, resulting in a more uniform current distribution and preventing electric field overheating caused by current concentration. Simultaneously, controlling the surface impurity concentration of the base region can optimize the surface electric field distribution and reduce the risk of surface breakdown.
Emitter structure optimization also significantly improves the electric field distribution. Emitter regions employing porous or mesh-like designs can increase emitter perimeter, enhancing current carrying capacity while dispersing electric field intensity. Furthermore, deep junction diffusion processes in the emitter region, such as phosphorus pre-deposition followed by long-term diffusion, can create a uniform impurity distribution, reducing electric field concentration caused by impurity concentration gradients. Rounding or beveling at the emitter edge can also reduce the density of electric field lines, improving edge breakdown voltage.
Process control is crucial for ensuring optimized electric field distribution. The precision of diffusion, photolithography, and etching processes directly affects the device's geometry and impurity distribution. For example, the exposure dose and development time in photolithography must be precisely controlled to ensure the spacing between the field ring and the main junction meets design requirements. Anisotropic control in etching processes can prevent sidewall damage and reduce electric field concentration caused by surface defects. Additionally, cleaning and passivation processes can reduce impurity contamination and surface states, optimizing the surface electric field distribution.
Optimization of low-frequency and high-power transistors requires comprehensive consideration of the synergy of structure, materials, and processes. By employing field-loop technology, multilayer epitaxial structures, optimized design of the base and emitter regions, and high-precision process control, the internal electric field can be effectively dispersed, reducing local peak values and thus preventing breakdown. These measures not only improve the device's withstand voltage but also enhance its stability under harsh conditions such as high current and high temperature, ensuring the reliable operation of power electronic systems.
