Origin of 1/f noise in lateral PNP bipolar transistors

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Abstract

1/f noise was measured on lateral bipolar PNP transistors over a temperature range of 220<T<450 K. Noise power spectral density measurements were performed simultaneously across two resistors connected in series with base and collector. The equivalent base current noise source SIB has two dominant components. One is SIBE that is between the base and the emitter, in parallel with rπ. The other is SIBC coming from the surface recombination current at the neutral base, between the base and the collector. The extracted SIB exhibited a near square law dependence on base current IB. The noise remained nearly constant when the temperature was below 310 K. However, it presented strong temperature dependence when the temperature was beyond 310 K. Two different models are proposed for the noise in different temperature regions. For the high temperature region, the surface recombination velocity fluctuation model is proposed, which indicates that the noise is coming from the fluctuations in the surface recombination velocity at the neutral base surface. The tunneling assistant trapping model is responsible for the low temperature region, where the noise source is the carrier trapping–detrapping by the defects in the spacer oxide covering the surface of the depletion layer.

Introduction

There has been a renewed interest in lateral bipolar junction transistors (LBJTs) since the advancements in BiCMOS technologies. High gain LBJTs as well as lateral insulated gate bipolar transistors (LIGBT) have been developed [1], [2], [3]. As with other analog devices, the operation bandwidth and signal purity of LBJTs are restricted by their low-frequency noise. Since LBJTs are inherently surface conduction devices, the effects of surface states on current and noise are similar to that of MOSFETs. Some important results on 1/f noise exhibited by thin-film-silicon-on-insulator (TFSOI) BJTs have been reported before [4]. Yet, very little exists in the recent literature on measurements, analysis and modeling of low-frequency noise in advanced lateral PNP BJTs. In order to investigate the physical origin of these fluctuations, we performed variable temperature measurements on PNP LBJTs. In addition, a novel method of noise measurements was introduced where the cross-correlation noise power spectral density is measured between the base and the collector currents yielding more accurate information about the different fluctuation sources.

Section snippets

DC characteristics of the device under test

Since the base is lightly doped, the LBJT can be biased into the high-level injection at relatively low bias voltages. The high-level injection condition can be defined as the condition where the injected minority carrier density is comparable to or larger than the background doping density ND. This happens when the base–emitter forward bias |VBE| approaches the value [5],VHI=2kBTqlnNDniHere, kB is the Boltzmann constant, T is the absolute temperature, ND is the base doping, and ni is the

Experimental results

In our experiments, the emitter diameter of the lateral BJT is 7 μm. The emitter edge to collector distance is 30 μm. The cross-section of lateral BJT is presented in Fig. 2. The equivalent hybrid-π model for the variable temperature noise measurement is shown in Fig. 3 [7]. The noise signals measured from resistances RS=500 KΩ and RL=5 KΩ, respectively, were first amplified by EG&G PAR113 preamplifiers. Then, the power spectral density SVB and SVC were measured using a HP 3562A dual-channel

Theory and discussion

It is known that SIB originates from fluctuations in three different base current components. Fig. 2 shows the different current components in lateral PNP BJTs. The first one is the volume current IB1, which is the major component of the base current. In the event that this current component is the primary contributor to noise, according to Kleinpenning’s mobility fluctuation model, SIB should be linearly propotional to the base current, SIBIB, which is defined as [12]:SIB=qIBDnWE2αHnflnN(0)N(W

Conclusion

The temperature dependence of 1/f noise is studied on lateral PNP BJTs to investigate the underlying physical mechanisms. The equivalent base current noise power spectral density SIB is extracted using a novel cross-correlation technique between the base and the collector biasing circuitry. Two models are proposed for the origin of noise. For the high temperature region (310<T<450 K), surface recombination velocity fluctuations at the neutral base seem to be responsible for SIB. For

Acknowledgements

This work is partially supported by Texas Higher Education Coordinating Board Advanced Technology Program under grant #003656-0001-2001, and Legerity Corporation.

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