AN INVESTIGATION OF THE INCONSISTENCY IN BARRIER HEIGHTS FOR PNP AND NPN POLYSILICON EMITTER BIPOLAR-TRANSISTORS USING A NEW TUNNELING MODEL

A new tunneling model is proposed for polysilicon emitter bipolar tran sistors in which the interfacial layer is modeled as a wide band-gap s emiconductor. This model predicts that potential barriers are formed i n both the conduction and valence bands. The shapes and sizes of the b arriers are dependent upon the amount of band-bending within the inter facial layer, which is in turn dependent on the amount of dopant segre gation in the interfacial layer. Measurements of base current and emit ter resistance are made on pnp polysilicon emitter bipolar transistors with deliberately grown interfacial oxide layers. These electrical ch aracteristics are then modeled using the new heterojunction tunneling model, from which values of electron and hole barrier heights are extr acted. Specifically, a hole barrier height of 0. 31 +/- 0.02 eV is obt ained, and an electron barrier height of either > 0. 68 +/- 0.08 eV or > 0.44 +/- 0.06 eV, with the value depending on the band-gap narrowin g model used in the calculation. These values of electron and hole bar rier height are shown to be different than the values obtained when si milar measurements are made on npn devices. The new heterojunction tun neling model is used to model these devices, and it is shown that it i s capable of fully explaining this discrepancy between the values of b arrier height obtained from measurements on npn and pnp transistors.