Base-emitter Voltage Research Articles

Revised Method for Extraction of the Thermal Resistance Applied to Bulk and SOI SiGe HBTs

A revision is presented of the technique to determine the junction temperature and thermal resistance of bipolar transistors. It is based on the temperature sensitivity of the base-emitter voltage when biasing the device under constant emitter current. It accounts correctly for the self-heating of the device during the measurement. Results are obtained for devices fabricated on silicon-on-insulator (SOI) and bulk silicon having different emitter widths and lengths. An increment of the thermal resistance is found for SOI devices with respect to bulk.

Suppression of the burn-in effect in InGaP/GaAs heterojunction bipolar transistors by constant period of voltage stress

Two significantly abrupt increases of dc current gain (β) at the opposite extremes of base-emitter voltages (Vbe) linked by a relatively slight increase of β in the range of 1.25⩽Vbe⩽1.75 V are found in InGaP/GaAs heterojunction bipolar transistors (HBT’s). It has been proved that the burn-in effect directly causes the second abrupt increase of β occurring at Vbe>1.75 V instead of elsewhere. In addition, choosing Vbe>1.75 V, a higher base-emitter voltage results in a faster increase rate with improvement in current gain transient. Based on these observations, a constant period of voltage stress (CPVS) with Vbe=2.0 V and Vce=3.0 V for 5 min with a specific choice of Vbe>1.75 V (the voltage range corresponding to the second abrupt increase of β) is thus applied to promote the electrical performance of HBT’s. After applying a CPVS, the burn-in effect is substantially suppressed without showing any second abrupt increase of current gain. Although the current measured under both reverse and forward biases is greatly reduced for the base-emitter junction, a CPVS slightly degrades the electrical properties of a base-collector junction. Also, it has been proved that bulk recombination, rather than the surface recombination current, the base contact recombination, or the space-charge recombination, is the dominant base current component causing the burn-in effect. The electrical improvement of the base-emitter junction is due to the annihilation of H-related traps in the base region after a CPVS. Once H− ions form, we propose that these ions are too fast to drift toward the base-collector junction under the reverse bias of the base-collector voltage. After migration through the extrinsic base region, H− ions are supposed to be trapped near the base-collector space region, which results in the degradation of the base-collector junction after a CPVS.

Neutron, proton, and electron irradiation effects in InGaP/GaAs single heterojunction bipolar transistors

Neutron, proton, and electron irradiation effects in InGaP/GaAs single heterojunction bipolar transistors are investigated. Devices with different emitter sizes and grown by two different growth techniques are studied. At low fluences of proton and neutron irradiation, a small gain increase is observed at low base currents. At higher fluences, gain degradation is observed whose magnitude depends on the nature and fluence of the irradiation particle. In the case of electron irradiation, the change in gain is very small for electron fluence up to 7/spl times/10/sup 15/ e/cm/sup 2/ although a careful analysis shows a slight gain increase at low base currents and a slight gain degradation at higher base currents. The gain increase at small base currents and low fluence is believed to be caused by the ionization damage in the polyimide passivation layer. The gain degradation at higher fluence and high base currents is due to the displacement damage in the emitter-base junction region. In addition to the gain degradation, neutron irradiation causes a shift of the emitter-base offset voltage, which is caused by the displacement damage in the base-collector region. The correlation of the gain degradation coefficients for the different particles is discussed in the framework of the NIEL theory.

Minimization of the mechanical-stress-induced inaccuracy in bandgap voltage references

This paper shows a systematic approach to calculate and to minimize mechanical-stress-related problems in bandgap voltage references. Mechanical stress is the main cause of long-term drift and packaging-induced inaccuracy in bandgap voltage references. Especially, low-cost epoxy packaging can cause a large mechanical stress, being in the range from -200 to +200 MPa. This stress shows local variations over the chip area and changes over time or during thermal cycling. Due to the piezojunction effect, this stress causes changes and drift in the base-emitter voltages of bipolar transistors and consequently in the output voltage of bandgap references. In this paper, it is shown that, even when using low-cost IC and packaging technology, a bandgap reference with a high immunity for the effects of mechanical stress can be realized.

A model for multi-finger HBTs including current gain collapse effects

A common-emitter equivalent circuit model which represents both the self-heating and the current collapse as feedback from the collector current to the base–emitter voltage is developed for multi-finger InGaAs/GaAs HBTs. The modified Ebers–Moll model is verified by comparing the simulated and measured results. Good agreement is also achieved for the scattering parameters and I– V characteristics confirming the validity of the model for high frequency applications.

High-frequency oscillations in an α-Si/Si(p)/Si(n) device

An α-Si/Si(p)/Si(n) heterojunction transistor exhibits current oscillations, which depending on the applied base–emitter voltage are either periodic or chaotic. The frequency dependence of the oscillations on the collector–emitter and the base–emitter voltage as well as on the load resistance, are presented.

サーミスタにより発振周波数を補償した電流帰還型磁気マルチバイブレータの解析

An analysis is presented of our original current-feedback magnetic multivibrator. The realized circuit has a constant oscillation frequency provided by a thermistor. Though current-feedback magnetic multivibrators are highly efficient and reliable, the oscillation frequency normally varies according to the load current. In the proposed multivibrator, a new frequency compensation circuit cancels the dependence of junction voltages on the load current without any additional transformer. In this paper, the oscillation frequency is analyzed using our models of bipolar junction transistors. These models account for power dissipation, because the base-emitter voltage depends on the junction temperature and other factors.

Experimental investigation of piezojunction effectinsilicon and its temperature dependence

Owing to the piezojunction effect, mechanical stress is a dominant cause of inaccuracy and long-term instability in the base-emitter voltage of integrated bipolar transistors. This effect directly affects the accuracy of important basic circuits such as bandgap voltage references and temperature sensors. Novel results are presented of an experimental investigation of the piezojunction effect and its temperature dependence for vertical bipolar transistors produced in a standard {001}-oriented silicon wafer. It is found that the piezojunction effect can be well characterised with a second-order equation including the piezojunction coefficients. Experimental investigation shows that the piezojunction coefficients decrease with increasing temperature.

RF modeling approach to determining end-of-life reliability for InP-based HBTs

Reliability testing for high-speed, compound semiconductor, heterojunction bipolar transistors (HBTs) has focused on determining device lifetimes based on changes in the DC characteristics. The work reported here considers aging effects on the RF characteristics of InP-based HBTs. In addition, an intended system life of 15 years has been assumed and we have attempted to stress the devices to approximate 15 years of circuit operation. As such, SPICE models were generated for the devices before and after the stress. The basic differences between these models are that the aged model shows increases in the emitter resistance ( R e) and in the base–collector capacitance ( C jc). The-end-of life model can be obtained from the standard model by increasing R e from 3.6 to 5.7 Ω and C jc from 20.1 to 23.4 fF. These changes lead to decreases in: transistor gain, F t, and F max, and an increase in the base–emitter voltage at typical operating conditions. No increase in low current base–emitter voltage occurs, indicating the lack of Be diffusion into the emitter. The changes in the device parameters generally fall within the normal process variations.

A new sensor structure using the piezojunction effect in PNP lateral transistors

Due to the piezojunction effect, the I C( V BE) characteristics of bipolar transistors are affected by mechanical stress. It appears that this effect is the dominant source of inaccuracy of many analogue circuits, such as smart temperature sensors and band-gap references. On the other hand, advantage can be taken from this effect to realize new mechanical sensors. This paper shows how to minimize or to maximize the influence of the piezojunction effect of lateral PNP (L-PNP) transistors on [1 0 0] silicon wafer. The L-PNP transistors are available in any standard bipolar or CMOS process and are suited to realize low cost sensor implementation. As compared to sensors based on the piezoresistive effect, the new sensor offers the advantage that it is very small, has a reduced power consumption and can also provide a temperature measurement using the base–emitter voltage ( V BE). The current mirror gauge factor amounts to 176; the nonlinearity is less than ±1% and the temperature coefficient of the gauge factor (TCGF) is 1.75×10 −3°C −1.

Random telegraph signal noise mechanisms in reverse base current of hot carrier-degraded submicron bipolar transistors: Effect of carrier trapping during stress on noise characteristics

We investigate physical mechanisms of random telegraph signal (RTS) noise in reverse base current of hot carrier-degraded polysilicon emitter bipolar junction transistors. RTS noise, analyzed in the time domain, is studied as a function of reverse base-emitter bias, temperature, and additional reverse-bias stress. Two-level RTS with a relative amplitude as high as 100% is observed at room temperature. The RTS amplitude varies exponentially with the applied reverse base-emitter voltage and depends weakly on temperature. The additional hot carrier stress is observed to induce changes in RTS amplitude and mean pulse widths (independent or correlated), and a disappearance/reappearance of the RTS fluctuations. The results are interpreted by a model where the RTS noise is caused by fluctuations of generation-recombination (g-r) parameters (i.e., capture cross sections and energy position in the gap) of a stress-induced complex bistable defect (CBD) at the Si/SiO2 interface. The complex defect is assumed to be either a two-state fast interface state or an interacting pair of a fast interface state with a slow neighboring border trap. The RTS amplitude is well explained by fluctuations in a single-defect electric-field-enhanced g-r rate between a finite value and naught. The RTS amplitude-bias characteristics and their temperature dependence are satisfactorily accounted for by an expression for a phonon-assisted tunneling current via a single deep-level state. The model parameters are the g-r parameters of the defect and its spatial position in the base-emitter p–n junction. The stress-induced changes in the RTS noise are attributed to the influence of log-time trapping of hot carriers on border states laying in the vicinity of a CBD center. The charged border traps interact with a CBD, changing both its g-r parameters and the RTS switching behavior. The variations in RTS parameters are related to the microscopic nature of the interaction and are discussed for the two types of the CBDs.

Temperature dependence of surface recombination current in MOS transistors

Analytical expressions are derived for the temperature dependence of the peak current amplitude, peak gate voltage, and current-voltage linewidth of the basewell terminal current in metal-oxide-silicon transistors (MOSTs) due to electron-hole recombination at the interface traps. They are verified by experimental data. It is shown that temperature dependence of the basewell terminal current cannot distinguish a discrete energy level from a distribution of energy levels of the interface traps in the silicon energy gap, because the interface traps measured by surface recombination current are those traps with energy close to midgap. A demonstration is given for the accurate determination of the local temperature inside the transistor using the ideal exponential dependence of the collector current on the emitter-base forward-bias voltage.

A unified model for single/multifinger HBTs including self-heating effects

This paper presents a unified analytical large-signal model that includes self-heating effects. The model is applied to a single-finger AlGaAs/GaAs heterojunction bipolar transistor (HBT) and a multifinger InGaAs/GaAs HBT. The self-heating effect in the HBT is simulated as a feedback from the collector current to the base-emitter voltage. The main advantage of the circuit presented here is that additional analysis of coupling between electrical and thermal circuits is not required, as is the case with the existing models. The small-signal HBT model is implemented based on the S-parameters at multiple frequencies measured at multiple bias points. This model is verified by comparing the measured and simulated S-parameters. The large-signal model is based on the forward Gummel plot and is built over the small-signal model. This model is verified by comparing the simulated and measured dc I-V characteristics.

The piezojunction effect in NPN and PNP vertical transistors and its influence on silicon temperature sensors

This paper describes a test structure to characterize the piezojunction effect for the base-emitter voltage V BE and the PTAT voltage Δ V BE. The piezojunction effect directly affects the accuracy of temperature sensors and special types of pressure sensors. Packaging is a source of mechanical stress in electronics circuits. Measurements have been performed for two types of vertical bipolar transistors. Firstly, an NPN transistor of a BiCMOS technology and secondly, a PNP substrate transistor of a CMOS technology, both of them using a [100] silicon wafer geometry. It has been found that the sensitivity for the uniaxial stress of the V BE of the PNP is four times less than that of the NPN transistor. In both cases, the PTAT voltage appears to be hardly stress-sensitive.

The piezojunction effect in bipolar transistors at moderate stress levels: a theoretical and experimental study

The piezojunction effect changes the current–voltage characteristics of a bipolar transistor when mechanical stress is applied. A good insight into this effect will facilitate its application in mechanical sensors, and also help to minimise errors in bandgap references due to parasitic stress. For the stress range and orientation in those applications, the piezojunction effect is modelled by taking into account piezoresistivity, band edge shifts, and stress-induced changes in the valence band densities of states. Experimentally, it is evaluated by stressing npn and pnp transistors from −155 to +155 MPa and measuring the collector current changes. The change in the equivalent base-emitter voltage ranges from −1.5 to 3.0 mV and shows a quite strong second-order nonlinearity. Up to high injection, it is independent of the bias. The measured curves are lower than those from the model, but share all qualitative features.

Simulation of the electrical characteristics of AlGaN/GaN heterojunction bipolar transistors

Heterojunction bipolar transistors fabricated from the III–V nitrides offer improved linearity for high-power and high-frequency amplifiers. We report the results of two-dimensional simulations of the characteristics of AlGaN/GaN heterojunction bipolar transistors (HBTs) using the MEDICI® simulator. The simulated performance of single-heterojunction devices predict that it is possible to realize an amplification coefficient β=1130, an emitter-base turn-on voltage V BCO =3 V , a collector saturation current I Csat =3.5 kA / cm 2 , and a collector breakdown voltage BV CEO =55 V . The simulated frequency response of these devices indicate a maximum cutoff frequency f T =18 GHz .

On the effects of hot carriers on the RF characteristics of Si/SiGe heterojunction bipolar transistors

This work for the first time experimentally investigates the hot carrier effects on the RF characteristics (up to 30 GHz) of Si/SiGe heterojunction bipolar transistors (HBT's). Reverse base-emitter voltage stresses were applied at room temperature on BiCMOS compatible, sub-micron transistors. The main observed degradation is a decrease of S/sub 21/. It was found that this degradation is minimized (maximized) when biasing at constant collector (base) current. These results may be valuable indications also for degradations induced by ionizing radiations.

An experiment to demonstrate the canonical distribution

We describe a simple experiment, suitable for an undergraduate laboratory, in which the collector current in a transistor is measured as a function of the base–emitter voltage at various temperatures. The experiment gives a very convincing demonstration of the canonical distribution of statistical mechanics, in which the probability of occupancy of a state of energy E is proportional to e−E/kT.

Nondestructive determination of beryllium outdiffusion in AlGaAs/GaAs heterojunction bipolar transistor structures by low-temperature photoluminescence

In this work, low-temperature photoluminescence (PL) is used for the investigation of beryllium (Be) dopant outdiffusion in AlGaAs/GaAs single-heterojunction bipolar transistors (HBTs). Near the typical emission peak at ∼1.92 eV which is due to the band-to-band transition from AlGaAs emitter, an additional emission peak is found in the PL spectrum. This peak is found to be caused by the Be-related transition from AlGaAs emitter due to the Be outdiffusion from the GaAs base to the AlGaAs emitter. The concentration of Be outdiffused into the emitter can be estimated from the energy separation between these two emission peaks in AlGaAs range based on the band gap narrowing effect. The measured dc current gain and the emitter-base turn-on voltage of the HBTs fabricated on different wafers with different growth conditions were found to correlate well with the PL results. Our results demonstrate that low temperature PL technique is an efficient method in identifying Be outdiffusion in HBTs and is useful in HBT growth and device structure optimization.

Direct observation of hot-electron energy distribution in silicon metal–oxide–semiconductor field-effect transistors

We have directly observed the energy distributions of hot electrons in Si metal–oxide–semiconductor field-effect transistors. We used a lateral hot-electron transistor employing two potential barriers (an emitter barrier and a collector barrier) that divided the Si surface into three regions (the emitter, base, and collector). For an emitter-base voltage of −1.04 V, hot electrons with an excess energy of 0.7 eV were detected at the collector. The ratio of hot electrons to the injected electrons was 1.3%.