Bipolar n-p-n transistor geometrical parameters are optimized based on the principle of minimizing the perimeter-to-area ratio (P/A). Three types of radiation-resistant n-p-n transistors are developed and fabricated in the 20 V bipolar process. The first is emitter-base junction hardened n-p-n transistor. The second has heavily boron doped base ring. And the last uses both radiation-resistant measurements. The experimental results indicate that after irradiated by the radiation of total dose of 1 kGy, in current gain, the common n-p-n(unhardened) transistor reduces about 60%65%, while the first two hardened n-p-n transistors increases 10%15%: the last hardened n-p-n transistors are 15%20% greater than the common n-p-n transistors in current gain.