ANALYTICAL MODELING OF POLYCRYSTALLINE SILICON EMITTER BIPOLAR-TRANSISTORS UNDER HIGH-LEVEL INJECTION

We derived an analytical model, focused on injected charge storage, for electron current density J(n), hole current density J(ps), current gain beta, and forward transit time tau(f), for heavily doped base and emitter with non-uniform band structures. This model can apply in high-level injection regions before the onset of the Kirk effect. By comparing with those results obtained from Sukuki's low-level injection model, it is observed that a new high-level injection term, proportional to the square of the hole current density, appears in the injected hole concentration in the polycrystalline silicon emitter and this term vanishes in the limit of low-level injection. As bias increases, J(n) and J(ps) of high-level injection model are less than those of the low-level injection model. In addition. for low-level injection model, beta, tau(f), the base transit time tau(b), the crystalline silicon emitter transit time tau(se), and the polycrystalline silicon emitter transit time tau(pe) keep constant for all injection region. However, for high-level injection model, beta and tau(b) decreases as bias increases, while tau(se) and tau(pe) increase with increasing bias. Tau(f) reaches a minimum and then increases. We conclude that the high-level injection model offers significant information for device operations at various electron current levels.