Epitaxial Transistors Research Articles

Computer simulation of radiation induced emitter crowding

Radiation induced emitter crowding effects can precipitate an anomalous photocurrent response. Such a response would not be predicted by the usual computer simulation using a charge control representation of a bipolar transistor equipped with a primary photo-current source. Radiation induced emitter crowding effects are prevalent in large epitaxial power transistors which operate with low impedance loads where large photocurrent s are observed as a result of irradiation. A model has been developed which simulates radiation induced emitter crowding and it is applicable to most existing circuit analysis codes. Experimental data obtained from pulsing 50 watt silicon epitaxial power transistors with 50 nanosecond and 8 microsecond linear accelerator pulses in the electron beam mode show excellent agreement with the predicted results.

A high input impedance buffer amplifier using bipolar transistors

Abstract The input impedance of conventional emitter follower circuits is limited due to the finite value of the passive emitter resistance, shunting effect of biasing resistors and that of intrinsic collector to base feedback admittance and also to the fall in current amplification factor at low operating currents. Further, the input admittance is frequency dependent because the device parameters involved therein are themselves frequency dependent. However, the shunt positive feedback incorporated in such circuits minimizes the shunting effect of the biasing network and also that of the intrinsic feedback admittance. The simulation of negative capacitance across the input terminals nullifies the effect of the presence of the otherwise positive capacity. This technique extends the bandwidth over which the input impedance remains constant. A typical buffer amplifier circuit employing five conventional epitaxial planar bipolar silicon transistors has been described in the present communication. The input im...

Anomalous Photocurrents in Low-Power Epitaxial Transistors

In this paper, we give additional experimental results concerning the anomalous photocurrent problem. Anomalous photocurrents were observed in various epitaxial NPN transistors for different operating modes and bias voltages. It is particularly shor that the devices switch on a stable I(V) characteristic that agrees closely with that given by the avalanche injection mode obtained experimentally with a fast rising high current pulse generator. The triggering mechanism is investigated and emitter injection is found to be a necessary condition. This experimental approach allows us to predict accurately the anomalous photocurrent value without radiation tests and seems to be quite useful in hardening studies.

Integrated circuit model for lateral PNP transistors including isolation junction interactions†

Abstract A large signal equivalent circuit for integrated circuit lateral PNP transistors including substrate interactions is presented. The model is useful for d.c, linear and non-linear transient time circuit analysis. Measurement methods to determine equivalent circuit parameters are presented. These consist of transistor d.c. current voltage relationships, current gain and small-signal cut-off frequency measurements in various transistor operating modes. The short-circuit current transfer ratio is shown to vary with frequency as 1/ffor the thin epitaxial transistor and as 1/√f for the thick epitaxial transistor. The equivalent circuit of a typical integrated lateral PNP—P substrate transistor (emitter: 50 μm dia., emitter-collector mask separation: 12·5 μm, lateral base width : approx. 6 μm, junction depth : 2·7 μm, epitaxy : resistivity 0·47 ohm-cm, epitaxial layer thickness : 6·4 μm) is determined. The validity of this equivalent circuit is confirmed by comparing the transient time response measurem...

Role of Emitter Area in Neutron-Hardened Transistor Design

Using Hauser's equation for the current dependence of effective emitter area in Hahn's epitaxial transistor model, it is predicted that emitter debiasing appreciably increases the equilibrium collector resistance of a transistor having a structure typical of neutron-hardened transistors. To experimentally determine whether debiasing is as severe as predicted, a small transistor and an adjacent emitter-shaped resistor were fabricated on the same chip. Near coincidence of the emitter-shaped resistor I-V characteristic with the transistor saturation locus, before and after irradiation to neutron fluences up to 5× 1014 n/cm2 (E > 10 KeV), demonstrates that emitter debiasing is not important in determining the saturation locus (onset of hFE falloff with collector current) in transistors having a moderate emitter stripe width (2 mils). It is concluded that extremely narrow emitter stripe widths are not needed for neutron-hardened transistors.

The saturation characteristics of n-p-ν-n power transistors

The transport equations and charge-control concepts are applied in an analysis of a static conductivity-modulation mechanism occurring in the collector region of n-p-ν-n power transistors. This results in an expression for collector-emitter saturation voltage as a function of terminal currents and device parameters. An expression is derived which describes the current gain characteristics of saturated epitaxial and triple-diffused devices. The analysis is also used to illustrate the relationship between the emitter metallization resistance, collector charge storage, and the turn-off crowding mechanism experienced by high-frequency saturating transistor switches. An analysis of a time-dependent collector conductivity modulation process is used to derive an expression which describes the repetition frequency dependence of the collector-emitter saturation voltage of an epitaxial (or triple-diffused) transistor switching a square wave of collector current. It is concluded that frequency-dependent edge-crowding mechanisms occur only at much higher frequencies than those considered in this study.

Resistivity changes caused by gold diffusion in epitaxial n-p-n transistors

Differential capacitance was used to determine resistivity changes in high-BV CEO n-p-n epitaxial transistor structures caused by gold diffusion and subsequent heat cycles. Resistivities corresponding to 8 × 1014to 2 × 1015phosphorus atoms/ cm3were found to be compensated to 1012/ cm8after a 15-min gold diffusion at 1150°C, with the degree of compensation diminishing after heat cycling at lower temperatures. Capacitance readings after gold diffusion were determined at forward bias to measure increments of depletion width. Gold concentrations were determined by radiochemical analysis and were correlated with diode lifetime measurements. A minimum lifetime value of 2 ns after gold diffusion could be increased to 14 ns by a low-temperature phosphorus emitter diffusion.

Electric Field Strength Dependence of Surface Damage in Oxde Passivated Silicon Planar Transistors

A dependence of surface degradation induced by ionizing radiation in matched oxide-passivated silicon planar epitaxial transistors on junction fringing electric field strength present during exposure is reported. The electric field strength and gamma dose dependence are investigated of the decrease in the forward current gain, hFE (as reflected by the increase in the surface recombination current component), the increase in the surface recombination velocity (as reflected by the increase in the reciprocal of the minority carrier lifetime), and the increase in junction capacitance. Empirical prediction equations have been derived, for matched devices, correlating the normalized base current increase and the normalized surface recombination velocity increase with the average junction electric field strength present during irradiation and the total gamma dose. The ionizing radiation induced surface recombination and surface channel components are analyzed from a study of the reciprocal slope term, "n", obtained from forward and inverse configuration current-voltage data.

Effect of collector design on hot-spot formation and second breakdown in transistors

Abstract : Hot-spot formation was studied as a function of collector resistivity and thickness variations for a similar group of epitaxial and triple-diffused silicon planar transistors. An infrared radiometer was used for locating and measuring hot-spot temperatures. Hot-spot thermal resistance is used in the analysis of both forward and reverse bias second breakdown performance. (Author)

Frequency and power limitations of Class-D transistor amplifiers

The transition and saturation loss mechanisms of the two basic tuned Class-D amplifiers are studied in detail, as functions of drive waveform, signal frequency, and type of transistor. The charge control approach is used to derive optimum drive waveforms, together with approximate expressions for the various contributions to transition time that are verified experimentally. It is found that for high-frequency operation, the current-switching system using simple epitaxial transistors provides the highest output power with least likelihood of transistor failure, although with a somewhat lower efficiency than the voltage-switch. The voltage-switching system is found to be preferable when simple epitaxial transistors are not used. An example is given of a current-switching amplifier designed to deliver a continuous power of 1 kW at 500 kHz with an overall system efficiency of greater than 90 percent into a variable load.

Design, fabrication, and characterization of a germanium microwave transistor

This paper summarizes the design, fabrication, and characterization of a p-n-p planar epitaxial germanium transistor for use as an amplifier in the 1-to 4-GHz frequency range and as a high-speed switch. The analytical basis for the geometry and impurity profile arrived at in the development of this transistor is presented in conjunction with experimental measurements. It is shown that good agreement between experiment and theory can be achieved even in the 1-to 6-GHz frequency region when sufficient attention is given to the formulation of an adequate equivalent circuit. For example, the calculated f T of this germanium microwave transistor is 5.7 GHz, which compares to a typical measured value of 5.6 GHz. The measured maximum available gain of the better transistors is 13.4 dB at 1.3 GHz (the corresponding calculated value is 13.9 dB) with a 2.7-dB noise figure at the same frequency.

On the static collector-emitter saturation voltage of a transistor with a lightly doped collector

A one-dimensional analysis which defines the static collector-emitter saturation voltage characteristic of epitaxial and triple-diffused transistors in terms of collector current, base current, and device parameters is given.

Radiation and Annealing Characteristics of Neutron Bombarded Silicon Transistors

Operating a silicon planar epitaxial transistor in the inverse configuration allows one to demonstrate clearly the importance of the neutron-induced base current component and its degradation of the emitter efficiency, and, because of the much larger depletion layer, to compute a volume dependent damage constant applicable to all silicon p-n junctions. The importance of minimizing the absolute change versus relative change in radiation hardening studies is clearly illustrated. Surface effects were found to be significant for transistors mounted in gas-filled cans. The diffusion potential was predicted, on theoretical grounds, to vary with neutron fluence, and the theory was experimentally confirmed. Isochronal and isothermal annealing data were obtained for the inverse configuration and from these data, it is concluded that the neutron-induced defect centers are field dependent.

On crowding effects and failure mechanisms in high power transistor switches

The relationship between current crowding and device failure in the high speed saturating epitaxial transistor switch is discussed. Agreement between theory and practice is demonstrated, using a composite large area device comprising an array of small area transistors.

Current mode second breakdown in epitaxial planar transistors

Current mode second breakdown is a type of voltage "switchback" observed in epitaxial transistors. The phenomenon is initiated when the emitter is injecting at a collector voltage in excess of the collector-emitter sustaining voltage, and is characterized by delay and voltage fall times on the order of a nanosecond. The device can be sustained in the low voltage state only as long as there is sufficient charge to produce conductivity modulation within the collector-base depletion region. When the available charge is exhausted, the collector voltage will recharge at a rate determined by the external circuit. At some critical current density, the collector-base depletion region collapses toward the high conductivity substrate. The electric field within the depletion region increases as the depletion region width narrows, until avalanche occurs. The sustaining voltage will be determined by the bulk base-to-collector avalanche voltage. A consequence of this behavior is that most epitaxial transistors cannot operate stably in the LV <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CER</inf> mode, and switching-off unclamped inductive circuits with the emitter-base junction terminated in some finite resistance will lead to second breakdown.

Second breakdown and crystallographic defects in transistors

A study was conducted to determine the effect, on second breakdown in transistors, of some of the crystallographic defects that can develop during handling and fabricating procedures. X-ray diffraction microscopy was used to detect these defects. The susceptibility to second breakdown of about 1500 epitaxial planar silicon transistors diffused on two wafers was measured and the site of the current constriction of second breakdown was registered on most of these transistors. These data were then compared with the X-ray topographs. Any effect of the observed gross dislocations was masked by other factors, such as apparent surface induced effects, that were not discernible hi the X-ray topographs. It is proposed that a faulted emitter structure, that has been correlated with the emitter diffusion step, may be of importance in affecting the susceptibility of the transistor to second breakdown. The heating in the transistor at the second breakdown current constriction site to temperatures above 600°C and recrystallized laser-induced melt regions smaller that about 30 µm in diameter produced no crystallographic changes nor any frozen-in strain fields detectable in the X-ray topographs.

Second breakdown in the forward and reverse base current region

At a constant forward or reverse base current, the pulse length of a collector current pulse, which forces a transistor under test into second breakdown, is measured. For all transistors measured, the pulse length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> is plotted vs. the collector pulse height, with the forward or reverse base current as a parameter. According to our observations, one can distinguish three types of transistors, as far as their secondary breakdown characteristics are concerned. 1) Good epitaxial transistors show an approximately linear dependence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> to I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> with a negative slope. The curves for reverse base current are displaced parallel to the ones for forward base current, the energy of the pulses not being much different in both cases. 2) Bad epitaxial transistors show an S-shaped curve of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> vs. I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> for reverse base currents. A deep minimum in second breakdown energy is observed at low collector currents. These transistors will readily fail in practical circuits, where the base current is allowed to reverse. 3) With homogeneous material, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> vs. I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> curves for forward and reverse base currents practically coincide. This method has revealed that a large number of transistor types, including overlay structures, show a decrease of energy handling capability of two to three orders of magnitude at collector currents of about 100 mA when the base current is reversed. It has also proved to be an indispensable tool in the development of high-frequency high power devices, which combine good electrical and second breakdown properties.

Application of new semiconductor elements in multichannel measuring analog-to-digital converters

1. Analog-to-digital conversion can be made at frequencies of several and even tens of me gahertz at errors of ≥2% of the maximum measured current of 5 mA, and at frequencies of tens and hundreds of kilohertz with an error of <2%. 2. The problem of contactless commutation of low-level signals is solved by using semiconductor switches with epitaxial silicon transistors made of a single piece of semiconductor material and located in a single casing. The switches can be controlled directly by the triggers. 3. Contactless switching by means of measuring amplifiers consisting of differential dc amplifiers with planar silicon transistors and by using ultrahigh-speed analog-to-digital converters provides information-feeding devices with a speed of operation approaching that of universal computers and ensures the interaction of these devices in a real time scale.

Calculation of Electrical and Radiation Storage Time in Transistors

The conventional viewpoint of saturation in junction transistors, from a device point of view, considers the excess minority carriers in the base region. A different viewpoint considers the majority carriers in the base. The important elements then are the number of these carriers stored in the transistor and whether they are stored in the active base region, the extrinsic base region, the collector body, or the epitaxial layer. The storage of carriers in the active base region plays a relatively minor role in the storage effect for most modern transistors because of the small volume of semiconductor material involved. In alloy, diffused, and epitaxial transistors the storage of carriers (electrons in a PNP transistor) is shown to be in the last three regions named above, respectively; in none of these transistor types is the storage region identifiable as the active base region. This new viewpoint leads to well-accepted storage-time expressions for alloy and diffused transistors and to new storage-time expressions for epitaxial transistors. Detailed calculations of the primary photocurrent have been made for three practical transistor construction types that relate this current to basic device parameters and to the electrical storage time. In addition, the radiation storage time has been calculated in terms of the electrical storage time for epitaxial transistors. Examples of these calculations are given and the results presented.

Design of a low-noise, uhf (450-Mc) receiver using RCA silicon planar transistors

This paper describes the application of the RCA 2N2857 (uhf) and 2N2708 (vhf) transistors in the rf-amplifier, mixer, local-oscillator, and if-amplifier stages of a 450-megacycle communications receiver. Block diagrams and circuit schematics are presented and described, the noise figure of the system is calculated, impedance matching is discussed, and data on “overload” signals are presented. Design examples in terms of admittance parameters are given for the rf stages, and calculations based on transmission-line relationships are made for the tuned circuits of the receiver. Receiver sensitivity, selectivity, and automatic gain control are also discussed. The 2N2857 and 2N2708 transistors are two of the most recent low-noise silicon planar epitaxial transistors developed by RCA for uhf and vhf applications. These transistors are packaged in a four-lead case which has the same dimensions as the TO-18 package, and their collectors are isolated. The fourth lead of the package, which is attached to the case, reduces feedback capacitance and rf radiation when its free end is connected to circuit ground.