PURPOSE: To improve the electric characteristics of a semiconductor element by shortening the base forming step converting a forming method for the base of a bipolar transistor.

CONSTITUTION: A P type impurity diffusion is performed at relatively low temperature such as 850∼1,000°C, a glass layer formed in the diffusing step is then removed, a product is inserted into a core tube of a furnace of low temperature state of 700∼850°C, and the furnace temperature is raised at the rate of 1∼ 10°C/min. Thereafter, H2-O2 burning at 900∼1,050°C or oxidation of 5∼50min with dry O2 is performed for the purpose of controlling the layer resistance. Subsequently, the furnace temperature is raised at the rate of 1∼10°C/min, inert gas such as N2 is fed at high temperature of 1,100∼1,250°C for 10∼900min, thereby deciding the diffusion depth. Thereafter, the furnace temperature is lowered at the rate of 1∼10°C, the oxidation is performed with H2-O2 burning at 900∼1,050°C for 5∼300min, thereby forming oxidation becoming the diffusion mask of the next step, and then the furnace temperature is lowered at the rate of 1∼10°C, thereby producing a product from the furnace.