Measurement of the base resistance of bipolar transistors

By measuring the transistor parameters hfe and yfb as functions of frequency, it is possible to determine the base resistance of bipolar transistors. This method has proved to be fast and accurate over a relatively large current range for integrated-circuit transistors, as well as for many types of discrete transistors.     

Measurement of the low-current base and emitter resistances of bipolar transistors

A new ac method is proposed to measure the emitter and base resistances of bipolar transistorsat low current levels at which the effective transistor geometry is given by the processing and is unaffected by the changes induced by high currents. The technique is based on a measurement of the input impedance at frequencies below about 50 MHz. It is particularly suited for the measurement of the physical emitter resistance of scaled transistors. The method is illustrated on microwave transistors with metal contacts and on self-aligned digital transistors with polysilicon contacts. A comparison of the results obtained using this method with those from dc methods operating at high currents can be used to explore the current dependencies of the resistances. The technique is applicable both for homojunction and heterojunction transistors.     

Photo carrier generation in bipolar transistors

Anomalous substrate currents have been observed in SiGe bipolar NPN-transistors, dependent on the collector bias, at high current levels. These currents appear to originate from light that is generated in the collector base junction when it is reverse biased. This light generates electron hole pairs in the n⁺ buried layer-substrate diode, yielding a considerable substrate current. This paper will show that these substrate currents can be used as a useful monitor for the occurrence of avalanche multiplication and high-level injection (Kirk effect) in heterojunction bipolar transistors (HBTs)     

Effects of base doping and carrier lifetime on differential current gain and temperature coefficient of 4H-SiC power bipolar junction transistors

Considering the circumstance of heterogeneous voice flows, first, by applying Markov chain, this paper proposes an unsaturated analytical model for the IEEE 802.11e EDCA protocol, which considers the condition of non-ideal transmission channel and the character of the occurrence of backoff countdown at the beginning of time slot in EDCA protocol. Furthermore, according to the proposed model, the media access delay and throughput of a flow are analysed, and the flow-oriented call admission control (CAC) scheme is proposed. Finally, the simulation results are shown to confirm that the proposed CAC scheme can guarantee the requirements of throughput and delay of voice flows, and can admit more voice flows to improve the utilisation efficiency of network resources by choosing the appropriate values of the minimum contention window or the appropriate varieties of voice flows.     

Switch-on transient of shallow-profile bipolar transistors

This paper presents 2-D computer simulations of the switch-on characteristics of shallow-profile silicon bipolar transistors with a focus on the current distribution and the base charge storage time calculations. From the simulations, we learned that 1) although the initial turn-on current predominantly flows through the edge of the emitter, after the transient, most current flows under the emitter, not through the edge; 2) a fast rise in collector current can be accomplished by a small voltage overdrive which causes a peak transient current larger than the final dc value at the emitter edge and 3) the charge storage time derived from the transient simulation is not necessarily the same as the one derived from the quasi-static method. The latter does not take the nonuniform current distribution into account.     

Measurement of minority carrier lifetime and diffusion length in silicon epitaxial layers by means of the photocurrent technique

A new method for the determination of minority carrier lifetime and diffusion length in thin silicon epitaxial layers was developed. Using a transparent MIS structure the surface recombination velocity was reduced below 25 cm/s. This method makes possible to determine minority carrier lifetime and also diffusion length much greater than the thickness of the epitaxial layer.     

Collector recombination lifetime from the quasi-saturation analysisof high-voltage bipolar transistors

A novel method of measuring the collector recombination lifetime, which is independent of emitter effects, is presented by extending the quasi-saturation analysis of high-voltage bipolar transistors to the high-current-density regime. The technique is supported by theory, and experimental results are presented on transistors fabricated with different emitter properties. This is a nondestructive method and gives the lifetime values at the current densities normally encountered when the transistor is in actual operation. The values for the collector recombination lifetime obtained by the present method are independent of the properties of the emitter region     

Measurement of the onset of quasi-saturation in bipolar transistors

The onset of quasi-saturation in a bipolar transistor is accompanied by h_FE- and f_T-fall-off and by an increase in h.f. distortion (intermodulation and cross-modulation). We show that the boundary between normal and quasi-saturated operation is easily established experimentally by measuring the l.f. third harmonics of the collector current, as a function of bias voltage and current. Examples are given for ohmic, tepid and hot carrier current flow in the epitaxial collector region.     

Mapping charge carrier density in organic thin-film transistors by time-resolved photoluminescence lifetime studies

The device performance of organic transistors is strongly influenced by the charge carrier distribution. A range of factors effect this distribution, including injection barriers at the metal-semiconductor interface, the morphology of the organic film, and charge traps at the dielectric/organic interface or at grain boundaries. In our comprehensive experimental and analytical work we demonstrate a method to characterize the charge carrier density in organic thin-film transistors using time-resolved photoluminescence spectroscopy. We developed a numerical model that describes the electrical and optical responses consistently. We determined the densities of free and trapped holes at the interface between the organic layer and the SiO₂ gate dielectric by comparison to electrical measurements. Furthermore by applying fluorescence lifetime imaging microscopy we determine the local charge carrier distribution between source and drain electrodes of the transistor for different biasing conditions. We observe the expected hole density gradient from source to drain electrode.     

Transient base dynamics of bipolar transistors in high injection

The transient behavior of a bipolar transistor in high level injection is analyzed both through simulations and an analytic model based on the quasi-neutral base approximation. It is found that, unlike the situation for low injection, transient operation can be influenced by the base majority carrier mobility and by the characteristics of the extrinsic base. While the quasi-neutrality approximation frequently remains valid, cases are presented in which it fails. In these cases, the transient conditions cause at least some small region of the normally quasi-neutral base to develop a space charge. The reclaimable fraction of the stored base charge is determined and discussed     

Direct measurement of base drift field in bipolar transistors

The authors have developed a method to measure an effective base drift field and the base transit-time reduction factor of bipolar transistors, by measuring the excess phase of the base transport factor. This technique relies on measuring small-signal characteristics of the transistor at a low frequency and following the phase of the transconductance at the frequency approaching and exceeding the unit current gain frequency (f_T). With this technique, the authors verify that the effective drift inside the base of Si bipolar transistors decreases with increased base implantation energy and thermal treatment. Such directly measured drift-dependent base transport provides additional insight for optimizing processing used in bipolar technology development     

Hydrogen-induced transient effect in carbon-doped InGaP hetero-junction bipolar transistors

The transient effect on the current gain of InGaP hetero-junction bipolar transistors was studied. It was found that this transient effect was caused by hydrogen and eliminated after three or more cycles of thermal annealing at 250 °C in nitrogen. A model based on the carbon-hydrogen complex dissociation by thermal anneal is proposed to explain this observation.     

Measurement of series collector resistance in bipolar transistors

A new method has been devised to measure directly the series collector resistance (r/SUB c/) of monolithic bipolar n-p-n transistors. The method uses the parasitic substrate p-n-p and the reverse n-p-n associated with each integrated n-p-n transistor to detect the internal collector-base voltage. The effects of temperature, conductivity modulation, and mobility on the collector resistance can be measured directly. Measurements on a range of devices indicate standard techniques such as the forced beta method measure only a fraction of the total collector resistance. The present technique yields results in good agreement with theoretically calculated values of r/SUB c/. This method is amenable to automated measurement systems.     

On the base region minority carrier concentration in narrow-base transistors

An empirical current- and voltage-dependent boundary condition for the minority carrier concentration at the base edge of the collector-base depletion layer in a one-dimensional narrow-base transistor structure is presented. The limitations for narrow-base structures of the standard, voltage-only-dependent condition are discussed. DC and small-signal ac analyses using the new boundary condition are also presented.     

Determination of the minority-carrier base lifetime of junction transistors by measurements of basewidth-modulation conductances

A method for determining the minority-carrier lifetime in the base of a bipolar transistor is described that involves measurement of the low-frequency, small-signal output conductance Goand reverse transconductance Gr, which arise from basewidth modulation. It involves also determining the base transit time either from calculations based on the base doping profile or from measurements at the transistor terminals. To illustrate the method, it is applied to transistors having considerably different base lifetimes.     

Empirical expression for base transit time in bipolar transistors

A new and compact formula for the base transit time, τ _b , of a modern high speed npn bipolar transistor with exponential base doping profile is derived considering doping dependence of mobility, bandgap narrowing effect, high injection effect and carrier velocity saturation at the base edge of the collector–base junction. The collector current density, J _c , and minority carrier stored charge, Q _b , in the base are separately expressed as a function of the injected electron density n _o in the base in order to find an empirical expression for τ _b . The modelling of J _c , Q _b and τ _b is essential for the design of high-speed bipolar transistor. The expressions are applicable for arbitrary injection before the onset of the Kirk effect and they are simple and straight forward to give insight into device operation. The base transit time calculated analytically is compared with simulation results in order to demonstrate the validity of the assumptions made in deriving the expression. The closed form expressions for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.     

Measurement of differential carrier lifetime in vertical-cavitysurface-emitting lasers

Measurements of differential carrier lifetimes on gain-guided proton-implanted vertical-cavity surface-emitting lasers with device size as a parameter are reported. The lifetimes were obtained from laser impedance measurements and from small-signal modulation optical response at subthreshold currents. A simple small-signal equivalent circuit was used to correct the optical data and to extract the carrier lifetimes from the impedance data. Carrier lifetimes ranged from 4.2 ns at 0.04 mA, to about 0.6 ns at a bias close to threshold. The measured carrier lifetimes were used to calculate the corresponding threshold carrier density (n_th~6×10¹⁸ cm^-3) and recombination parameters     

Non-ideal base current in bipolar transistors at low temperatures

Bipolar transistors havetraditionally been considered not useful in low-temperature applications. This assumption, however, is based upon an incomplete physical understanding of bipolar device physics at low temperatures. This paper shows experimentally that recombination mechanisms play a substantially larger role in determining base current at low temperatures than at room temperature. The results are explained and quantitatively modeled using conventional Shockley-Read-Hall theory, with the addition of the Poole-Frenkel high field effect. It is concluded that trap levels in the silicon bandgap due to bulk traps or interface states are very important in determining bipolar transistor base currents at low temperatures. Non-ideality factors larger than 2 are often observed. Such trap levels will have to be carefully controlled if low-temperature operation of bipolar transistors is to be considered.     

RTS noise in submicron SiGe epitaxial base bipolar transistors

In this paper, we report a comprehensive study of Random Telegraph Signal (RTS) noise in SiGe epitaxial base bipolar transistors. We analyse the multilevel fluctuations of base and emitter forward currents before and after reverse stress on the emitter-base junction. We show the influence of the chemical treatment preceeding polysilicon emitter deposition on noise properties. We identified that RTS noise arises from different regions in the device : the silicon/polysilicon interface if an oxidizing surface preparation is used, and the emitter periphery after stress-induced degradation. Temperature and bias dependent measurements allowed us to characterize these defects (activation energy, defect type), to analyse their impact to the low frequency noise properties of these transistors and to discuss the role of hot carrier stressing.     

Measurement of minority carrier lifetime profiles in silicon

The influence of finite surface recombination velocity on the measurement of minority carrier lifetime has been studied. The method is based on measuring the phase shift between the a.c.-photocurrent of a Schottky contact and the incident light. The limit of spatial resolution has been shown by theoretical calculations to be about one diffusion length. Lifetime profiles have been measured by the use of a mercury capillary contact.