On the operation configuration of SiGe HBTs based on power gain analysis

The power gain difference, under different device stability conditions, between common-emitter (CE) and common-base (CB) bipolar junction transistors (BJT) is analyzed comprehensively. The analysis reveals that the CB configuration offers higher maximum available power gain than the CE configuration in the device's high operation frequency range, while the inverse relation holds in the very low frequency range. In the intermediate frequency range, the base resistance value, mainly affected by the base doping concentration, determines which configuration offers higher maximum stable power gain (MSG). These analyses have explicit implications on the operation configurations of SiGe heterojunction bipolar transistors (HBTs). Employing a typical doping profile of Si bipolar junction transistors with a trapezoidal Ge profile in SiGe HBTs usually results in a larger base resistance than the emitter resistance. For these devices, the CE configuration exhibits higher MSG than the CB configuration. Employing a higher base doping concentration than the emitter with a box-type Ge profile considerably reduces the base resistance and thus favors the CB configuration for power amplification in this frequency range. The analysis are quantitatively verified with simulation and measurement results from SiGe HBTs of representative Ge and base doping profiles.     

Voltage stress-induced hot carrier effects on SiGe HBT VCO

This paper presents the hot carrier (HC) induced performance degradation in a 10 GHz voltage controlled oscillator (VCO) with SiGe heterojunction bipolar transistors (HBTs). SiGe device characteristics due to HC stress are examined experimentally. The vertical bipolar inter-company (VBIC) model parameters extracted from measured data are used in Cadence SpectreRF simulation to verify the HC effect on the VCO. The VCO shows significant vulnerability to hot carriers.     

Degradation of AlGaAs/GaAs HBTs induced by hot carriers

In AlGaAs/GaAs HBTs, the instability of the surface states of the extrinsic base, which is revealed by mesa-etching and passivated by Si₃N₄, affects reliability. In this study the reverse constant current stress in an avalanche regime is applied across the emitter–base junction in order to test the stability of the heterojunction and the surface state of the extrinsic base. It has been identified that the surface of the extrinsic base is vulnerable to hot carriers. A new degradation mechanism is suggested and verified by numerical simulation. In addition, a way to improve the reliability is proposed based on the experimental results.     

An Ebers-Moll model for the heterostructure bipolar transistor

An Ebers-Moll model for the heterostructure bipolar transistor (HBT) is developed. The model describes both single and double heterojunction transistors with or without band spikes and applies to uniform or graded base HBTs. Model parameters are directly related to device parameters such as doping densities, dimensions and band spikes. Junction velocities are introduced to describe the transport of carriers across the junctions. Results demonstrate that even for compositionally graded junctions, transport across the junctions may limit HBT performance if the base is graded. Use of the model is illustrated by examining a recently proposed technique for extracting conduction band spikes by comparing forward and inverted I-V characteristics.     

A phenomenological approach to estimating transit times in GaAsHBTs

A method is proposed for estimating transit times in heterojunction bipolar transistors (HBTs) which are fabricated from semiconductor materials in which the conduction band can be represented by a two-valley model. The transition times for exchange of electrons between the conduction band valleys are treated as phenomenological parameters and are shown to be linked by the electric field in the device. Incorporation of the transition rates into the continuity equations for upper and lower valley electrons yields a set of equations which can be solved under transient conditions to yield directly the transit times of carriers across the base and the base-collector space-charge region. Results from this approach are compared with Monte Carlo calculations and shown to exhibit good agreement     

Nonequilibrium electron transport in HBTs

Nonequilibrium electron transport in heterojunction bipolar transistors (HBTs) becomes clearly observable as their vertical dimensions are reduced, This is the reason for the superior tradeoff relationships between the device parameters and the resultant high-speed performance of HBTs fabricated with III-V semiconductor materials. This paper reviews the hot carrier effect in the base and the velocity overshoot effect in the collector and discusses the roles these effects play in reducing carrier traveling times and improving device performance     

A reliability comparison of InGaP/GaAs HBTs with and without passivation ledge

InGaP/GaAs heterojunction bipolar transistors (HBTs) with and without passivation ledge in the extrinsic base region were investigated. Gummel plot changes before and after reliability testing were compared. The experimental results demonstrated that the devices featuring the lower quality of the extrinsic base surface are more sensitive to a temperature–current stress. The HBTs with a passivation ledge have an activation energy of 1.41 eV and a mean time to failure (MTTF) of 10⁶ h whereas the HBTs without passivation ledge have an activation energy of 1.24 eV and a MTTF of 10⁵ h.     

Optimization of characteristics related to the emitter-base junction in self-aligned SEG SiGe HBTs and their application in 72-GHz-static/92-GHz-dynamic frequency dividers

Characteristics related to the emitter-base junction of self-aligned selective-epitaxial-growth SiGe heterojunction bipolar transistors (HBTs) were optimized for use with a highly-doped base. The thickness of the Si-cap layer affected both the emitter-base junction concentration and space-charge width, so the dc and ac characteristics of the SiGe HBTs were in turn dependent on this thickness. With a 4/spl times/10/sup 19/-cm/sup -3/ boron-doped base, a 131-GHz cutoff frequency and ECL gate-delay time of 5.4 ps were achieved for the optimized SiGe HBTs. A static frequency divider with a maximum operating frequency of 72.2 GHz and a dynamic frequency divider with a maximum operating frequency of 92.4 GHz were developed for optical-fiber link and millimeter-wave communication systems of the future.     

Resolving degradation mechanisms in ultra-high performance N-p-nheterojunction bipolar transistors

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown Al_xGa_1-xAs/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBTs). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current ratio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V _BE uniformity and long-term reliability of HBTs     

A nonfundamental theory of low-frequency noise in semiconductordevices

A general low-frequency noise theory based on the fluctuation in the number of carriers is presented. In this theory, the low-frequency noise is attributed to the traps within the bandgap of a semiconductor, which are the sources of the generation-recombination noise. The cumulative effect of the generation-recombination noise from each trap center generates a 1/f type noise. It is shown that in fact, 1/f noise may have any frequency dependence between 1/f⁰-1/f². If not masked by thermal noise, the low-frequency noise generated from these traps becomes 1/f² at very high frequencies. Also, if the lifetime of the carriers in the semiconductor under nonequilibrium condition is finite, at very low frequencies, the noise spectral density reaches a plateau. While this theory can be applied to any semiconductor device, only heterojunction bipolar transistors (HBTs) were considered in detail. Based on this theory, a model for low-frequency noise in the base of HBTs is derived. Frequency and current dependence of low-frequency noise are modeled. Results of the base noise measurements in AlGaAs/GaAs HBTs were found to agree with the noise theory presented here. This significant theory, for the first time, proves the possibility of the number fluctuation model as a general 1/f noise cause without a need for specific and nonrealistic carrier lifetime probability functions     

Si/GexSi1-x heterojunction bipolartransistors with the GexSi1-x base formed by Geion implantation in Si

We have fabricated n-p-n, Si/Ge₂Si_1-x heterojunction bipolar transistors (HBTs) with the Ge_xSi_1-x base formed by high-dose Ge implantation followed by solid phase epitaxy. The fabrication technology is a standard self-aligned, double polysilicon process scheme for Si with the addition of the high-dose Ge implantation. The transistors are characterized by a 60 mn-wide neutral base with a Ge concentration peak of ≈8 at.% at the base-collector junction. The HBTs show good electrical characteristics and compared to Si homojunction transistors show lower base resistance, larger values of current gain, and a lower emitter-to-collector transit time     

A base transport model for ultra-thin-base SiGe HBT

The characteristics of parameters such as carrier temperature and diffusion coefficient in ultra-thin-base SiGe heterojunction bipolar transistors (HBTs) are analysed according to the solution of the Boltzmann equation; a new model of base transport in SiGe HBTs, different from traditional ones is described.     

InGaAs/InP heterobipolar transistors for integration on semi-insulating InP substrates

InGaAs/InP npn heterojunction bipolar transistors (HBTs) have been fabricated from LPE layers grown on semi-insulating InP substrates for application to integrated circuits. The inverted emitter configuration is used, which allows the growth of the active layers without any additional steps. The HBTs show stable characteristics without any hysteresis and with current gains up to 25 for 0.7 ?m base width. To our knowledge this is the first report of InGaAs/InP HBTs on a semi-insulating substrate.     

Effect of bulk recombination current on the current gain ofGaAs/AlGaAs heterojunction bipolar transistors in GaAs-on-Si

The effect of bulk recombination current on the current gain of heterojunction bipolar transistors (HBTs) in GaAs-on-Si has been studied. A thin AlGaAs emitter layer is used to prevent an exposed extrinsic base region, which has previously masked the effect of the bulk recombination current on the current gain in HBTs. It is found that once the surface recombination current is removed, the current grain due to bulk recombination alone is approximately ten times lower for the HBTs in GaAs-on-Si than for HBTs in GaAs. The difference in the current gain is probably due to electrically active dislocations in the base in the HBTs in GaAs-on-Si, where the density is about 10⁸ cm^-2 as measured by transmission electron microscopy     

New collector undercut technique using a SiN sidewall for low basecontact resistance in InP/InGaAs SHBTs

A new collector undercut process using SiN protection sidewall has been developed for high speed InP/InGaAs single heterojunction bipolar transistors (HBTs). The HBTs fabricated using the technique have a larger base contact area, resulting in a smaller DC current gain and smaller base contact resistance than HBTs fabricated using a conventional undercut process while maintaining low C_bc. Due to the reduced base contact resistance, the maximum oscillation frequency (f_max) has been enhanced from 162 GHz to 208 GHz. This result clearly shows the effectiveness of this technique for high-speed HBT process, especially for the HBTs with a thick collector layer, and narrow base metal width     

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     

Base Charge Dynamics of Abrupt Base–Emitter Junction HBTs and Its Representation in Transistor Models

Base charge dynamics in abrupt heterojunction bipolar transistors (HBTs) are determined by the thermionic/field emission boundary condition. As a result, their small and large signal models differ from those of bipolar junction transistors (BJTs), which obey the Shockley boundary condition. There is comparison of the base charge dynamics in the abrupt HBT to the BJT case and derive the small and large signal models of the abrupt HBT     

Pulsed current stress of Berillium doped AlGaAs/GaAs HBTs

The major concern for reliability of Berillium doped HBTs is the diffusion of the base dopant towards the emitter. This degradation can be enhanced by the device self-heating and/or by REID mechanism. In order to separate the thermal and REID components to the Berillium outdiffusion we performed a pulsed current stress on AlGaAs/GaAs HBTs. In this paper we report on results obtained with different values of the duty cycles for this current.     

Long-term base current instability in AlGaAs/GaAs HBTs: physical mechanisms, modeling, and SPICE simulation

This paper provides an overview of the long-term base current instability in the AlGaAs/GaAs heterojunction bipolar transistor (HBT), which is a main mechanism governing the HBT long-term current gain drift and thus a major concern for the HBT reliability. Topics covered include: (1) types of base current instability and their underlying physical mechanisms; (2) leakage currents in the HBT and their relevance to the reliability; (3) electrothermal interaction and their impact on the HBT reliability; (4) analytic model for predicting the HBTs mean time to failure (MTTF); and (5) SPICE implementation and simulation of HBT circuit reliability. Measurements and device simulation results are also included in support of the modeling and analysis.