Radiation Induced Forward Emitter Current Gain Degradation of Lateral and Vertical PNP Power Transistors in Voltage Regulators

Transcription

1 1188 PIERS Proceedings, Xi an, China, March 22 26, 2010 Radiation Induced Forward Emitter Current Gain Degradation of Lateral and Vertical PNP Power Transistors in Voltage Regulators Vladimir Vukić 1 and Predrag Osmokrović 2 1 Institute of Electrical Engineering Nikola Tesla, Belgrade, Serbia 2 Faculty of Electrical Engineering, University of Belgrade, Belgrade, Serbia Abstract Voltage regulators National Semiconductor LM2940CT5 and STMicroelectronics L4940V5 were examined in gamma radiation field. It was perceived that voltage regulators LM2940CT5, manufactured by the use of conventional monolithic bipolar process, characterized by lateral pnp transistors with round emitters, became unfunctional after absorption of low doses of radiation. On the other hand, voltage regulators L4940V5, BiCMOS integrated circuits created with implementation of local oxide side isolation process, showed much higher radiation hardness. Detailed examinations of voltage regulator L4940V5 pointed on significant reduction of serial transistor s forward emitter current gain, causing the great increase of quiescent current. The main reason of high radiation susceptibility of examined vertical serial pnp transistor is implementation of interdigitated emitter, with high perimeter-to-area ratio. Level of vertical pnp power transistor s forward emitter current gain degradation was similar to the current gain degradation of transistors with round emitters in circuits LM2940CT5, with small perimeter-to-area ratio. Experiment showed similar influences of emitter geometry and technological process of pnp transistor creation on degradation of power transistor s characteristics, yet the functioning of two types of integrated circuits was completely different. 1. INTRODUCTION In the previous papers were presented results of the examination of low-dropout voltage regulators National Semiconductor LM2940CT5 and STMicroelectronics L4940V5 in ionizing radiation fields [1 3]. It was perceived that voltage regulators LM2940CT5, manufactured by the use of conventional monolithic bipolar process with lateral pnp transistors with round emitters, became unfunctional after absorption of low doses of radiation (less then 300 Gy (SiO 2 )). On the other hand, voltage regulators L4940V5, BiCMOS integrated circuits created with implementation of local oxide side isolation process, showed much higher radiation hardness (more then 10 kgy (SiO 2 )). Before experiment it was assumed that local oxide may cause creation of parasitic MOSFET, i.e., radiationinduced leakage currents between collector and emitter of serial transistor in voltage regulators L4940V5. However, implementation of isolated collector in integrated circuit made by HDS 2 /P 2 Multipower 20 V process prevented negative influence of local oxide isolation. Hypothesis was that the main cause of L4940V5 s high radiation hardness was small degradation of serial vertical pnp transistor s forward emitter current gain, mainly due to the shift of current flow from the surface towards the substrate, while the assumption of L2940CT5 s low radiation hardness was that it is caused by the rapid loss of lateral pnp transistor s forward emitter current gain in ionizing radiation field. In the new series of experiments were measured voltage regulator s output current, output voltage and quiescent current, obtaining the possibility to calculate the serial pnp transistor s forward emitter current gain. 2. EXPERIMENT Integrated 5-volt positive commercial-off-the-shelf voltage regulators ST Microelectronics L4940V5 and National Semiconductor LM2940CT5 were tested in Institute of Nuclear Sciences Vinča, Belgrade, in Metrology-dosimetric laboratory. 60 Co was used as a source of γ radiation and it was situated in the device for the realization of γ-field, IRPIK-B. Accepted mean energy of γ-photons is E γ = 1.25 MeV. Samples were irradiated in the mouth of collimator. Exposition doses measurement was exerted with the cavity ionizing chamber Dosimentor PTW M23361, volume m 3, with uncertainty of measurement ±2%. With cavity ionizing chamber, reader DI4 was used [4].

2 Progress In Electromagnetics Research Symposium Proceedings, Xi an, China, March 22 26, Samples of voltage regulators LM2940CT5 and L4940V5 were irradiated in groups of four circuits. Ten meters long cables supplied devices. Beside the supply cables were laid sense cables of the same length. Current and voltage measurements were carried out with laboratory instruments Fluke 8050A and Hewlett-Packard 3466A. All measurements and the irradiation of components were performed on room temperature of 20 C. The main values used for detection of voltage regulator s degradation due to exposure to ionizing radiation were forward emitter current gain and maximum load current. Measured electrical values were voltage regulator s output current, output voltage and quiescent current. Examination of maximum collector current change was performed in the following way: for the constant input voltage equal to 8 V, load current was increased until output voltage dropped to 4.7 V. Lower output voltages are unacceptable for voltage regulator, since the device is beginning to shutdown [5]. The next step was measurement of output voltage and quiescent current for unloaded voltage regulator, with input voltage of 8 V. In voltage regulators with serial pnp power transistor, quiescent current represents the sum of control circuit s internal consumption current and serial transistor s base current. Measurement of quiescent current for unloaded voltage regulator obtains value of internal consumption, with minor influence of serial transistor s base current. Subtraction of unloaded circuit s quiescent current from quiescent current of maximally loaded device, for the same input voltages, gives the value of serial transistor s base current. Additional measurement of output current, i.e. serial power transistor s collector current, obtains possibility to calculate the forward emitter current gain of serial transistor, both during the irradiation and after the absorption of specified amount of total dose. Devices had been irradiated until predetermined total doses were reached. To avoid the effects of recombination in semiconductor after irradiation, all measurements were performed up to half an hour after the exposure. Devices in γ radiation field were exposed to total dose of 500 Gy (SiO 2 ), with dose rate of 4 cgy (SiO 2 )/s. More details about experiment, sources of ionizing radiation, test procedure and technological processes implemented in manufacture of examined circuits are provided in references [1 4]. 3. RESULTS Data presented in Figures 1 3 were obtained by the tests of circuits LM2940CT5 from batch PM44AE, made by National Semiconductor s subcontractor in China. Circuits were packaged in Malacca, Malaysia. It was perceived earlier [1, 3] that voltage regulators LM2940CT5 became unfunctional after absorption of low doses of radiation. The main reason for circuit failures for low total doses was not loss of forward emitter current gain, but the degradation of error amplifier circuit [3]. The very beggining of irradiation of voltage regulators LM2940CT5 brought rapid decrease of maximum output current, but also increase of serial transistor s forward emitter current gain for unbiased devices and circuits operating with low currents. (γ, γ, Figure 1: Change of mean maximum output current in voltage regulator LM2940CT5 under influence of γ radiation. Figure 2: Change of serial transistor s forward emitter current gain in voltage regulator LM2940CT5 under influence of γ radiation.

3 1190 PIERS Proceedings, Xi an, China, March 22 26, 2010 In the beggining of irradiation of voltage regulator LM2940CT5, due to the slow formation of interface traps, oxide traps had dominat influence, causing the rapid increase of electric field generated by the positive trapped charge in the oxide. Electric field caused by the positive trapped charge rises quickly in the highly contaminated oxides [6]. Under the influence of external electric field starts transport of holes and hydrogen ions towards the interface oxide-semiconductor and generation of interface traps. Owing to the influence of electrostatic barrier, generated by space charges in oxide, hole and hydrogen ion transport towards the interface was very slow, causing the dominant influence of oxide trapped charge on lateral pnp transistor. This effect induced abrupt reduction of emitter efficiency and decrease of the base current due to reduction of space-charge region in the base area [4]. Specified interpretation relates to operation with small emitter currents. When high current flows through the emitter, due to the degradation of lightly doped emitter, caused by the positive trapped charge in the oxide, space-charge region spreads deep into the p-type emitter. Examined samples with load current of 500 ma operate with significantly lower forward emitter current gain, with high carrier injection into emitter. In the case of lateral pnp transistors, high holes current flows through the n-type base, suppressing the influence of positive trapped charge in the oxide on surface accumulation in the base area. Increase of interface traps concentration proportional to total dose causes additional rise of base current and degradation of forward emitter current gain. High current flow through the lateral pnp transistors does not cause significant recombination of positive trapped charge in the oxide because the minority carriers in the pnp transistor s base area are holes. Forward emitter current gain degradation of serial pnp transistor is less than expected for lateral pnp transistors with lightly doped emitters (decrease up to 40% in regard with current gain before irradiation). Anyhow, it appeared that older technological process, based on round, not interdigitated emitters, is the primary reason for moderate degradation of forward emitter current gain of power transistor. On the other hand, small perimeter-to-area ratio had significant effect on degradation of emitter injection efficiency, especially during the operation with high currents, reducing the maximum current supplied to the load. Therefore, information of forward emitter current gain in operating point was not sufficient data for evaluation of serial transistor s radiation hardness, demanding also information of maximum current. In Figures 4 6 are presented data of examinations obtained for voltage regulators STMicroelectronics L4940V5 from batch WKOOGO 408, made in China. Despite the verified great radiation hardness of voltage regulators L4940V5, from Figures 5 and 6 can be noticed significant decrease of forward emitter current gain, reaching more than 50% in regard with initial values. During the first series of experiments it was assumed that the main reason for high radiation tolerance of voltage regulators L4940V5 was small influence of ionizing radiation on vertical pnp transistor s forward emitter current gain. The main reason why radiation susceptibility of examined vertical serial pnp transistor was γ Figure 3: Relative change of serial transistor s forward emitter current gain in voltage regulator LM2940CT5 under influence of γ radiation. Figure 4: Change of mean maximum output current in voltage regulator L4940V5 under influence of γ radiation.

4 Progress In Electromagnetics Research Symposium Proceedings, Xi an, China, March 22 26, higher than expected is implementation of interdigitated emitter, with high perimeter-to-area ratio, applied in order to increase the emitter efficiency during the operation with high currents. In total, 36 groups of elementary pnp transistors occupy high chip area, with very high perimeter-to-area ratio [2]. Consequently, positive trapped charge in oxide has great impact on emitter injection efficiency and spread of space-charge region deep into the emitter area. After the initial rapid decrease, forward emitter current gain fell on one half of initial value (for samples with low load currents) after absorption of total dose 200 Gy (SiO 2 ), remaining in saturation for higher doses. Yet, for the exact evaluation of forward emitter current gain degradation are necessary data of serial transistor s base-emitter voltage (V BE ). Although the base and emitter currents are known values, voltage V BE during the measurement of serial transistor s collector and base current remained unknown, since it was not possible to measure this value during the experiment. Only if values of current gain was known in various conditions for constant values of V BE, reconstruction of characteristic β (I E ) could be possible. Therefore, conclusion is that owing to the nagative feedback reaction the serial transistor s operating point moved away from maximum of characteristic β (I E ), not that for all values of emitter current appeared twofold decrease of forward emitter current gain. Examinations of maximum output current (Figure 4) pointed on small change of this parameter, although the forward emitter current gain was halfed in some cases. Examinations of maximum output current demand serial transistor operation in high level injection mode (ideality factor n = 2). In this mode serial transistor operates in far right part of characteristic β (V BE ), where change of V BE has the least influence on forward emitter current gain. Some authors consider reduction of voltage regulator s maximum output current proportional to serial transistor s forward emitter current gain [7], so the change of forward emitter current gain in high-level injection mode of operation may consider basic for mutual comparison of sample s radiation tolerance. In the worst case, decline of maximum output current of voltage regulator L4940V5 didn t exceed 7%. Increase of base current, also as stable regulation of output voltage pointed on primary significance of driver transistor and control circuits (especially voltage reference and error amplifier) on voltage regulator s radiation hardness, while the current gain loss of serial pnp transistor had less influence. Mentioned effects didn t have influence on proper functioning of circuit L4940V5, even after absorption of very high total doses. The only similarity in radiation responses between two types of low-dropout voltage regulators was response of samples that operated with high load current during the irradiation. In serial transistors operating in conditions of high-level carriers injection in emitter (total circuit s current of 500 ma), noticed decline of lateral serial transistor s forward emitter current gain (voltage regulator LM2940CT5) was some 40%, while the current gain decline of vertical transistor, situated in circuit L4940V5, was about 30%. Mean Forward Emitter Current Gain, β Unbiased (I = 0 A, V in = 0V) Biased (I = 1 ma, V in = 8V) Biased (I = 100 ma, V in = 8V) Biased (I = 500 ma, V in = 8V) L4940V5 batch WKOOGO 408 CHINA γ (Co 60, 1,25 MeV) dd/dt = 4 cgy(sio 2 )/s V in = 8V, V out = 4,7V γ Dose, D [Gy(SiO 2 )] Figure 5: Change of serial transistor s forward emitter current gain in voltage regulator L4940V5 under influence of γ radiation. Figure 6: Change of relative value of serial transistor s forward emitter current gain in voltage regulator L4940V5 under influence of γ radiation.

5 1192 PIERS Proceedings, Xi an, China, March 22 26, CONCLUSION Voltage regulators National Semiconductor LM2940CT5, made by the use of conventional monolithic bipolar process with lateral pnp transistors and round emitters, even due to the use of older technological process and creation with highly contaminated isolation oxide, showed notable degradation of lateral pnp transistor s forward emitter current gain, but much less than expected. However, all samples had functional failure after exposure to low total doses of medium-dose-rate ionizing radiation. On the other hand, voltage regulators STMicroelectronics L4940V5, BiC- MOS integrated circuits created by the use of vertical process with side local oxides, demonstrated exceptionally high radiation hardness. Increase of absorbed total dose caused higher electron injection from the base into the emitter area of lateral pnp transistors, having impact on increase of lateral pnp transistor s base current during the irradiation of voltage regulators LM2940CT5. Detailed examinations of voltage regulator L4940V5 pointed on significant degradation of serial transistor s forward emitter current gain, that exerted by the great increase of voltage regulator s quiescent current. Nevertheless, this effect didn t affect device s proper functioning. The main reason for noticed sensitivity of examined vertical serial pnp transistor was application of interdigitated emitter, with great perimeter-to-area ratio, used to increase the emitter efficiency in conditions of opereation with high current density. Beside the assumed high radiation hardness of vertical pnp transistors, primarily due to the relocation of current flow from the surface towards the substrate, experiment pointed on the great influence of emitter geometry on characteristics of power transistor, comparable with the influence of implemented technological process. Also, it was noticed that changes of serial transistor s parameters don t need to be the decisive factor in the correct operation of voltage regulators in the radiation environment. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technological Development of Republic Serbia under project G, Physics of electromagnetic and radiation compatibility of electrical materials and components. REFERENCES 1. Vukić, V., P. Osmokrović, and S. Stanković, Influence of medium dose rate X and gamma radiation and bias conditions on characteristics of low-dropout voltage regulators with lateral and vertical serial PNP transistors, 8th European Conference on Radiation and Its Effects on Components and Systems RADECS 2005, Cap d Agde, France, September 19 23, Vukić, V. and P. Osmokrović, Total ionizing dose response of commercial process for synthesis of linear bipolar integrated circuits, Journal of Optoelectronics and Advanced Materials, Vol. 8, No. 4, , Vukić, V. and P. Osmokrović, Total ionizing dose degradation of power bipolar integrated circuit, Journal of Optoelectronics and Advanced Materials, Vol. 10, No. 1, , Vukić, V., Radiation tolerance of analog bipolar integrated circuits, Ph.D. Thesis, University of Belgrade, 2008 (in Serbian). 5. McClure, S. S., J. L. Gorelick, R. L. Pease, and A. H. Johnston, Dose rate and bias dependency of total dose sensitivity of low dropout voltage regulators, IEEE Radiation Effects Data Workshop, , Shaneyfelt, M. R., R. L. Pease, J. R. Schwank, M. C. Maher, G. L. Hash, D. M. Fleetwood, P. E. Dodd, C. A. Reber, S. C. Witczak, L. C. Riewe, H. P. Hjalmarson, J. C. Banks, B. L. Doyle, and J. A. Knapp, Impact of passivation layers on enhanced low-dose-rate sensitivity and thermal stress effects in linear bipolar IC s, IEEE Transactions on Nuclear Science, Vol. 49, , Pease, R. L., S. McClure, J. Gorelick, and S. C. Witczak, Enhanced low-dose-rate sensitivity of a low-dropout voltage regulator, IEEE Transactions on Nuclear Science, Vol. 45, , 1998.