Enhai Zhao

An Investigation of Dose Rate and Source Dependent Effects in 200 GHz SiGe HBTs

We present an investigation of the observed variations in the total dose tolerance of the emitter-base spacer and shallow trench isolation oxides in a commercial 200 GHz SiGe HBT technology. Proton, gamma, and X-ray irradiations at varying dose rates are found to produce drastically different degradation signatures at the various oxide interfaces. Extraction and analysis of the radiation-induced excess base current, as well as low-frequency noise, are used to probe the underlying physical mechanisms. Two-dimensional calibrated device simulations are employed to correlate the observed results to the spatial distributions of carrier recombination in forward- and inverse-mode operation for both pre- and post-irradiation levels. Possible explanations of our observations are offered and the implications for hardness assurance testing are discussed

On the high-temperature (to 300/spl deg/C) characteristics of SiGe HBTs

A comprehensive investigation of the high-temperature characteristics of advanced SiGe heterojunction bipolar transistors (HBTs) is presented, and demonstrates that, contrary to popular opinion, SiGe HBTs are potentially well-suited for many electronics applications operating at temperatures as high as 300/spl deg/C.

The effects of radiation on 1/f noise in complementary (npn+pnp) SiGe HBTs

We present the first study of the effects of radiation on low-frequency noise in a novel complementary (npn+pnp) silicon-germanium (SiGe) HBT BiCMOS technology. In order to manipulate the physical noise sources in these complementary SiGe HBTs, 63.3 MeV protons were used to generate additional (potentially noise-sensitive) trap states. The base currents of both the npn and pnp SiGe HBTs degrade with increasing proton fluence, as expected, although in general more strongly for the npn transistors than for the pnp transistors, particularly in inverse mode. For the pnp SiGe HBTs, irradiation has almost no effect on the 1/f noise to proton fluence as high as 5.0/spl times/10/sup 13/ p/cm/sup 2/, while the npn SiGe HBTs show substantial radiation-induced excess noise. In addition, unlike for the pnp devices, which maintain an I/sub B//sup 2/ bias dependence, the 1/f noise of the post-irradiated npn SiGe HBTs change to a near-linear dependence on I/sub B/ at low base currents following radiation. That suggests a fundamental difference in the noise physics between the two types of devices.

Assessing reliability issues in cryogenically-operated SiGe HBTs

We assess SiGe HBTs for emerging mixed-signal cryogenic circuits designed to operate on the Moon without ambient heating or cooling (from +120C to as low as -230C), focusing of potential reliability issues. Comprehensive mixed-mode reliability stress data for these SiGe HBTs were measured from 300 K to 85 K. We extract the thermal resistance over temperature to evaluate the impact of the self-heating at low temperatures, explore the low-frequency noise performance at room temperature and cryogenic temperatures as a function of stress condition, and examine the impact of cooling on breakdown voltage and operating point instabilities for mixed-signal circuits.

An investigation of low-frequency noise in complementary SiGe HBTs

We present a comprehensive investigation of low-frequency noise behavior in complementary (n-p-n + p-n-p) SiGe heterojunction bipolar transistors (HBTs). The low-frequency noise of p-n-p devices is higher than that of n-p-n devices. Noise data from different geometry devices show that n-p-n transistors have an increased size dependence when compared with p-n-p transistors. The 1/f noise of p-n-p SiGe HBTs was found to have an exponential dependence on the (intentionally introduced) interfacial oxide (IFO) thickness at the polysilicon-to-monosilicon interface. Temperature measurements as well as ionizing radiation were used to probe the physics of 1/f noise in n-p-n and p-n-p SiGe HBTs. A weak temperature dependence (nearly a 1/T dependence) of 1/f noise is found in both n-p-n and p-n-p devices with cooling. In most cases, the magnitude of 1/f noise is proportional to I/sub B//sup 2/. The only exception in our study is for noise in the post-radiation n-p-n transistor biased at a low base current, which exhibits a near-linear dependence on I/sub B/. In addition, in proton radiation experiments, the 1/f noise of p-n-p devices was found to have higher radiation tolerance than that of n-p-n devices. A two-step tunneling model and a carrier random-walk model are both used to explain the observed behavior. The first model suggests that 1/f noise may be caused by a trapping-detrapping process occurring at traps located inside IFO, while the second one indicates that noise may be originating from the emitting-recapturing process occurring in states located at the monosilicon-IFO interface.

Substrate bias effects in vertical SiGe HBTs fabricated on CMOS-compatible thin film SOI

A comprehensive investigation of substrate bias effects on device performance, thermal properties, and reliability of vertical SiGe HBTs fabricated on CMOS-compatible, thin-film SOI, is presented for the first time. Calibrated 2D MEDICI simulations are used to support our explanations, and the resultant device design trade-offs encountered in building SiGe HBTs on thin-film SOI are quantitatively assessed.

On the suitability of SiGe HBTs for high-temperature (to 300/spl deg/) electronics

The first comprehensive investigation of the high-temperature operation of SiGe HBTs is presented, and demonstrates that, contrary to popular opinion, SiGe HBTs are well-suited for many electronics applications operating at temperatures as high as 300/spl deg/C.

A New Device Phenomenon in Cryogenically-Operated SiGe HBTs

A new negative differential resistance (NDR) effect is reported for the first time in cryogenically-operated SiGe HBTs. A physical explanation based on heterojunction barrier effect (HBE) is presented, and confirmed using calibrated 2-D TCAD simulations. The ac consequences of this NDR effect and the impact of technology scaling on the phenomenon are also addressed

Modeling of broadband noise in complementary (npn + pnp) SiGe HBTs

We present the first comprehensive investigation of broadband noise in a complementary (npn + pnp) SiGe (C-SiGe) HBT BiCMOS technology. A base-transit time based simple noise model with its origins in linear noisy two-port theory is presented, which takes into account the fundamental base and collector shot noise in a bipolar transistor, and their cross-correlation. The minimum noise figure of the npn and the pnp SiGe HBT in this technology at 2.0 GHz is measured to be 1.0 dB and 1.4 dB, respectively. This noise model is compared with the conventional SPICE noise model and the measured differences in the noise behavior of the npn and pnp SiGe HBTs are analyzed.

On the mechanisms of low-frequency noise in vertical silicon pnp BJTs

We present an investigation of low-frequency noise in advanced vertical pnp bipolar junction transistors (BJTs) with differing interfacial oxide thicknesses (10Å, 12Å, and 14Å). Low-frequency noise is observed to exhibit a cubic dependence on IFO thickness. Devices were measured across the temperature range of 90 K to 450 K. From 90 K to 250 K, the magnitude of the low-frequency noise is found to decrease with temperature, but from 250 K to 450 K the noise actually increases with temperature. Devices were hot-carrier (electrically) stressed, and the low-frequency noise was found to be almost unchanged with the addition of stress-induced traps. The transparency fluctuation model is suggested as a possible explanation for the operative noise mechanism, due to the similar dependence of base current and low-frequency noise on interfacial oxide thickness.