Charge storage in the polysilicon emitter

The effect of excess charge storage on the dynamic behavior of the polysilicon emitter has been studied. The excess charge is a manifestation of minority-carrier confinement, resulting in a lower base current and therefore improved gain. The diode reverse recovery technique was used in the study. This technique is based on the fact that the initial phase during the reverse current transient of a diode is a function of the excess minority carriers stored inside the diode. The devices were fabricated using n-p⁺ epitaxial wafers and the contact to the n-side was made either by diffusion from n⁺ (As) polysilicon or by direct implantation into single-crystal silicon. The structures therefore simulate the emitter-base junction of an n-p-n bipolar transistor with the base more heavily doped than the emitter. The storage phenomenon in the polysilicon contact diodes has been modeled by solving the time-dependent continuity equation. A closed-form analytical solution has been obtained, allowing the first extraction of the surface recombination velocity of polysilicon contact from the transient data. It was found that: (1) there is indeed a considerable increase in the excess stored charge in polysilicon emitter; (2) the interfacial native oxide cannot account for the observed behavior; (3) all the observations agree with the predictions of the thermionic-diffusion model; and (4) the measured storage times can be explained quantitatively by the above model