Modelling of bonding pads and their effect on the high-frequency-noise figure of polysilicon emitter bipolar junction transistors

This paper presents detailed experimental results on the input impedance of bonding pads, and a simple electrical model that accurately describes the impedances of these pads as a function of frequency for five different types of pad structures. It also describes the effects of the bonding pads on the minimum high-frequency-noise figure (NF MIN ) of polysilicon emitter npn bipolar junction transistors as functions of collector current density and emitter areas. It was found that for devices with larger emitter areas (A E > 48 μm 2 ) and relatively low base resistance (R B < 30 Ω), the effects of the same bonding pads on the noise figure was not as pronounced as for the smaller area devices with larger base resistances. For a device with A E = 3.2 μm 2 operating at 1 GHz, biased with a collector density of 0.15 mA μm −2 , neglect of the effects of the bonding pads results in too low NF MIN (by 1–2 dB) when calculated values were compared to measurements. Finally, for devices with the same emitter areas, bonding pads with smaller impedance results in a larger NF MIN compared to measurements on similar transistors with pads of larger impedances.