Diffusion profile measurements in the base of a microwave transistor

A technique is presented for the measurement of the diffusion profile in the active base region of a microwave transistor. This profile is a very important parameter since it determines to a large degree the power gain, noise factor, and transition frequency. Previous profiling techniques have severe limitations for measurements in the active base. Thin layers of silicon are removed by the growth of an anodic oxide; MOS C(V) measurements are made after each oxidation. After each measurement, the oxide is removed, a new oxide grown, and another MOS C(V) measurement made The process is repeated a number of times. For each MOS C(V) curve the maximum width of the space charge layer, Wm, is determined. An experimental plot is made of Wmvs. Xx, the total thickness of silicon removed. The base impurity profile is found by calculating, for a series of theoretical profiles, the curve Wm(Xs,). The profile whose Wm(Xs) curve matches the experimental curve is taken as the correct profile.