Base Doping Concentration Research Articles

Optimum base-doping profile for minimum base transit time considering velocity saturation at base-collector junction and dependence of mobility and bandgap narrowing on doping concentration

We used variational calculus, the trial function, and the iterative procedure method to evaluate the optimum base doping concentration profile that will yield the minimum base transit time /spl tau//sub B/. All methods were extended to include the velocity saturation at the base collector junction, and the dependence of mobility and bandgap narrowing on the base doping concentration. We showed that all methods produce almost the same /spl tau//sub B/, although the profiles differ. Among them, the iterative method always produces the minimum /spl tau//sub B/ and variational calculus clearly shows the dependence of /spl tau//sub B/ on physical parameters.

Pnp InGaAsN-based HBT with graded base doping

A grading of the base doping concentration is used to establish an electric field in the base of a Pnp AlGaAs/InGaAsN heterojunction bipolar transistor (HBT). An improved peak current gain value (β ≃ 60) is observed over a device with uniform base doping, due to better hole transport in the base. A cut-off frequency of 15 GHz measured from a 3 × 12 µm2 non-self aligned HBT demonstrates the practical impact the electric field has on the transit time across the base.

Base transit time in abrupt GaN/InGaN/GaN HBT's

Base transit time in an abrupt GaN/InGaN/GaN HBT is reported. Temperature and doping concentration dependence of low field mobility is obtained from an ensemble Monte Carlo simulation. Base transit time, /spl tau//sub b/, decreases with increasing temperature. The low temperature /spl tau//sub b/ is dominated by the diffusion constant or, in other words, transport within the neutral base region. However, at elevated temperatures base transit time is dependent more upon the base-collector junction velocity or, in other words, by the transport across the heterointerface. /spl tau//sub b/ increases with In-mole fraction showing a stronger dependence at lower temperatures. Unity gain current cut-off frequency, f/sub T/, is a strong function of temperature and base doping concentration. An f/sub T/ of 20 GHz is obtained for a 0.05 /spl mu/m HBT.

Analysis of conduction mechanisms in annealed n-Si1−xCx:H/p-crystalline Si heterojunction diodes for different doping concentrations

We fabricated and characterized annealed n-type amorphous Si1−xCx on p-type crystalline silicon heterojunction diodes with three different base doping concentrations: NA∼1016, 1018, and 1020 cm−3. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that the diodes with low doping concentrations (1016 cm−3) are ideal, because the phosphorous slightly diffuses into the crystalline silicon, whereas diodes with higher doping concentrations (1018–1020 cm−3) are dominated by multitunneling capture emission. The increase in the base acceptor doping concentration also causes excess current over the ideal diode current at low forward bias and an increase in the leakage reverse current.

Determination of polysilicon solar cell parameters using electrical short-circuit current decay method

The electrical short-circuit current decay (ESCCD) method is enlarged to a polycrystalline silicon solar cell with grains fibrously oriented. The current decay through the n–p junction of a short-circuited cell, after the sudden cut-off of a constant forward voltage, is derived and numerically computed. A theoretical study of the influence of grain boundaries and base doping concentration on ESCCD, not noticed earlier, is made. A graphical evaluation of GB recombination velocity S ng and bulk lifetime τ n is performed in the case of a BSF cell. The accuracy of the ESCCD method in the extraction of these solar cell parameters is found to be very sensitive to the ratio of base width to grain size H b/ X g.

Profile design considerations for minimizing base transit time in SiGe HBT's

A unified closed form analytical model for base transit time of SiGe HBT's for uniform and exponential base dopant distributions with different Ge profiles in the base (e.g., box, trapezoidal, triangular) is reported. The model is subsequently used to study the design of Ge profile for different base doping profiles, including that of epitaxial base transistors. Consistent with the reported results, our unified model predicts that beyond a certain total Ge content, there is very little reduction in /spl tau//sub b.SiGe/. It is further demonstrated that the trapezoidal Ge profile with X/sub T//spl sim/0.8W/sub B/ gives near optimal base transit time for all doping profiles considered. Our analysis shows that 1) for a given base width and intrinsic base resistance, the exponential base doping profile with Ge yields the least value of /spl tau//sub b, SiGe/ and 2) for a given peak base doping concentration and the intrinsic base resistance, the uniform base doping with Ge gives minimum /spl tau//sub b, SiGe/. Also, the need for keeping the total base Ge content constant while optimizing the Ge profile in the base is emphasized by showing that a false minimum for /spl tau//sub b, SiGe/ may appear if the total Ge content is not kept constant.

Quantitative modelling of current gain and cut-off frequency at high injection levels at 77 and 300K in polysilicon emitter contact bipolar transistor

Existed experiments and theories have demonstrated that the polysilicon emitter contact bipolar transistor (PET) is suitable for low-temperature operation. However, an overall theory for the current gain and the cut-off frequency of PET is still absent, particularly for the quantitative modelling at high injection levels. The paper presents the quantitative relationships between the current gain, the cut-off frequency and the collector current density at 77 K and 300 K. The main physical mechanisms for the current gain and the cut-off frequency at high injection levels are also analysed. Finally, the influence of the maximum base and emitter doping concentrations on the current gain, the base and emitter transit times in polysilicon emitter contact transistors is also analysed at low temperatures. These may provide a rational basis for the design of low-temperature bipolar transistors.

On buried oxide effects in SOI lateral bipolar transistors

We report buried oxide effects on the Silicon-On-Insulator Lateral Bipolar Transistor (SOILBT) performance by two-dimensional (2-D) numerical simulation and experiments. An early punchthrough is observed in SOILBT compared to the bulk due to the presence of buried oxide. In addition to dopant segregation into the buried oxide, the presence of buried oxide also diverts some electric field lines emanating from collector toward substrate, due to 2-D distribution of field, leaving fewer across the base region and hence increased depletion widths and punchthrough. One-dimensional (1-D) depletion approximation fails to predict this punchthrough. To establish the evidence of buried oxide induced punchthrough without dopant segregation effect, simple and yet novel measurement techniques are proposed to extract effective base width and base doping concentration near the buried oxide-silicon film interface using the parasitic MOSFET in SOILBT. Good agreement between 2-D simulation and experimental results was observed. Finally design curves are generated using 2-D numerical simulation for different base doping and buried oxide thicknesses on SOI substrates.

Analytical modeling of the recombination current in the base-emitter space charge region of heterojunction bipolar transistors

An analytical modeling of the recombination current in the base-emitter space charge region of heterojunction bipolar transistors is derived. The recombination rates as functions of distance in the space charge region are calculated for different base doping concentrations. The relations between the space charge recombination current and the applied voltage have been determined.

Quantitative modelling of DC and transient characteristics at high injection level at 77 and 300K in silicon bipolar transistor

The current gain of a silicon bipolar transistor has been quantitatively modelled by the Early effect, the conductivity modulation effects in the base and in emitter, the effective base widening effect and the emitter current crowding effect at 77 and 300K involved in the carrier recombination currents in the emitter-base space charge region and in the base. On this basis, the transit times, the total transit time from the emitter to collector and the cutoff frequency are modelled as functions of the collector current at 77 and 300K. The results obtained are in agreement with the experimental data. Besides this, the influence of the maximum base doping concentration on the maximum current gain and the maximum cutoff frequency has been also analysed. The main results show that the current gain is mainly determined by the conductivity modulation effect and the emitter current crowding effect at high injection level at 77K, but at 300K it is mainly determined by the effective base widening effect. The cutoff frequency is mainly determined by the minority-carrier base transit time at 300K, and at 77K the minority-carrier emitter transit time may be dominant for the large bandgap narrowing effect in emitter. The bandgap narrowing effect is the main reason for the degradation of the cutoff frequency with lowering temperature, instead of the low-temperature trapping effect by the compensated impurities with the shallow energy levels.

Fluorine effects in n-p-n double-diffused polysilicon emitter bipolar transistors

We demonstrate that fluorine incorporation in the polysilicon emitter of n-p-n double-diffused bipolar transistors during BF/sub 2/ implantation at a dose of 1/spl times/10/sup 15/ cm/sup -2/ significantly alters the device electrical characteristics. In particular, tunneling emitter/base currents are observed at both forward and reverse voltages, due to excessive base dopant concentration at the junction. Fluorine-enhanced interfacial oxide break-up and epitaxial realignment of the poly-Si emitter are shown to be responsible for these results.

Electrical determination of bandgap narrowing in bipolar transistors with epitaxial Si, epitaxial Si/sub 1-x/Ge/sub x/, and ion implanted bases

The apparent bandgap narrowing in bipolar transistors with epitaxial Si, epitaxial SiGe and ion implanted bases is measured from the temperature dependence of the collector current density J/sub c/(T). A graph of InJ/sub c/(T)/J/sub o/(T) as a function of reciprocal temperature is plotted, and the apparent bandgap narrowing obtained from the slope. For epitaxial base transistors, in which the boron base profiles are abrupt, a linear J/sub c/(T)/J/sub o/(T) characteristic is obtained, which allows the unambiguous determination of the apparent bandgap narrowing. The measured values for epitaxial Si bases are in good agreement with the theoretical model of Klaassen over a range of base doping concentrations. For Si/sub 0.88/Ge/sub 0.12/ and Si/sub 0.87/Ge/sub 0.13/ epitaxial base heterojunction bipolar transistors (HBT's), values of bandgap narrowing of 119 and 121 meV are obtained due to the presence of the Ge, which can be compared with theoretical values of 111 and 118 meV. For the implanted base transistor, the J/sub c/(T)/J/sub o/(T) characteristic is not linear, and its slope is larger at high temperatures than at low. This behaviour is explained by the presence of a tail on the ion implanted profile, which dominates the Gummel number of the transistor at low temperatures.

Correlation of effective recombination velocity of a backsurface field with a boron concentration profile

The efficiency of silicon solar cells can be increased by reducing the recombination of minority carriers at the backsurface of a cell. This can be achieved by application of a low-high junction, commonly called a backsurface field (BSF). The dependence of the effective backsurface recombination velocity Seff of a BSF on the BSF doping profile and the base doping concentration was studied experimentally. The doping profiles were analyzed using sheet resistance measurements, stripping Hall measurements, and secondary ion mass spectrometry. The effective recombination velocity was obtained from photoconductivity decay probed with microwaves on symmetrical p+pp+ structures, and from photocurrent decay measurements on n+pp+ solar cells. The measured values of Seff were compared with calculated values, based on the measured doping profiles. Both from photoconductivity and photocurrent decay measurements, Seff was found to decrease with an increasing difference in acceptor concentration between the BSF and the base of the solar cell. The measured values of Seff, obtained with the two experimental techniques, are in agreement with each other.

Analytical and experimental characteristics of SiGe heterojunction bipolar transistors with thin α-Si:H emitters

SiGe heterojunction bipolar transistors (HBTs) with thin n + hydrogenated amorphous Si (α-Si:H) emitters are discussed. A first order analytical model is used to illustrate the leverage of the structures in terms of current gain, resulting from the barrier height created by the thin α-Si:H emitters, and high Early voltage resulting from optimizing the Ge profile and doping concentration in the base. Two-dimensional simulations were carried out to investigate device performance parameters. Devices with 50 Å α-Si:H emitters and various base doping concentrations and Ge distribution profiles were fabricated. A current of 100, current gain-Early voltage product larger than 11040 and unity gain cutoff frequency f T of 14.5 GHz were obtained for devices having base doping concentrations of 1 × 10 19 cm −3, a base Ge grading of 0–20% and an emitter size of 2 × 4 μm 2.

Molecular beam epitaxy growth for a heavily-doped thin base layer of heterojunction bipolar transistors used for high-speed integrated circuits

The technique of molecular beam epitaxy (MBE) growth has been developed for high-speed AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with a heavily beryllium-doped base layer. Doping limit, DC current gain and reliability were examined under various growth conditions. By the growth under the conditions that substrate temperature was 580°C and V/III flux ratio was 27, the authors have developed HBTs in which the base doping concentration was set to as heavy as 8 × 10 19 cm −3 and the base thickness was set to 45 nm or 90 nm. Using these HBTs, they have fabricated ICs which can operate at 40 GHz.

High current gain InGaP/GaAs heterojunction bipolar transistors grown by multi-wafer gas-source molecular beam epitaxy system

We report for the first time on the high quality and highly uniform InGaP/GaAs heterojunction bipolar transistors with a carbon-doped base grown by a multi-wafer gas-source molecular beam epitaxy (GSMBE) system, which was developed by ourselves. A large-area uniform growth of p-type GaAs and InGaP has been realized. The variation in layer thickness of these layers was less than ± 1.8% across three 3-inch wafers. A large-area (200 × 200 μm 2) heterojunction bipolar transistor (HBT) with a base-sheet resistance of 350 Ω showed a high current gain of 190. This is effectively the highest value considering the base sheet resistance, and is comparable to the best data of AlGaAs/GaAs HBTs reported so far. The small signal current gains showed an excellent uniformity of 3% across one of five 2-inch wafers. The wafer-to-wafer variation of current gains for the two 2-inch wafers, in the same growth run, was less than 3%. The base sheet resistances also showed an excellent uniformity of 0.86%. In addition the average base sheet resistances of the two 2-inch wafers was the same. The device also showed an excellent high-frequency characteristics. The cut-off frequency ( f t ) of 61 GHz, and the maximum-oscillation frequency ( f max ) of 94 GHz were obtained for the transistor having a base dopant concentration of 4 × 10 19 cm −3. These promising results demonstrate the high potential capability of multi-wafer GSMBE for the production of InGaP/GaAs HBTs.

Ultrahigh f/sub T/ and f/sub max/ new self-alignment InP/InGaAs HBT's with a highly Be-doped base layer grown by ALE/MOCVD

We report on a new self-alignment (SA) process and microwave performance of ALE/MOCVD grown InP/InGaAs heterojunction bipolar transistors (HBT's) with a base doping concentration of 1/spl times/10/sup 2/0 cm/sup -3/. We obtained f/sub T/ of 161 GHz and f/sub max/ of 167 GHz with a 2/spl times/10 /spl mu/m emitter. These high values indicate the best performance of InP/InGaAs HBT's ever reported, in so far as we know. These values were attained by reducing the base resistance using ALE/MOCVD and base-collector capacitance using a new SA process. These results indicate the great potential of these devices for ultrahigh-speed application. >

Bandgap narrowing effects on the open circuit voltage in Si, GaAs and InP solar cells

Bandgap narrowing affects semiconductor device operation. Solar cells are pn junction devices for which the energy bandgap is a very important parameter. The aim of this paper is to investigate how bandgap narrowing can influence the open circuit voltage of silicon, gallium arsenide and indium phosphide solar cells. The increase in the base doping concentration for improving device operation is limited by bandgap narrowing effects. A quantitative study of the open circuit variation with base doping concentration and ideality factor is presented and the critical values for these parameters are determined for these materials.

A CAD procedure for optimizing bipolar devices relative to BiCMOS circuit delays

Proposes an optimization procedure for the bipolar base doping profile for a high-speed BiCMOS circuit. This procedure aims to achieve the minimum propagation delay time, taking the power supply, the load capacitance, and the bipolar device layout into consideration. For a small load capacitance, the peak base doping concentration and the base width should be as small as possible. However, for a high load capacitance, the optimized base doping profile can vary under the emitter area constraint. Utilizing this optimization procedure, it will be shown that the propagation delay time can be reduced by more than 30% at the same MOS current (I/sub MOS/=1 mA) through the coordinated reduction of both the vertical doping profile and the horizontal dimensions of the bipolar transistor, with the power supply voltage scaled from 5 to 3.3 V. >

Profile Tail Effects on Low Temperature Operation of Silicon Bipolar Transistors

We present a detailed investigation, in the temperature range between 340 K and 80 K, of the static properties of advanced single-polysilicon bipolar transistors covering a wide range of base doping concentrations. Special attention is paid to the temperature dependence of the intrinsic current gain β. We find that the rate of degradation of β with temperature below about 150 K is nearly independent of the peak or average base doping concentration. Our data do not match the conventional theory of the temperature dependence of β using available band-gap narrowing models for p-type silicon. We show that these, as well as other recently published related results, can be understood considering the importance of profile tail regions in controlling the operation of bipolar transistors at low temperature.