Collector-base Space Charge Region Research Articles

Limiting Effects on the Design of Vertical Superjunction Collectors in SiGe HBTs

The implementation of a “superjunction” collector design in a silicon–germanium heterojunction bipolar transistor technology is explored for enhancing breakdown performance. The superjunction collector is formed via the placement of a series of alternating the p/xn-doped layers in the collector-base space charge region and is used to reduce avalanche generation leading to breakdown. An overview of the physics underlying superjunction collector operation is presented, together with TCAD simulations, and a parameterization methodology is developed to explore the limits of the superjunction collector performance. Measured data demonstrate the limitations explored in simulation.

Impact of Breakdown Voltage on Gamma Irradiation Effects in 0.13- $\mu \text{m}$ and 0.25- $\mu \text{m}$ SiGe HBTs

We have investigated the ionization damage by 60Co gamma irradiation in 0.13- and 0.25- $\mu \text{m}$ SiGe heterojunction bipolar transistors (HBTs). Both technologies feature high-speed HBTs (HS-HBTs) together with high-voltage HBTs (HV-HBTs). Base current degradation with increasing total irradiation dose is studied. An identical behavior of corresponding HS-HBT and HV-HBT is found for operation in forward mode probing the emitter–base junction. In reverse mode where the collector base junction is determining the base current degradation, HV devices exhibit larger degradation than their HS counterparts. The increased width of the collector–base space charge region in HV devices leads to enhanced interface recombination at the adjacent Si/oxide interfaces and stronger base current degradation. TCAD simulations of device degradation suggest a linear relationship between total irradiation dose and radiation-induced interface state density $N_{\mathrm{ it}}$ .

Graded Applications of NQS Theory for Modeling Correlated Noise in SiGe HBTs

In this paper, we develop a correlated noise model for bipolar transistors from an accurate nonquasi-static model. The proposed noise model includes the signal delay through base-collector space-charge region and is implemented using four extra nodes. We also present a simplified version of the same model that requires only two extra nodes. A further simplified version that uses only one extra node is found to be identical with a state-of-the-art correlated noise model. When compared with the device simulation data, our proposed models show improved accuracy compared with the existing state-of-the-art noise models.

Physical and Electrical Performance Limits of High-Speed SiGeC HBTs—Part I: Vertical Scaling

The overall purpose of this paper (including Part I, in this issue) is the prediction of the ultimate electrical high-frequency performance potential for SiGeC heterojunction bipolar transistors under the constraints of practical applications. This goal is achieved by utilizing most advanced device simulation tools with parameters calibrated to existing experimental results. This Part I outlines the overall scaling procedure and then focuses on the vertically scaled structure. According to isothermal device simulation, the “ultimate” doping profile yields a peak transit frequency <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fT</i> of almost 1.5 THz, a BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CEO</sub> above 1 V (dependent on BE bias) and a zero-bias internal base sheet resistance of about 3 kΩ/sq. The reasons for achieving a higher product f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CEO</sub> (>; 1.5 THzV) than anticipated from the classical Johnson limit are explained. Finally, it is found that <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fT</i> is limited by the minority charge stored in the BE junction and that BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CEO</sub> is mainly determined by the tunneling mechanisms in the base-collector space-charge region.

Design and Optimization of Superjunction Collectors for Use in High-Speed SiGe HBTs

After reviewing the various mechanisms causing breakdown in bipolar transistors, we present a novel collector design for silicon-germanium heterojunction bipolar transistors (SiGe HBTs). The design improves the well-known speed/breakdown voltage tradeoff in SiGe HBTs for radio-frequency (RF) and millimeter-wave applications. Applying multiple alternating p- and n-type layers (a superjunction) deep in the collector-base (CB) space-charge region (SCR) alters the electric field and electron temperature in the CB junction. Consequently, impact ionization is suppressed, whereas the width of the CB SCR is not increased, and therefore, the breakdown voltages BVCEO and BVCEO are increased, with no degradation in the device speed or RF performance. For a fixed alternating-current performance, BVCEO is improved by 0.33 V, producing a SiGe HBT with fT = 101 GHz, fmax = 351 GHz, and BVCEO = 3.0 V, as predicted by calibrated DESSIS technology computer-aided design simulations. Concerns with regard to the influence of thermal cycles associated with fabrication are considered, and a more practical doping profile is proposed to simplify the use of superjunctions. The proposed structure is also contrasted with other approaches from the literature.

Modeling the input non-quasi-static effect in small signal equivalent circuit based on charge partitioning for bipolar transistors and its impact on RF noise modeling

This paper models the input non-quasi-static (NQS) effect of bipolar transistors using charge partitioning. The input NQS effect associated with the base minority carrier transport is modeled with a RC network, while the input NQS effect associated with the base-collector space charge region (CB SCR) carrier transport is modeled with a RLC network. With the proposed input NQS equivalent circuit model, Y-parameters and RF noise parameters using the van Vliet model are successfully modeled for frequencies up to f T . The transport noise model is extended to represent the van Vliet model by using two noise related time constants, which eliminates the need of Y-parameter in the description of the noise source. The input NQS effect for such model is verified to be important only for frequencies above f T /3. Analytical Y-parameter and noise solutions of a 1-D bipolar transistor at low injection level are used for validation.

Discussions and extension of van Vliet’s noise model for high speed bipolar transistors

The general van Vliet noise model for transistors was derived for base minority carriers only under adiabatic boundary conditions. This paper extends the model by including the emitter minority carrier noise and the base–collector space charge region (CB SCR) effects based on the mathematical framework developed by van Vliet. Both the finite surface recombination velocity at polysilicon emitter and the finite carrier exit velocity at CB SCR are considered. It is proved that the van Vliet model can be directly used to include the emitter minority carrier noise, and the model holds when the two finite velocity boundary conditions are imposed. A new set of equations are derived to include the effect of CB SCR delay on noise.

A compact model for SiGe HBT on thin-film SOI

Heterojunction bipolar transistor (HBT) fabrication on thin-film silicon-on-insulator (SOI) has been recently demonstrated. Due to the space volume constraint (thin film) for the device fabrication, the HBT structure is different from bulk HBT. In fact, compared to a bulk device, the buried layer has been suppressed and a lateral collector contact configuration is introduced. This device features a vertical expansion followed by a lateral expansion of the base-collector space charge region. This nonconventional charge behavior induces a kink in the base-collector junction capacitance characteristics, and as a consequence a modified Early effect. Furthermore, the low current transit time is modified compared to a bulk HBT. In this paper, all these effects are analyzed and a compact model for SOI-HBT is proposed. The model is validated on real SOI-HBTs with different collector doping levels.

Observation of fast erase due to enhanced generation of holes caused by electron impact ionization

This letter describes an enhanced erase mechanism in flash memory cells due to impact ionization induced generation of holes. The increased population of holes is initiated by the impact ionization of electrons in the collector-base region of a parasitic bipolar transistor. Electrons injected from the emitter (drain) of a parasitic n-p-n bipolar transistor into the base (substrate) can drift to the collector (source) where the high electrical field in the collector-base space charge region causes impact ionization and carrier multiplication. The impact ionization generated holes that gain enough energy to overcome the oxide barrier can be injected into the floating gate, resulting a very fast erase.

Electric field effects associated with the backside Ge profile in SiGe HBTs

A comprehensive investigation of electric field effects associated with the backside Ge profile in SiGe heterojunction bipolar transistors (HBTs) is conducted using calibrated simulations. We show for the first time that the backside Ge retrograde can alter the local electric field distribution in the base–collector space-charge region near the SiGe to Si heterojunction, thereby affecting the impact ionization and the apparent neutral base recombination (NBR) ( I B( V CB)/ I B( V CB=0)) of SiGe HBTs. The changes in the electric field induced by the Ge-induced band offsets contributes to a decrease of the observed impact ionization between comparably doped SiGe HBTs and Si bipolar junction transistors (BJTs), as well as an improved V CB dependence of I B (apparent decrease in NBR). Experimental data on SiGe HBTs with various Ge profiles and a Si BJT control are used to support our claims.

Materialprobleme der Basiskontakte von schnellen integrierten Si/SiGe-Heterobipolartransistoren

Base contact issues of a Si/SiGe heterojunction bipolar transistor with a shallow implanted base are described. A silicide, required for low resistivity contact, could reach the base-collector space-charge region due to silicide roughness. Cobaltsilicide offers a solution to the contacting of the base. In this case the reaction of the cobalt with a cap-layer has to be reduced and the roughness of the cobaltsilicide has to be optimized by well chosen process parameters.

Optimization of SiGe HBTs for operation at high current densities

A comprehensive investigation of the impact of Ge profile shape as well as the scaling of collector and base doping profiles on high-injection heterojunction barrier effects in SiGe HBTs has been conducted over the -73-85/spl deg/C temperature range. The onset of Kirk effect at high current densities is shown to expose the Si/SiGe heterojunction in the collector-base space charge region, thereby inducing a conduction band barrier which negatively impacts the collector and base currents as well as the dynamic response, leading to a premature roll-off in both /spl beta/ and f/sub T/. In light of this, careful profile optimization is critical for emerging SiGe HBT circuit applications, since they typically operate at high current densities to realize maximum performance. We first explore the experimental consequences and electrical signature of these barrier effects over the 200-358 K temperature range for a variety of Ge profiles from an advanced UHV/CVD SiGe HBT technology. We then use extensive simulations which were calibrated to measured results to explore the sensitivity of these barrier effects to both the Ge profile shape and collector profile design, and hence investigate the optimum profile design points as a function of vertical scaling.

An investigation of the spatial location of proton-induced traps in SiGe HBTs

The effects of 46 MeV proton irradiation induced trap generation and its impact on the electrical characteristics of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) from an advanced ultrahigh vacuum/chemical vapor deposition (UHV/CVD) SiGe BiCMOS technology are examined and discussed for the first time. At proton fluences as high as 10/sup 14/ p/cm/sup 2/ the peak current gain of the devices degraded by less than 8% compared to the pre-irradiated samples. The maximum oscillation frequency and cutoff frequency of the SiGe HBTs showed only minor degradation after 10/sup 14/ p/cm/sup 2/. Calibration of 2-D device simulation (MEDICI) to measured data in both forward and inverse modes of operation was used to infer the spatial location of the proton-induced traps. Traps in the collector-base space charge region appear as generation/recombination (G/R) centers in the inverse emitter-base region and are the result of displacement damage. Traps at the emitter-base spacer oxide interface appear as G/R centers in the forward emitter-base space charge region and are the result of ionization damage. Taken together, these results suggest that UHV/CVD SiGe HBT technology is robust to proton fluences at least as high as 10/sup 13/ p/cm/sup 2/ without radiation hardening.

Influence of a base epitaxial regrowth on first order and low frequency noise measurements on GaInP/GaAs HBTs

A comparison of GaInP/GaAs HBTs with and without a base epitaxial regrowth is undertaken via static and low-frequency noise measurements. Current gain values and behaviour versus emitter area are different in both types of transistor. From low-frequency noise measurements emitter series resistances are extracted. Values of resistances are much smaller by about a factor of ten on samples without regrowth. The and g-r noise components are then analysed versus bias and geometry. For samples without regrowth, the and the g-r noise sources are mostly located at the extrinsic base surface and are due to recombinations. These recombinations are suppressed on samples with a base regrowth. On these devices, the noise sources related both to intrinsic and extrinsic phenomena are present. The g-r noise sources are located within the intrinsic transistor and more particularly in the base-collector space-charge region. This is partly explained by a thermal treatment induced by the process technology. Finally, it is shown that the base epitaxial regrowth brings a correlation between base and collector currents.

Gummel–Poon model for Npn heterojunction bipolar phototransistors

A Gummel–Poon model for abrupt, single heterojunction Npn bipolar phototransistors is described including the effects of the dc base bias on the current and optical gains. Initially, the excess electron concentration at the emitter end of the quasineutral base is determined by matching the thermionic field emission across the emitter–base heterojunction with the diffusion current at the emitter end of the base and including the effects of optical generation. The result is used in determining the electron profile in the base from which the base charge and the electron component to the emitter and collector currents are calculated following the Gummel–Poon model. The photocurrent’s components due to optical absorption in the quasineutral base, the base–collector space charge region, and the collector region are determined taking into account the nonuniform optical generation assuming topside illumination. A comprehensive description of the recombination current components is incorporated including the effects of optical absorption on recombination. The model is then used to calculate the dc and small signal current gain and the device’s optical gain, and to examine the effects of dc biasing and the optical power level. The simulation results are compared with the available experimental results and reasonable agreement is found.

A thermionic-field-diffusion model for Npn bipolar heterojunction phototransistors

This article describes the development of a thermionic-field-diffusion model for use in the analysis and design of abrupt, single heterojunction bipolar phototransistors (HPTs) for use in HBT-based optical receivers. In particular, included in this approach are the effects of a dc base bias on the optical gain and device performance. Taking into account the optical generation, the excess electron concentration at the emitter end of the quasi-neutral base is initially determined using a matching of the thermionic field emission across the emitter-base heterojunction with the diffusion current at the emitter end of the base. At the collector end of the base region, a finite electron concentration is used based on the collector current density. The results are used to determine the electron profile in the quasi-neutral base region and the diffusion components to the emitter and collector currents. To determine the device’s optical gain, the photocurrent and the small signal current gain are calculated. The dominant component in the photocurrent is found to be due to optical absorption in the base-collector space charge region. For backside illumination, the collector component becomes larger. The base width and electron diffusion length control the current gain and thereby the optical gain. The model is used to quantify the effects of the device’s structure on the optical gain.

Inclusion of tunneling and ballistic transport effects in an analytical approach to modeling of NPN InP-based heterojunction bipolar transistors

We describe an analytical approach to modeling NPN InP-based heterojunction bipolar transistors using a Gummel Poon model. Starting from an initial, physical description of the device's epitaxial structure and geometry, this physics-based model is utilized to simulate the device's operation using a thermionic-field-diffusion model for carrier injection at the emitter and diffusive transport across the base. This approach incorporates recent advances in understanding of device operation and improved models of parasitics. The cutoff frequency and maximum frequency of oscillation are calculated as a function of collector current density and compared with experimental measurements. For the cutoff frequency, the results of this model underestimate the high frequency performance of the device by as much as 50% at the higher collector current levels. To improve the model and to allow simulation of narrower base widths, a more rigorous analysis of tunneling through the emitter spike and ballistic transport across the base and base-collector depletion regions has been included. Quantum mechanical tunneling of electrons through the emitter-base spike has been taken into account for accurate description of the energy distribution of electron injection into the base. The resulting electron flux distribution is used as an initial distribution in a regional Monte Carlo simulator to model electron transport from the emitter edge of the base to the subcollector. The results are then used to calculate an average base transit time and base-collector space charge region delay time which can be incorporated in the analytical model.

Gunn effect in heterojunction bipolar transistors

The Gunn effect in III-IV heterojunction bipolar transistors is investigated using hydrodynamic simulations. It is shown that Gunn domains nucleate and propagate in the collector drift region of an npn AlGaAs/GaAs transistor in which the electric field at the base-collector space charge region is properly engineered.

Analytical study of collector-base capacitance and cutoff frequency of n+- p- n- n+ bipolar junction transistors

Modern high-speed bipolar junction transistors (BJTs) tend to operate with saturated carrier velocity in the base-collector space-charge region and at high current density. In the past, there have been several discussions on the analytical expressions for the collector-base capacitance and the transition frequency for n- p- n- n + BJTs. However, the expressions did not complely take into account the effect of velocity saturation. Taking into account this effect completely, analytical expressions for these device quantities of n +- p- n- n + BJTs are presented. We found that the effect of velocity saturation appears for devices with shorter epitaxial collector. In the model, the device parameters can be calculated analytically using external voltages.

Prediction of impact-ionization-induced snap-back in advanced Si n-p-n BJT's by means of a nonlocal analytical model for the avalanche multiplication factor

The authors point out that when a triangular shape for the electric field in the base-collector space-charge region of an n-p-n Si BJT (bipolar junction transistor) is assumed, the electron mean energy can be calculated analytically from a simplified energy-balance equation. On this basis a nonlocal-impact-ionization model, suitable for computer-aided circuit simulation, has been obtained and used to calculate the output characteristics at constant emitter-base voltage (grounded base) of advanced devices. Provided the experimental bias-dependent value of the base parasitic resistance is accounted for in the device model, the base-collector voltage at which impact-ionization-induced snap-back occurs can be accurately predicted. >