Annealing Characteristics of Neutron Irradiated Silicon Transistors

When a transistor is subjected to neutron irradiation, a component of base current proportional to neutron fluence is induced. From the effects of annealing on the base and collector currents, the conclusion was drawn that there is an apparent difference in the annealing characteristics between the neutral and the space-charge regions of the semiconductor device. This study of the anomalous annealing indicates that the neutron-induced component of base current is a result of one, or a combination, of the following mechanisms: a quasi-tunneling recombination phenomena in the emitter-base space-charge region, or an influence of the p-n junction electric field on the formation, annealing, and electronic behavior of the neutron-induced defect centers. A field dependence of the formation and annealing of the neutron-induced defects appears to be present both during the introduction and annealing of the neutron-induced defect centers. It could not be finally determined whether or not the quasitunneling phenomena occurred although it can be shown on theoretical grounds that it is possible for such phenomena to occur. The annealing characteristics of the defects, as represented by changes in the collector and base currents, have been obtained. Three sets of devices were irradiated and then annealed, with one set having a forward bias during annealing, one set having no bias, and one set having a reverse bias. The dependence of the field on annealing is present but appears quite complex.     

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.     

Neutron Radiation Damage in Silicon Transistors

In the investigation of base and collector current as a function of the emitter-to-base voltage, previous studies have shown that neutroninduced base current has components originating in the emitter space charge region as well as the neutral base region. This study shows that while the low injection level neutron-induced base current is dominated by the space charge component, the high injection behavior appears to be controlled by recombination in the neutral base region. Additional experiments performed in special tetrode transistors and van der Pauw-type samples indicate that changes in collector current are dominated by recombination in the neutral base, while changes in base doping and mobility have only a secondary effect. These conclusions are reached from experiments on transistors with a ring emitter, on tetrode-type test transistors, and on special Hall-effect devices, and by a detailed analysis of the emission crowding characteristics of a "ring-dot" geometry device.     

High-Energy Radiation Damage in Silicon Transistors

An experimental investigation of electron and gamma ray damage in silicon transistors is presented. At low values of fluence (?e < 1014 electrons/cm2), loss in common-emitter dc current gain of medium frequency n-p-n planar transistors at collector currents of one to 10 milliamperes is attributed to changes in the surface recombination velocity. Displacement-induced recombination centers in the base region cause a reduction in gain when ?e is greater than 1014 electrons/cm2. A technique of saturating the surface damage with low energy electrons (E = 125 kev) so as to permit a separation of surface and bulk damage is demonstrated. The minority-carrier lifetime-damage constant, K?, has been estimated from the separated bulk-damage curve. It agrees with the value determined from electron irradiation of a low frequency (f?b = 1.25 Mc/sec) mesa n-p-n transistor which is shown to suffer degradation in gain only from bulk recombination current losses within the base region. Surface damage from both electrons and gamma rays is annealed at 250°C or by injecting emitter currents of 200 milliamperes which generate a high internal temperature. In contrast to this behavior, electron irradiation of p-n-p transistors caused loss in gain which is attributed to bulk damage. Damage constants, K?, determined from the data show that p-n-p transistors suffer bulk radiation damage about five times greater than n-p-n transistors.     

Short-Term Annealing in p-Type Silicon

A series of experiments was performed to study the effects of acceptor-type, carrier-concentration, and material-growth techniques on the short-term annealing of neutron damage to p-type silicon. No significant dependence of the annealing on these parameters was observed. The parameter most important to the annealing rate is the injected minority-carrier concentration (?n). The use of extremely low injection concentrations, a very short neutron burst, and early time measurements has resulted in the detection of anneal factors as high as 50 in the tests reported here. No anneal factors of this magnitude have ever been reported before.     

Displacement Damage in Silicon and Germanium Transistors

An experimental and theoretical analysis of neutron damage to germanium and silicon transistors is presented. A combination of experimental data and first order theory has been used to determine lifetime damage constant as a function of neutron energy. Two separate effects attributable to the recombination centers have been identified. At low bias levels recombination centers present in the emitter diode field region cause a very large decrease in current gain. At higher levels of emitter bias current recombination centers present in the base region cause a reduction in current gain. The temperature dependence of current gain is stronger when emitter region recombination is the dominant process than when base region recombination is most effective. The damage thresholds of silicon transistors can be increased by operating at high ambient temperatures. The characteristics of the recombination centers have been determined from the experimental data and agree well with values previously obtained by measurements on bulk semiconductor samples. Two discrete recombination levels in silicon and two discrete recombination levels in germanium are shown to determine the dependence of transistor current gain on neutron radiation.     

Short-Term Annealing in Transistors Irradiated in the Biased-Off Mode

A set of experiments was performed at the White Sands Missile Range Fast-Burst Reactor Facility in which measurements were made of the annealing of three types of transistors (two NPN and one PNP) which were irradiated while biased off. Anneal factors as high as 5 or 6 were observed in the NPN devices when turned on tens of milliseconds after the neutron burst. The PNP device (2N2875) showed an anneal factor of nearly 4 when turned on 29 msec after the radiation pulse. The injection dependence of the annealing is clearly evident.     

Short-Term Annealing in Electron-Irradiated p-Type Silicon

Studies of short-term annealing of minority carrier lifetime in bulk p-type silicon following ~1.4 MeV pulsed electron irradiation are reported. Investigations were performed on vacuum-float-zone boron-doped material in the temperature range 235-3860K and the maximum fluence employed was ~5 x 1012 electrons/cm . Simple exponential recovery under isothermal conditions, a characteristic of first order reaction kinetics, was observed. The amount of unstable damage varied as exp (-t/?R), where ?R is the characteristic recovery time. The temperature dependence of ?R yielded an activation energy of 0. 32 ± 0. 03 eV for the recovery process. The current results, when extrapolated to lower temperatures, compare quite closely with Watkins' EPR data for similar material. It is concluded that the present data is consistent with the observation of neutral vacancy annealing.     

High Dose Rate Burnout in Silicon Epitaxial Transistors

This paper presents experimental results for ionizing radiation-induced burnout in n-p-n transistors. The results indicate a dose-rate and a collector bias threshold for transistor burnout. The collector voltage threshold was shown to be on the order of 2/3 BVCEO and the dose-rate threshold was shown to be on the order of 2 × 1010 rad(Si)/s. The triggering mechanism was postulated to be caused by high current injection which results in avalanche breakdown fields developing across the n-n+ substrate boundary.     

Short-Term Charge Annealing in Electron-Irradiated Silicon Dioxide

The existence of a rapid annealing phase in the decay of space charge induced in silicon dioxide by pulsed irradiation has been demonstrated. This effect has been observed in MOS structures prepared from both wet and dry thermal oxides and also in several commercial N-channel MOSFET's. A simple model involving thermal release of the trapped positive charge from a distribution of oxide trapping levels conveniently approximates the major features of short-term annealing.     

Recombination Statistics for Neutron Bombarded Silicon Transistors

This paper presents a recombination statistical model for the neutron-induced base current component reported previously. The derivation was based on the following: 1) the current equation for the induced current component developed previously; 2) the Shockley-Read-Hall statistics for holes and electrons; and 3) the recombination statistics derived by Sah, Noyce and Shockley for sites in the bulk space-charge region. The recombination statistics model depends on the diffusion potential, the junction voltage, the activation energy, temperature, and the ratio of capture cross-sections for holes and electrons. The utility of such a recombination statistical model is illustrated by using measured parameters to predict the neutron-induced base current for p-n junction transistors and by comparing the results with measured base currents. Further, the temperature variation of the reciprocal slope term is calculated from the model and found to agree well with experiment.     

Annealing of 10 MeV Electron Damage in Silicon

Annealing studies were performed to investigate the recovery of both the carrier concentration and lifetime in silicon following room temperature irradiation with 10-MeV electrons. Both n- and p-type material containing the more common dopants and varying amounts of oxygen were employed to evaluate the effect of these impurities on the annealing behavior. The recovery of carriers in n-type material was found to depend very strongly upon oxygen. A well-defined recovery stage was observed at intermediate temperature (~110° to 180° C) in oxygen-free samples but not in pulled material. The position of the stage appeared to depend upon the dopant, and for this reason it is attributed to the breakup of donor-vacancy complexes. In contrast, no appreciable carrier recovery was obtained in p-type material after anneals up to 253° C, regardless of the dopant or the oxygen content. Annealing of lifetime changes was particularly interesting. A pronounced annealing stage at ~225° C was observed in all but one of the n-type samples regardless of the oxygen content and in p-type samples containing oxygen. The detailed behavior due to this stage was dependent upon the radiation dose. Oxygen-free p-type samples exhibited completely different annealing behavior. A prominent reverse annealing stage at~144° C was observed in four of these samples and no subsequent positive recovery was observed up to 283° C.     

Radiation Induced Leakage Currents in Silicon on Sapphire MOS Transistors

Measurements are reported in which photodepopulation and thermal bleaching techniques have been applied to the study of radiation induced trapped positive charge in the Al2O3 substrate of n-channel MOS/SOS devices. Photodepopulation data indicates an optical depth for the hole traps of 2.5 eV. Thermal bleaching studies yield a corresponding thermal depth of 0.75 eV. Some preliminary thermally stimulated current measurements which reveal additional low temperature stable traps are also reported.     

Irradiated Silicon Gate MOS Device Bias Annealing

N-channel silicon gate MOS transistors exhibit positive threshold shifts when bias annealed following ionizing irradiation exposure. The threshold shift is probably due to electrons tunneling into the oxide and recombining at trapping centers near the Si/Sio2 interface. The threshold shifts are large enough in some cases to cause functional failures of MOS integrated circuits.     

Modeling Rapid Annealing in Digital Integrated Circuits

An analytical model for the effects of rapid annealing in narrow base bipolar transistors has been developed. This model utilizes transistor base-emitter voltage and an empirical curve to calculate annealing factor with time. The model has been incorporated into the TRAC circuit analysis code and used to predict the time-dependent response of a low-power Schottky TTL NAND gate and a four-bit shift register as a function of neutron fluence and operating condition.     

Calculation of Reverse Annealing in Neutron Damaged p-Type Silicon

A calculation of the reverse annealing process in neutron-burst irradiated p-type silicon at 300°K was performed, utilizing a variation of the equilibrium cluster theory of Gregory. Material conditions which were considered included 1, 10, and 100 ?-cm material with an injection ratio of 10-6 , and 1 ?-cm material with injection ratios of 10-7 and 10-8. This calculation demonstrated the relationship between the peak and time of occurrence of the damage constant versus time curve and the material resistivity and injection level. It was also shown that the reverse annealing rate depends upon the capture rate of defects in the cluster, rather than the rate of transport of defects from the depletion region to the cluster. These carrier capture rates were such that the forward annealing process usually began before the clusters reached charge saturation. Finally, it was concluded that an injected minority carrier pulse lasting less than 5×10-6 sec after burst will not appreciably affect the reverse annealing process.     

Injection Dependence of Transient Annealing in Neutron-Irradiated Silicon Devices

Studies have been performed to explore accurately the injection level and temperature dependence of transient annealing in neutron-irradiated P- and N-type silicon. In P-type material, the annealing factor in the 0 to 0.1 second time interval is very sensitive to the minority carrier injection level. For example, by varying the injection level from 10-5 to 10-1 the annealing factor at 0.001 second can be reduced from 10 to approximately 2. In contrast to the P-type results, the injection dependence observed in N-type silicon is very small and, furthermore, is in the opposite sense; i.e., an increase in the injection level causes an increase in the annealing factor. However, this study shows that this seemingly different behavior can be correlated on the basis of the hole-to-electron ratios of the different material types and resistivities. Annealing measurements performed in the temperature range from 180°to 300°K reaffirm the 0.3 eV activation energy previously found in P-type silicon and establish a value of 0.17 eV for N-type silicon.     

A Study of the Neutron-Induced Base Current Component in Silicon Transistors

In a previous paper a neutron-induced component of base current was reported that increases in proportion to integrated neutron flux and varies with base-to-emitter voltage, VBE, as exp(q/nkT VBE), n being approximately 1.5. A component of base current which varies similarly with base-to-emitter voltage has previously been reported to be of surface-perimeter origin. For this reason it is significant to unambiguously locate the origin of the neutron-induced component. For this, a detailed study of the current-voltage characteristics, an analysis of the deviation of the characteristics from an exponential caused by emission concentration for "ring-dot" structures, and an analysis for the transverse bias dependence of the base current components for a special "tetrode-type" test structure were performed. The results indicate that the small "1.5 component" of current initially present is indeed of surface-perimeter origin while the added "1.5 component" of current induced by neutron bombardment is of bulk space charge region origin. To determine whether this effect is universal, an investigation was made of many different types of silicon transistors, and all were found to exhibit a similar neutron-induced current component. The number n in the exponential term is found to be a function of temperature decreasing with increasing temperature. For example at -50°C, n = 1.6, and at +100°C, n = 1.3. Annealing studies reveal an apparent difference in annealing rates for the neutral bulk base region and the high-field region of the emitter-base junction.     

Model for Short-Term Annealing of Neutron Damage in p-Type Silicon

A model is presented for the short-term annealing of a damage claster resulting from neutron radiation in p-type silicon. The damage cluster is assumed to be a disordered core imbedded in an otherwise undisturbed lattice, as suggested by Gossick. The disordered core is characterized in this model by two defects. The dominant defect immediately after the damaging event is assumed to be the vacancy, which is represented by a two-level defect (donor-acceptor) in the forbidden gap. The density of this defect decreases with time as a function of the densities of its different charge states. The second defect is the divacancy which is also represented by a two-level defect in the forbidden gap. The density of the divacancies increases as the vacancies anneal. The analysis is made for a single, spherically symmetric cluster located at the center of a sphere of radius R of undamaged material. The problem is solved by the PN code which approximates the exact continuity, Poisson's, and generation-recombination equations in finite difference form. Annealing factors are obtained as a function of time by simulating a steady, uniform ionization rate throughout the sphere R. An effective lifetime for the volume R is calculated as the instantaneous average density of excess minority carriers in the sphere divided by the ionization rate. Time histories of the annealing factors are compared to experimental data obtained from shortcircuited solar cells.     

The Effects of Gamma Radiation on Glass Coated Silicon Transistors

It has previously been found that semiconductor devices employing various gas encapsulation techniques exhibited significant alterations of their electrical responses under gamma and neutron exposure. Since the neutron bombardment produces bulk or volume damage, the device surface environment has little effect on the neutron induced changes. It has been observed that gamma exposures at doses well below the threshold for permanent damage have caused changes in several electrical characteristics of transistors and diodes which may be described as of surface origin. The parameters exhibiting significant change are the transistor ICBO and the reverse biased diode leakage current. This paper will describe the results of experiments performed to study the effects of exposure to gamma radiation on selected silicon transistors when coated with a low temperature melting glass. The surface sensitive parameters of the coated transistors degraded to a much lesser extent than observed for the gas encapsulated units, indicating a more favorable surface environment to minimize the effect of gamma radiation. Comparison of the glass coated transistors with devices employing a planar structure incorporating a silicon-oxide coating on the device material surface yielded comparable results. The results of these experiments are consistent with the model proposed to explain this effect and its various ramifications.