Base Transit Time Research Articles

Switching-on Delay Jitter Caused by Lateral Distribution of Current Channel of Avalanche Transistor

The stability of the avalanche transistor’s (AT’s) switching-on process is essential for its extensive application in power semiconductors. The switching-on process was typically described in one-dimensional terms, overlooking the effects of multi-dimensional structural variations on stability. This paper investigated the influence of the lateral distribution of current channels on the switching-on delay jitter in the AT. The lateral size of the current channel affects the transit time by changing the electron path in the base region, resulting in the switching-on delay jitter of the AT. An analytical formula for the lateral size of the current channel and the switching-on delay jitter has been proposed. The two-dimensional simulation model of the AT gave the distribution of current channels. The model’s accuracy was verified by comparing experimental and simulation data. The experimental data proved that the base transit time was the main component of the switching-on delay. The results show that the switching-on delay jitter can be significantly reduced by adjusting the current channel’s lateral size. In addition, the trigger signal’s characteristics also change the current channel’s lateral distribution and then affect the stability of the switching-on delay, which provides a new perspective for the design and application of ATs.

Overview of Aging Mechanisms in Sige Hbts

SiGe HBTs offer significant performance advantage over Si BJTs with improved base resistance and base transit time. However, each faster technology iteration usually comes at the cost of reduced breakdown parameters. As a result, devices are often restricted to smaller operation biases, and rely on fully utilizing these imposed limits to attain maximum circuit performance. Both high voltage and high current operation have been shown to take a toll on a device’s aging behavior. One unintended consequence of technology scaling is that it forces designers to consider alternate circuit architectures and operate these devices under non-traditional modes such as reverse active, off-state, and saturation. Although not as optimal for device performance as forward active operation, these modes enable improved radiation hardening, power savings and even simpler circuit design.A bipolar transistor’s operation can be categorized into four main modes as shown in the figure below based on the applied voltage to the base-emitter (VBE) and base-collector (VCB) junctions. Due to the asymmetrical junction engineering for vertical HBTs, the forward-active mode is usually the most optimized for device performance. Because it is the most utilized mode, it is also the most characterized for device reliability. In this paper, in addition to the mixed mode stress used to characterize SiGe bipolars, we will discuss some of the recent learning in SiGe HBT reliability in the other less-understood modes of operation. We begin with a brief overview of SiGe HBT reliability physics including hot carrier generation (Auger vs. Impact-Ionization), the temperature dependence of competing mechanisms and the physical damage responsible for electrical degradation. Using measurements and simulations, we discuss in detail the device degradation under off-state and saturation modes. We demonstrate how the hot carrier generation physics responsible for degradation in the forward-active mode is still applicable in the other regions of operation. Figure 1

A vertical silicon-graphene-germanium transistor

Graphene-base transistors have been proposed for high-frequency applications because of the negligible base transit time induced by the atomic thickness of graphene. However, generally used tunnel emitters suffer from high emitter potential-barrier-height which limits the transistor performance towards terahertz operation. To overcome this issue, a graphene-base heterojunction transistor has been proposed theoretically where the graphene base is sandwiched by silicon layers. Here we demonstrate a vertical silicon-graphene-germanium transistor where a Schottky emitter constructed by single-crystal silicon and single-layer graphene is achieved. Such Schottky emitter shows a current of 692 A cm−2 and a capacitance of 41 nF cm−2, and thus the alpha cut-off frequency of the transistor is expected to increase from about 1 MHz by using the previous tunnel emitters to above 1 GHz by using the current Schottky emitter. With further engineering, the semiconductor-graphene-semiconductor transistor is expected to be one of the most promising devices for ultra-high frequency operation.

Base transport factor and frequency response of transistor lasers

We report on a charge control analysis that relates the characteristic time constants of a three-port laser made of a quantum-well (QW) heterojunction bipolar transistor (HBT) to the electronic gain of the device. For this purpose, we take into account the linear variation of the base transport factor α with the HBT base current. Our approach enables us to obtain QW capture time, base recombination lifetime, and base transit time in terms of the experimental values of base current and of the transistor laser (TL) design parameters such as base width, QW width, and QW location. Whereas the base recombination lifetime is calculated to be a fraction of a nanosecond, the QW capture time is found to be of the order of a picosecond or less. The time constants obtained from our model are used to successfully reproduce the TL experimental optical frequency response.

Quantum well capture and base carrier lifetime in light emitting transistor

We carry out the physical modeling of the light emitting transistor (LET) operation with the focus on the carrier lifetime in the base, which is a key factor in the device speed performances. Our model is based on the observation of the degradation of the base transport factor caused by the LET heavy base doping and its variation with the base current. We revise the conventional charge control model of the bipolar junction transistor to account for these features and assess the concentration of the base minority carriers captured in the quantum wells (QWs). Our approach based on the Zhang and Leburton (Z-L) rate equation model enables us to obtain the device microscopic parameters, such as the capture time, the base lifetime and the base transit time in terms of the LET emitter current and the base current, as well as the design parameters such as the doping concentration, the base width, the QW width and number, and their location. Whereas the base recombination lifetime can be estimated to be of the order of a fraction of a nanosecond, the QW capture time is found to be of the order of a picosecond or less. Our simulation results agree well with the LET optical frequency response obtained experimentally.

New fast turn-on speed SCR device for electrostatic discharge protection

A new SCR with the variation lateral base doping (VLBD) structure (VSCR) is proposed to improve the turn-on speed for electrostatic discharge (ESD) protection. The turn-on speed of the SCR was determined mainly by the base transit time of the parasitic p-n-p and n-p-n transistors of the SCR, and the VLBD structure can reduce the base transit time of the bipolar transistors to improve the turn-on speed of the SCR. The experimental and simulation results show that the turn-on time of the VSCRs with the VLBD structure is 12% less than that of the MLSCR with the traditional uniform base doping without adding extra process masks and increasing the chip area.

Influence of temperature on transit times and microwave noise performances of SiGe HBT

The influence of temperature (300 K and 40 K) on intrinsic transit times and microwave noise performances of silicon germanium (SiGe) heterojunction bipolar transistors (HBTs) is investigated. At 300 K, we compared measured and modelled S-parameters and four noise parameters, and we found a good agreement. At 40 K, we compared measured and modelled S-parameters, and we deduced noise performances from the S-parameter measurements. The electric model includes correlated junction noise sources and a proper extraction of the transit times involved in these sources. Moreover, the microwave noise model considers all the physical phenomena that impact noise performances in SiGe HBTs. We analysed three devices having different Ge content (10%–20%, 10%–25% and 10%–30%). At 40 K, the device with 10%–25% reaches one of the lowest base transit times (τB), the lowest minimum noise figure (NFmin), and the lowest equivalent noise resistance (Rn), for operation frequencies up to the maximum device dynamic performances (f ≈ fT) These results demonstrate the excellent potential to develop cryogenic applications of SiGe HBTs.

A novel approach for justification of box-triangular germanium profile in SiGe HBTs

This work presents a unique and robust approach for validation of using the box-triangular germanium profile in the base of SiGe heterojunction bipolar transistors, where the methodology considers the simultaneous optimization of the p-type base doping profile in conjunction with the germanium profile in the base. The study analyses the electron motion across the SiGe base in SiGe HBTs, owing to different accelerating electric fields. The analysis first presents a figure of merit, to achieve the minimum electron transit time across the base in conjunction with the increased current gain in n—p—n-SiGe HBTs, which shows a general trend vis-à-vis the advantage of a trapezoid germanium profile, but with additional accuracy as we considered simultaneously optimized p-type base doping. The effect of minority carrier velocity saturation is then included to make the study more detailed. The analysis then investigates the shifted germanium profile in the base to further minimize the base transit time. Finally, it is shown that a shifted germanium profile eventually evolves into a box-triangular Ge-profile in the SiGe base, which could simultaneously minimize the base transit time and reduce emitter delay by virtue of the high current gain. The analysis verifies that for an average Ge-dose of 7.5% Ge across the base, a box-triangular germanium profile in conjunction with an optimum base doping profile has an approximately identical base transit time and a 30% higher current gain, in comparison with an optimum base doping and triangular Ge-profile across the whole base.

Investigation of effective base transit time and current gain modulation of light-emitting transistors under different ambient temperatures

In this report, the modulation of current gain of InGaP/GaAs light-emitting transistors under different ambient temperatures are measured and analyzed using thermionic emission model of quantum well embedded in the transistor base region. Minority carriers captured by quantum wells gain more energy at high temperatures and escape from quantum wells resulting in an increase of current gain and lower optical output, resulting in different I-V characteristics from conventional heterojunction bipolar transistors. The effect of the smaller thermionic lifetime thus reduces the effective base transit time of transistors at high temperatures. The unique current gain enhancement of 27.61% is achieved when operation temperature increase from 28 to 85 °C.

Quantum Well Saturation Effect on the Reduction of Base Transit Time in Light-Emitting Transistors

In this paper, we demonstrate the improvement of cutoff frequency of the light-emitting transistor (LET) due to quantum well (QW) band-filling phenomenon (i.e., saturation) along with the carrier capturing and escaping processes in the base region. Through microwave measurement followed by small-signal model analysis, we observe that the base transit time, τ t , of the LET is reduced evidently from 90 to 20 ps when the collector current density increases from 2.43 to 34.9 kA/cm 2 . The reduction of τ t via saturation effect can be explained by the electroluminescence spectrum and thermionic emission theory of QW.

A novel DTSCR with a variation lateral base doping structure to improve turn-on speed for ESD protection

The turn-on speed of electrostatic discharge (ESD) protection devices is very important for the protection of the ultrathin gate oxide. A double trigger silicon controlled rectifier device (DTSCR) can be used effectively for ESD protection because it can turn on relatively quickly. The turn-on process of the DTSCR is first studied, and a formula for calculating the turn-on time of the DTSCR is derived. It is found that the turn-on time of the DTSCR is determined mainly by the base transit time of the parasitic p–n–p and n–p–n transistors. Using the variation lateral base doping (VLBD) structure can reduce the base transit time, and a novel DTSCR device with a VLBD structure (VLBD_DTSCR) is proposed for ESD protection applications. The static-state and turn-on characteristics of the VLBD_DTSCR device are simulated. The simulation results show that the VLBD structure can introduce a built-in electric field in the base region of the parasitic n–p–n and p–n–p bipolar transistors to accelerate the transport of free-carriers through the base region. In the same process and layout area, the turn-on time of the VLBD_DTSCR device is at least 27% less than that of the DTSCR device with the traditional uniform base doping under the same value of the trigger current.

Microwave Determination of Quantum-Well Capture and Escape Time in Light-Emitting Transistors

We report the determination of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{In}_{0.20}\hbox{Ga}_{0.80}\hbox{As}$ </tex></formula> quantum-well (QW) capture and escape time by utilizing electrical microwave measurement on an InGaP/GaAs light-emitting transistor (LET) platform. The emitter-to-collector transit times <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau_{\rm ec}$</tex></formula> of the LET and the heterojunction bipolar transistor (HBT) are measured and extracted under the same bias condition ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{ \rm bc} = \hbox{0}\ \hbox{V}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$J_{c} = \hbox{22.6}\ \hbox{kA}/\hbox{cm}^{2}$</tex></formula> ). Small-signal model analysis shows that the LET exhibits a larger base transit time <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau_{t}$</tex></formula> of 25 ps compared with 5 ps of the HBT. The difference, i.e., 20 ps, hence is attributed to the carrier capturing/escaping processes caused by the QWs embedded in the base region of the LET.

Composition-Graded AlInP/AlGaAsSb/InP Type-II DHBTs With $f_{T}/f_{\rm MAX} = \hbox{450/510}\ \hbox{GHz}$

A type-II AlInP/AlGaAsSb/InP double heterojunction bipolar transistor (DHBT) with composition-graded Al content in the base is designed and grown by molecular beam epitaxy. Compared with composition-graded GaAsSb and InGaAsSb bases in DHBT, the graded AlGaAsSb base can generate a larger built-in electric field, thus reducing base transit time and improving device speed. We have fabricated AlGaAsSb graded base DHBTs and demonstrated f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> /f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> = 450/510 GHz for a 0.3 × 2 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device.

Technology Computer-Aided Design (TCAD) Feasibility Study of Scaling SiGe HBTs

Technology computer-aided design (TCAD) feasibility studies for for scaling a SiGe-based NPN with cutoff frequencies, fT/fMAX of 200/250GHz were conducted, in which the critical process and device design components were self-consistently addressed. The calibrated 200/250GHz calibration was verified by utilization for scaling to a 300GHz performance level, which was subsequently used as the baseline for scaling to a 500 and 630GHz fT performance level. Comparison of the transit time components reveal the base and collector-base transit times as being the predominant electron delay components for performance levels less than 500GHz. At 630GHz the device was observed to be limited primarily by the collector-base transit time.

Monte Carlo Analysis of Base Transit Times of InP/GaInAs Heterojunction Bipolar Transistors with Ultrathin Graded Bases

The base transit times of an InP/GaInAs heterojunction bipolar transistor (HBT) with an ultrathin and heavily doped base are investigated by carrying out a Monte Carlo (MC) simulation. The acceleration of electrons due to a conduction-band discontinuity between the emitter and base is taken into consideration. Scattering caused by the spontaneous emission of hole plasmons considerably increases the base transit time even when the thickness of the base is 20 nm. Band-gap grading in the base effectively sweeps slow electrons into the collector, and the increase in the base transit time is partially suppressed. The MC simulation shows that when the band-gap grading is just twice the thermal energy, the transit time of the 20-nm-thick base decreases by 40%.

Experimental study of transport in InAs Quantum Hot Electron Transistor

We study a Quantum Hot Electron Transistor, an original transistor made of InAs/AlSb heterostructures with high speed intrinsic transport. It is a unipolar vertical transport device, based on the concept of hot electron transistor. Devices using different heterostructure designs have been grown, processed and characterized. Both resonant and parasitic transport of electrons is studied by mean of the static output characteristics of the transistors. We demonstrated that a key parameter is the design and thickness of the collector barrier. For 15nm-thin barriers, we obtained static current gain of 5 associated to a base transit time of 140 fs.

Doping profile and Ge-dose optimization for silicon–germanium heterojunction bipolar transistors

The speed of silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs) has been dramatically increased. It is known that the speed of HBTs is dominated by the base transit time, which could be influenced by the doping profile in the base region and the Ge concentration. In this study, the design of the doping profile and Ge-dose concentration for SiGe HBTs are mathematically formulated and solved by a technique of geometric programming (GP). The solution calculated by the GP method is guaranteed to be a global optimal. The accuracy of the adopted numerical optimization technique is first confirmed by comparing with two-dimensional device simulation. The result of this study shows that a 23% Ge fraction may maximize the current gain; furthermore, a 12.5% Ge may maximize the cut-off frequency for the explored device, where a 254 GHz cut-off frequency is achieved.

Analytical models for the base transit time of a bipolar transistor with double base epilayers

The doping profile function of a double base epilayer is constructed according to drift-diffusion theory. Then an analytical model for the base transit time τb is developed assuming a small-level injection based on the characteristics of the 4H-SiC material and the principle of the 4H-SiC BJTs. The device is numerically simulated and validated based on two-dimensional simulation models. The results show that the built-in electric field generated by the double base epilayer configuration can accelerate the carriers when transiting the base region and reduce the base transit time. From the simulation results, the base transit time reaches a minimal value when the ratio of L2/L1 is about 2.

Tilted-charge high speed (7 GHz) light emitting diode

We demonstrate a higher speed form of light emitting diode (LED), an asymmetrical two-junction tilted-charge LED, utilizing an n-type buried “drain” layer beneath the p-type “base” quantum-well (carrier and photon) active region. The drain layer tilts and pins the charge in the manner of a heterojunction bipolar light emitting transistor (HBLET), selecting and allowing only “fast” recombination (recombination lifetime τB of the order of base transit time τt). The tilted-charge LED, simple in design and construction, is capable of operation at low current in spontaneous recombination at a 7 GHz bandwidth or even higher with more refinement.

Minoritary Transport in Heavily Doped p-type InGaAs

AbstractMinoritary carrier’s transport properties are of fundamental importance in the HBT’s physics. The base transit time is a key parameter to improve microwave figure of merit. Some recent minoritary electron mobilities measurements versus acceptor doping level using magneto transport method exhibit a dramatic increase at very high majority carrier concentration. This effect has been attributed to the coupling of polar optical phonons with hole plasmons (LOPC) which controls the balance between enegy gain by electric field acceleration and energy loss by polar optical phonon emission. We present minoritary mobilities as a function of majority carrier doping calculated in the frame of electrons and holes Monte Carlo modelling including LOPC.