Quantifying neutral base recombination and the effects ofcollector-base junction traps in UHV/CVD SiGe HBTs

This work quantifies neutral base recombination in UHV/CVD SiGe heterojunction bipolar transistors (HBTs) using calibrated two-dimensional (2-D) simulation. The simulated collector-base conductance through neutral base recombination (NBR) modulation is far below the experimentally observed values, and hence does not explain the measured output resistance degradation under forced-I_B operation. In spite of the output resistance degradation, these UHV/CVD SiGe HBTs have approximately the same base current as the silicon control, and hence higher current gain. Based on the observation of the majority carrier concentration limited recombination in the CB junction depletion layer, as opposed to the minority carrier concentration limited recombination in the neutral base, local presence of traps in the CB junction depletion layer is suggested. This explains the measured CB conductance modulation and the related output resistance degradation without compromising the current gain. Numerical simulations using traps locally introduced into the CB junctions successfully reproduced the measured collector-base conductance from simulation without appreciable degradation in current gain     

Experimental method to extract AC collector-base resistance fromSiGe HBT's

A simple experimental technique is presented that enables the accurate extraction of the collector-base resistance r_μ that models the impact of neutral base recombination on the ac output resistance of bipolar transistors. Measurements of ratios of r_μ to the output resistance r_o arising from the Early effect for SiGe HBTs allows for the quantitative determination of the impact of neutral base recombination on analog circuits that are designed to have a high output resistance. It is found that SiGe HBT structures indicative of those currently being used commercially in advanced analog processes exhibit neutral base recombination that is significant enough to severely degrade the output resistance of analog circuits, even when the output transistors experience ac base voltage drive conditions. Finally, it is shown how extraction of the ratio of r _μ to r_o can be a useful tool to determine the impact of parasitic potential barriers formed by boron out-diffusion at the collector-base junction on device and analog circuit performance     

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°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 β and f_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     

异质结位置对缓变集电结SiGe HBT性能的影响

研究了npn型SiGe HBT集电结附近的异质结位置对器件性能的影响.采用Taurus-Medici 2D器件模拟软件,在渐变集电结SiGe HBT的杂质分布不变的情况下,模拟了各种异质结位置时的器件直流增益特性和频率特性.同时比较了处于不同集电结偏压下的直流增益和截止频率.分析发现即使没有出现导带势垒,器件的直流和高频特性仍受SiGe层中性基区边界位置的影响.模拟结果对SiGe HBT的设计和分析都具有实际意义.     

Reliability of microwave SiGe/Si heterojunction bipolar transistors

The degradation behavior of NPN Si/SiGe/Si heterojunction bipolar transistors, grown by solid-source molecular beam epitaxy (MBE), has been studied by accelerated lifetime testing at different ambient temperatures. The degradations of the dc current gain and the microwave performance of the devices are explained in terms of recombination enhanced impurity diffusion (REID) of boron atoms from the base region and the subsequent formation of parasitic energy barriers at the base-emitter and base-collector junctions     

异质结位置对缓变集电结SiGe HBT性能的影响

研究了npn型SiGe HBT集电结附近的异质结位置对器件性能的影响.采用Taurus-Medici 2D器件模拟软件,在渐变集电结SiGe HBT的杂质分布不变的情况下,模拟了各种异质结位置时的器件直流增益特性和频率特性.同时比较了处于不同集电结偏压下的直流增益和截止频率.分析发现即使没有出现导带势垒,器件的直流和高频特性仍受SiGe层中性基区边界位置的影响.模拟结果对SiGe HBT的设计和分析都具有实际意义.     

基区复合电流对双极晶体管厄利电压的影响

推导了在考虑了基区复合电流后双极晶体管厄利电压的理论表达式。用该表达式计算了 Si/Si Ge异质结双极晶体管的厄利电压 ,并且与仿真结果进行了比较。比较结果表明 ,两种情况下计算出的厄利电压值符合良好     

On the profile design and optimization of epitaxial Si- andSiGe-base bipolar technology for 77 K applications. I. Transistor DCdesign considerations

The DC design considerations associated with optimizing epitaxial Si- and SiGe-base bipolar transistors for the 77-K environment are examined in detail. Transistors and circuits were fabricated using four different vertical profiles, three with a graded-bandgap SiGe base, and one with a Si base for comparison. All four epitaxial-base profiles yield transistors with DC properties suitable for high-speed logic applications in the 77-K environment. The differences between the low-temperature DC characteristics of Si and SiGe transistors are highlighted both theoretically and experimentally. A performance tradeoff associated with the use of an intrinsic spacer layer to reduce parasitic leakage at low temperatures and the consequent base resistance degradation due to enhanced carrier freeze-out is identified. Evidence that a collector-base heterojunction barrier effect severely degrades the current drive and transconductance of SiGe-base transistors operating at low temperatures is provided     

Electrical stress effect on RF power characteristics of SiGe hetero-junction bipolar transistors

In this paper, we investigate the electrical stress effects on both the high-frequency and RF power characteristics of Si/SiGe HBTs. Simultaneously applying a high collector current density and a high collector–base voltage upon the Si/SiGe HBTs, their hot carriers will induce device performance degradation. This stress condition is similar to the DC bias conditions of a current source RF power amplifier, and is termed as a “mixed-mode” stress. We find that not only the maximum oscillation frequency but also the output power performance of Si/SiGe HBTs are suffered by this electrical stress. In addition, the degradations of high-frequency and power characteristics are also worse under a constant base-current measurement than those under a constant collector-current measurement. Finally, we developed a commercial large-signal model to examine the degradations of the parasitic resistances and ideality factors of base and collector currents to explain the RF power and linearity degradations.     

Analytical modelling of current gain and frequency characteristics under high injection levels in Si/SiGe heterojunction bipolar transistors at 77 and 300 K

This paper reports an analytical modelling of current gain and frequency characteristics in Si/SiGe heterojunction bipolar transistors (HBTs) at 77 and 300 K. Important transistor parameters, such as current gain, transconductance, cutoff frequency and maximum oscillation frequency are calculated as a function of Ge concentration in the base under different injection levels. The main physical mechanisms for the current and cutoff frequency rolloff at high injection levels are also analyzed. It shows that the high-level injection effect is more pronounced in the SiGe HBTs as a result of the increasing minority carrier concentration in the base and the Ge concentration and distribution will have a decisive influence of device performance. The results may provide a basis for the design of low temperature operation SiGe HBTs.     

Numerical modeling of parasitic barrier formation at the SiGe/Si heterojunction due to p–n junction displacement

The formation and characteristics of a parasitic conduction band barrier located at a SiGe/Si heterojunction have been investigated using a commercial numerical simulator and a simple, three-region model of a heterojunction with a nearby p–n junction. The barrier’s formation is examined as a function of the displacement of the p–n junction from the heterojunction, but also found to depend on the germanium concentration, junction doping and the applied bias. The phenomenon is of interest for understanding the performance of SiGe/Si heterojunction bipolar transistors, where the p–n junction is intentionally displaced from the heterojunction at either the emitter or collector junctions or where boron outdiffusion from the base produces p–n junction displacement. The barrier is found to scale with the germanium mole fraction and to be significantly larger when the heterojunction is displaced into the p-side of the p–n junction. Beyond some minimum separation of the junctions, the barrier height rises with junction displacement and saturates. For a given displacement, the barrier’s height can be suppressed with reverse bias or enhanced by forward bias of the p–n junction. The results of the numerical simulations are compared with those from a simple analytical model as an aid in understanding the barrier’s formation and characteristics.     

Negative base current and impact ionization phenomena inAlGaAs/GaAs HBTs

Impact ionization phenomena in the collector region of AlGaAs/GaAs heterojunction bipolar transistors give rise to base current reduction and reversal. These phenomena can be characterized by extracting the M-1 coefficient, which can be evaluated by measuring base current changes. Measurements of M-1 are affected at low current densities by the presence of the collector-base junction reverse current I_CBO. At high current densities, three effects contribute to lower the measured M-1 value: voltage drops due to collector (R_C) and base (R_B) parasitic resistances, device self-heating, and lowering of the base-collector junction electric field due to mobile carriers. By appropriately choosing the emitter current value, parasitic phenomena are avoided and the behavior of M-1 as a function of the collector-base voltage V_CB in AlGaAs/GaAs HBTs is accurately characterized     

Low-frequency noise in UHV/CVD epitaxial Si and SiGe bipolartransistors

In this work a comprehensive investigation of low-frequency noise in ultrahigh vacuum/chemical vapor deposition (UHV/CVD) Si and SiGe bipolar transistors is presented. The magnitude of the noise of SiGe transistors is found to be comparable to the Si devices for the identical profile, geometry, and bias. A comparison with different technologies demonstrates that the SiGe devices have excellent noise properties compared to AlGaAs/GaAs heterojunction bipolar transistors (HBT's) and conventional Si bipolar junction transistors (BJT's). Results from different bias configurations show that the 1/f base noise source is dominant in these devices. The combination of a 1/Area dependence on geometry and near quadratic dependence on base current indicates that the 1/f noise sources are homogeneously distributed over the entire emitter area and are probably located at the polysilicon-Si interface. Generation/recombination (Gm) noise and random telegraph signal (RTS) noise was observed in selected Si and SiGe devices. The bias dependence and temperature measurements suggest that these G/R centers are located in the base-emitter space charge region. The activation energies of the G/R traps participating in these noise processes were found to be within 250 meV of the conduction and valence band edges     

RF power characteristics of SiGe HBTs at cryogenic temperatures

This paper investigates the temperature dependence (from 77 to 300 K) of dc, ac, and power characteristics for n-p-n SiGe heterojunction bipolar transistors (HBTs) with and without selectively implanted collector (SIC). In SiGe HBTs without SIC, the valance band discontinuity at the base-collector heterojunction induces a parasitic conduction band barrier while biasing at saturation region and high current operation at cryogenic temperatures. This parasitic conduction band barrier significantly reduces the current gain and cutoff frequency. For transistors biased with fixed collector current, the measured output power, power-added efficiency, and linearity at 2.4 GHz decrease significantly with decreasing operation temperatures. The temperature dependence of output power characteristic is analyzed by Kirk effect, current gain, and cutoff frequency at different temperatures. The parasitic conduction band barrier in SiGe HBTs with SIC is negligible, and thus the device achieves better power performance at cryogenic temperatures compared with that in SiGe HBT without SIC.     

The effects of base dopant outdiffusion and undopedSi1-xGex junction spacer layers in Si/Si1-xGex/Si heterojunction bipolar transistors

The effects of base dopant outdiffusion and nominally undoped Si _1-xGe_x spacer layers at the junction interfaces of Si/Si_1-xGe_x/Si n-p-n heterojunction bipolar transistors (HBTs) have been studied. It has been found that small amounts of boron outdiffusion from heavily doped bases of nonabrupt interfaces cause parasitic barriers in the conduction band, which drastically reduce the collector current enhancement in the HBTs. Undoped interface spacers can remove the parasitic barriers, resulting in a strongly improved collector current enhancement     

Simulation study of high injection effects and parasitic barrier formation in SiGe HBTs operating at high current densities

The effects of high carrier densities near the base–collector (B–C) heterojunction in npn SiGe heterojunction bipolar transistors during device operation at high current densities has been investigated using a commercial numerical device simulator. Due to electron velocity saturation in the B–C space charge region and the presence of the valence band discontinuity at the B–C junction, hole accumulation occurs at the collector end of the base at the onset of base pushout at high current densities. Formation of a parasitic barrier to electron flow in the conduction band occurs at the collector junction which increases recombination in the base and the base current and produces saturation in the collector current. Together, these effects produce an abrupt degradation in the transistor’s current gain and cutoff frequency with increasing emitter junction bias. In this paper, we investigate the onset and relationship of parasitic barrier formation and base pushout, and their dependence on device structure and biasing.     

DC characteristics and stability behaviour of high-speed Si/SiGe HBTs with undoped SiGe spacer between base and collector

The quality of MBE-grown Si/SiGe heterojunction bipolar transistors (HBTs) was improved by inserting a well-defined undoped SiGe layer between the neutral base and the collector. The DC characteristics and the long-term stability of these non-selfaligned devices proved to be excellent, and the transit frequency reached 33 GHz (V/sub CB/=1 V). Those features are essential preconditions for future application in high-speed ICs.<>     

Parasitic energy barriers in SiGe HBTs

Parasitic energy barriers can easily be introduced during processing. Measurements and calculations of experimental n-p-n HBTs (heterojunction bipolar transistors) are presented, showing that a parasitic conduction-band barrier at the base-collector junction reduces the collector current and the cutoff frequency. A simple analytical model explains the f_T degradation, caused by the reduction of the collector current and a pileup of minority carriers in the base. With the model the effective height and width of the barrier can also be derived from the measured collector current enhancement factor I_C(SiGe)/I_C(Si)     

Impact of collector-base junction traps on low-frequency noise in high breakdown Voltage SiGe HBTs

This work investigates the impact of collector-base (CB) junction traps on low-frequency noise in high breakdown voltage (HBV) SiGe HBTs. By comparing the base current and 1/f noise at the same internal emitter-base (EB) voltage of the standard breakdown voltage (SBV) and HBV devices, we show that the CB junction traps not only increase base current, but also contribute base current 1/f noise when high injection occurs. The individual 1/f noise contributions from the emitter-base junction traps and from the collector-base junction traps are separated. The dependence of the 1/f noise component on the corresponding base current component is determined, and shown to be different for the EB and CB junction traps. The dependence of the total 1/f noise on the total base current, however, remains the same before and after high injection occurs in the HBV device, which is approximately the same as that for the SBV device. The I/sub B/ contribution from the CB junction recombination current needs to be modeled for accurate I-V and 1/f noise modeling.     

Space-charge region recombination in heterojunction bipolartransistors

A new comprehensive model for space-charge region (SCR) recombination current in abrupt and graded energy gap heterojunction bipolar transistors (HBTs) is derived. It is shown that if a spike is present in one of the bands at the heterojunction interface, the SCR recombination current becomes interrelated with the collector current. A previously proposed charge control model for the HBT is modified to include the SCR recombination current. The model is used to study SCR recombination characteristics in HBTs