PREPARATIONOF SEMICONDUCTOR ELEMENT

PURPOSE:To prepare a bipolar transistor in a self-alignment manner by positioning a contact part and an emitter part by means of one mask and by making a window for diffusion or ion injection and a window for contact into an identical one. CONSTITUTION:On a semiconductor substrate 21, polycrystalline silicon 24 is made to grow and further a pattern 22 of a silicon nitride film is formed. Next, thermal oxidation is applied by using the pattern 22 as a mask and thereby a part of the polycrystalline silicon not covered with the pattern 22 is changed into an oxide film 23. Subsequently, the silicon nitride film 22-1 alone being left by a photolithographic method, impurity is diffused through the polycrystalline silicon 24-2 and thereby a side base layer 25 is formed. Then, an impurity ion is injected into the whole surface to form a main base layer 26, the silicon nitride film 22-1 is removed thereafter, and the impurity ion is injected only into the polycrystalline silicon 24-1 by using a mask material 27 as a mask. Lastly an emitter layer 28 is formed through the activating treatment and an electrode 29 is connected thereto, whereby the bipolar transistor is obtained.