1/f noise and generation/recombination noise in SiGe HBTs on SOI

A study is made of 1/f and generation/recombination (GR) noise in silicon-on-insulator (SOI) silicon-germanium heterojunction bipolar transistors fabricated using selective growth of the Si collector and nonselective growth of the SiGe base and n-type low doped Si emitter. A range of devices is studied in which different etch processes are used for the field oxide and different temperatures for the selective collector epitaxy. The results obtained are interpreted using Gummel plot measurements and two-dimensional device simulations. The 1/f noise dominates the base current noise spectra at high biases while the GR noise is the main noise component at low biases, and the noise levels in all devices are much lower than reported in previous bulk devices. The 1/f noise in small-area devices shows a different base current dependence than in large area devices and this dependence correlates with a turnover of the collector characteristic at high biases. This turnover is also more marked in devices where the overlay of the polysilicon over the emitter window is large. The analysis of the 1/f noise in small-area devices has shown that the collector current turn over effect observed is due to the voltage drop across the interfacial oxide layer resistance. Device simulations show two different current crowding mechanisms, with crowding in the centre of the device for large overlays and crowding at the periphery for small overlays. Analysis of the 1/f noise results indicates that the 1/f noise in small-area devices has a signature consistent with transparency fluctuations in the interfacial oxide at the polysilicon/silicon interface and in large area devices a signature consistent with recombination at the oxide/silicon surface. The GR noise is visible because of the low values of 1/f noise obtained and is shown to be due to recombination at deep levels in the emitter/base depletion layer, possibly due to residual damage from the extrinsic base implant. The noise in SOI devices is found to be comparable to that in control bulk devices, indicating that the buried oxide in SOI devices does not degrade the noise.

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