Parasitic Emitter Research Articles

On Common–Base Avalanche Instabilities in SiGe HBTs

This paper presents a detailed investigation of the key device-level factors that contribute to the bias-dependent features observed in common-base (CB) dc instability characteristics of advanced SiGe HBTs. Parameters that are relevant to CB avalanche instabilities are identified, extracted from measured data, and carefully analyzed to yield improved physical insight, a straightforward estimation methodology, and a practical approach to quantify and compare CB avalanche instabilities. The results presented support our simple theory and show that CB-instability characteristics are strongly correlated with the parasitic base and emitter resistances. The influence of weak quasi-pinch-in effects are shown to contribute additional complexity to the bias dependence of the CB-instability threshold. Measured data from several technology nodes, including next-generation (300-GHz) SiGe HBTs, are presented and compared. Experimental analysis comparing different device geometries and layouts shows that while device size plays an important role in CB avalanche instabilities across bias, these parameters are not sensitive to standard transistor layout variations. However, novel measurements on emitter-ring tetrode transistor structures demonstrate the influence of perimeter-to-area ratio on CB stability and highlight opportunities for novel transistor layouts to increase .

VERTICAL SCALING OF TYPE I InP HBT WITH FT > 500 GHZ

We have fabricated the high-speed InP/InGaAs -based single heterojunction bipolar transistors (SHBTs) with current gain cutoff frequency, fT from 166GHz to over 500GHz by the approach of vertical scaling. Collector thickness is reduced from 3000Å to 750Å and the peak current density is increased up to 1300kA/cm2. In this paper, device rf performance has been compared with respect to materials with different vertical dimensions. The scaling limitation is also studied by analytical approach. The extracted physical parameters suggest that the parasitic emitter resistance is the major limit on further enhancing ultra-scaled HBT intrinsic speed due to the associated RECBC delay. The cut-off frequency of a 500Å collector SHBT has been measured and the results indicate a dramatic drop on fT, supporting the conclusion projected by model analysis. It is also commented that for deeply downscaled HBTs, impact ionization could be another degrading mechanism limits device bandwidth.

Distortion analysis of high-frequency log-domain filters using Volterra series

An approach to estimate the distortion in log-domain filters is presented. The models used for the bipolar transistors include the C/sub /spl pi// and C/sub sub/ parasitic capacitors, the beta effect, and the parasitic emitter resistances. Simple closed-form expressions describing the effect of each nonideality on distortion are derived and compared to simulations. A general method, which could be extended to analyze higher order filters, based on Volterra series, is used.

Analysis and compensation of log-domain biquadratic filter response deviations due to transistor nonidealities

A translinear log-domain integrator, the powerhouse of log-domain filtering techniques, is analyzed to determine its deviations due to major time-invariant bipolar transistor imperfections, such as parasitic emitter and base resistances, finite beta, Early effect, and area mismatches. The results are used to predict the transfer function deviations of biquad filters, for both lowpass and bandpass filter responses. It will be shown that the errors can be understood and categorized into integrator scalar and negative feedback errors, and how they can be electronically compensated. SPICE simulations, both large and small-signal analysis, are performed to verify the results.

Distortion compensation in log-domain filters using state-space techniques

A new state-space method is proposed for the synthesis of "quasilog-domain" filters where the parasitic emitter resistance of transistors is built into the equation formulation. The explicit exponential maps for log domain filters are replaced with implicit exponential maps yielding nodal equations for active filters. The theory, which unifies a class of externally linear filters including log domain filters as a subset, is shown to be applicable to general higher order filters. Circuit realization is discussed, some example circuit topologies are proposed, and simulation results are offered to show the potential of the approach in improving log-domain filter performance.

Extrapolated f/sub max/ of heterojunction bipolar transistors

It is shown that the extrapolated f/sub max/ of heterojunction bipolar transistors (HBT's) can be written in the form f/sub max/=/spl radic/f/sub T//8/spl pi/(RC)/sub eff/, where f/sub T/ is the common-emitter, unity-current-gain frequency, and where (RC)/sub eff/ is a general time constant that includes not only the effects of the base resistance and collector-base junction capacitance, but also the effects of the parasitic emitter and collector resistances, and the dynamic resistance 1/g/sub m/, where g/sub m/ is the transconductance. Simple expressions are derived for (RC)/sub eff/, and these are applied to two state-of-the-art devices recently reported in the literature. It is demonstrated that, in modern HBT's, (RC)/sub eff/ can differ significantly from the effective base-resistance-collector-capacitance product conventionally assumed to determine f/sub max/.

Parameter-extraction method for heterojunction bipolar transistors

A method for determining the microwave equivalent-circuit element values of a small-signal heterojunction bipolar transistor (HBT) is described. Most important is the ability to separate the parasitic emitter resistance from the junction resistance. No use of 'cold' measurements or test patterns is required. The method may also be applicable to homojunction bipolars.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Extremely high peak specific transconductance AlGaAs/GaAs heterojunction bipolar transistors

Self-aligned AlGaAs/GAs heterojunction bipolar transistors with peak specific transconductances as high as 25 mS/ mu m/sup 2/ of emitter area are discussed. These are the highest specific transconductances ever reported for a bipolar transistor. These devices, which contain no indium in the emitter, display specific parasitic emitter resistances of less than 1*10/sup -7/ Omega -cm/sup 2/. This low parasitic resistance is attributed to an improved n-type contact technology, in which a molybdenum diffusion barrier and a plasma-enhanced chemical vapor deposition SiO/sub 2/ overlayer are used to achieve low specific contact resistivities. >