- Email: [email protected]
Operation of guard rings on the ohmic side of n+–p–p+ diodes
Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Operation of guard rings on the ohmic side of n>—p—p> diodes夽 N. Egorov , V. Eremin *, S. Golubkov , K. Konkov , Yu. Kozlov , Z. Li, A. Sidorov Russian Institute of Material Sciences and Technology, Zelenograd, Russia Ioffe Physico-Technical Institute RAS, St-Petersburg, Russia Brookhaven National Laboratory, Upton, NY, USA
Abstract Detectors from high-resistivity p-type silicon with multi guard rings structure located on the ohmic side have been investigated. The processed detectors were constructed with four p>-rings surrounding the central p>-pad. The n>—p junction opposite to the contact with rings had not any protection and was cut-through by chip scribing. Investigation of the potential distribution between the floating p>-rings has shown that the potential difference arises just after the depletion of the detector bulk and is accompanied by the detector leakage current saturation. The saturated current is mainly a leakage current flowing in a scribed periphery. Grounding of one of the p>-rings to collect the detector leakage current allows the separation of the bulk and the surface components of the current, which reduces the current of a central p>-pad contact down to tens nA/cm. A model based on the field effect in the gap between the neighboring p>-rings is proposed. The experiments carried out in this study show that construction of the ohmic side of Si detectors with multi guard rings is a perspective approach to design the detectors operational at high biases. Detectors processed in this way allow one to apply biases up to 600 V. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Detectors; MGR structures; n>—p—p> diodes
1. Introduction The main efforts in the development of semiconductor detectors with p—n junction were concentrated on the elaboration of a design that avoids p—n junction breakdown at operational biases and reaches a stable I—» characteristic with minimum leakage current. This kind of design focuses on the
夽
This investigation was supported in part by the U.S. Department of Energy: Contract No: DE-AC02-98CH10886. * Corresponding author.
reduction of the maximum value of a local electric field, which appears near the border of p—n junction and can lead to impact ionization and avalanche breakdown. There are two main approaches in the literature to minimize this electric field: (1) metallization of the junction area, which overlaps the SiO passivating layer (field plate) [1—3] and (2) surrounding of the junction by multi guard ring (MGR) termination structure [4—6]. The first approach allows the reduction of the electric field at the border of heavily doped junction area that is in contact with the accumulated n-layer under SiO induced by the fixed oxide charge. The
0168-9002/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 4 9 2 - 2 VI. GaAs AND SILICON DEVICES
198
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
second approach makes it possible to redistribute the potential around a junction over a wide area covered by MGR, which reduces the electric field component that is parallel to the detector surface. Both approaches are successfully used in the design of all type of Si planar detectors including pad and strip ones, and drift detectors. The physics of MGR structure operation on the junction side of detectors has been investigated in detail in the literature. The properties of these structures can be summarized as: (1) the rings are floating; (2) the potential spreads from a central ring to the outer ones; (3) the potential in a ring appears when the space charge region touches it (pouch-through) and (4) the potential drop between the neighboring rings depends on the gap between them. The configuration of MGR on a junction side can be optimized by a computer simulation of the electric field distribution at the p>—n junction taking into account the resistivity of silicon used and characteristics arisen from planar technology, such as the doping profile and fixed charge density in the SiO passivating layer between rings [6]. In this paper we investigate an alternative approach for the field termination in planar detectors, which deals with the backside of the detector with ohmic, non-injecting contact (ohmic side). This approach was not considered in detail until now, although some papers in the literature contain data confirming that this approach is realistic enough. As an example, it can be found in the study of neutron irradiated Si detectors processed from the n-type silicon and designed with MGR on the p>contact [7,8]. For high resistivity (*4 k) cm) Si detectors, at fluences over 10 n/cm the sign of a space charge in depletion region, initially positive (concentration of ionized donors is larger than that of acceptors), changes to negative (space charge sign inversion, SCSI, or simply inversion). Conse-
quently the junction of the detector moves to the back side and is formed now by the n>-contact and the acceptor-type space charge region in the bulk silicon. In this situation the p>-contact with MGR operates as non-injecting ohmic contact. Investigation of the reverse current of these detectors has shown that no breakdown is observed at bias of hundreds of volts in spite of n>—p junction heavily damaged by scribing. One of the possible causes can be attributed to MGR effect on the ohmic side. Below the detailed investigation on operation of MGR on the ohmic side is presented.
2. Device and experimental technique Detectors studied here are diodes processed from p-type silicon wafers with a resistivity of about 10 k) cm and 400 lm in thickness. A cross-section of the diode is shown in Fig. 1 and geometrical parameters of MGR structure are listed in Table 1. The n>-contact on the junction side was processed by phosphorus implantation into the entire wafer surface followed by a metallization using Al. On the ohmic side the p>-contact was formed by boron implantation through a thin SiO layer. The central p>-pad contact had an area of 10;10 mm and
Fig. 1. Cross-section of the detector with MGR.
Table 1 Parameters for detectors used in this study Distance between (mkm)
PAD — Ring A 25
Rings A—B 25
Rings B—C 35
Rings C—D 55
Ring C-cut 110
Width of (mkm)
Ring A 40
Ring B 40
Ring C 40
Ring D 45
CUT 400
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
was surrounded by 4 square-shaped p>-rings. The p>-pad area and p>-rings were metallized. The gap on the ohmic side between the neighboring detectors used for detector chip scribing was boron implanted and metallized as well. Processed wafers were scribed by a diamond saw. No additional treatment of the scribed periphery of individual detector chips was used. Diced detectors were then attached to the supporting board by conducting glue on the n>-side, and all p>-contacts (pad, 4 rings and metallized area at the border of a chip) were wire bonded to the contacts on the board. All measurements were performed in air at room temperature with no humidity control.
3. Experimental results In Fig. 2a, the schematics used for current—voltage characteristic measurements are shown. The ohmic side was at low potential and the junction side was positively biased. This allows one to measure the I—» characteristics at different modes of p>-rings grounding or floating, and to separate the two components of the current — the current flowing through a pad contact and that through
Fig. 2. Schematics of the detector biasing schemes: (a) for I—» measurements, and (b) for measurements of potentials on the MGR on the ohmic side.
199
one of the rings. The amount of working p>-rings in MGR structure can be varied by parallel connection of pad electrode with some neighboring rings. This mode was used to simulate the operation of detector with reduced number of rings. In the extreme case, all rings and the p>-contact (the latter named here as CUT) were connected together to the ground leaving the detector with no protection against low voltage breakdown at all. In Fig. 3, the I—» characteristic of the detector with all p>-rings active (not grounded and/or connected with pad contact) and CUT floating is shown. At a bias below 100 V the reverse current increases near linearly with bias, which corresponds to a leakage resistance of about 1.6;10 ). At higher biases the current saturates at a value of about 50 lA and remains nearly constant in the bias range of 100 to 600 V. The current measured from the CUT contact (with the pad and all rings are floating) shows the same value and the same dependence on the bias in the range of 0 to 100 V. Comparison of these two characteristics leads to the conclusion that the origin of the high leakage current is the damaged layer on the chip border caused by scribing. For the explanation of the current saturation, the full depletion voltage (» ) of the detector was measured by C—» method. The value of » , about 97 V, is very close to the begin ning of the current saturation. Hence, the effect of current saturation is due to the MGR, which become active after the full depletion of the detector. This is in agreement with I—» characteristic of pad current with the inner p>-ring grounded (Fig. 4). After full depletion the surface leakage current is separated from the pad current, and the residual low value of 7 nA (see fragment in the Fig. 5) is the bulk generation current component. The potential distribution between the p>-rings (Fig. 5) has been measured using a scheme shown in Fig. 2b. At biases (» no potential difference between the pad and any ring was detected as all p>-rings were effectively connected with pad contact through non depleted p-type bulk material. At higher biases rings become activated in a narrow voltage range (100—105 V) when the front of space charge region approaches the ohmic side of the detector. At over depletion the MGR on the ohmic side act as a linear divider of potential between the
VI. GaAs AND SILICON DEVICES
200
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Fig. 3. Current—voltage characteristics for a detector with MGR on the ohmic side when the current is measured from the pad contact with active MGR structure (curve “PAD current”), and from the CUT contact with MGR and pad contact floating (curve “CUT current”).
p>-rings (the divider coefficient is not affected by the bias applied to the detector) in the wide range of over depletion bias.
4. Discussion Results described above show that MGR structure on the ohmic side is a main factor of the stable I—» characteristic of these detectors. The MGR adjust their equivalent resistor under the bias applied to the detector to maintain the stable leakage current in the wide range of bias. Phenomenological MGR structure can be considered as a sequence of resistors distributed between the pad electrode and the detector periphery, each of them is associated with gaps between neighboring p>-rings. The properties of the individual resistor are determined by the structure of the inter-ring gap. The simplified model of the gap includes the following factors: (1) SiO passivating layer containing the positive fixed charge;
(2) n-type silicon layer, which is induced by the positive fixed charge of SiO layer; (3) non depleted layer of neutral p-type silicon (starting material) and (4) the space charge region originating from the main, reverse-biased n>-p junction. It is easy to see that the electron accumulation layer cannot conduct the current between the p>-neighboring rings because the potential of the inner p>-ring is more positive than the outer one, so that the left junction in each gap between the p>-doped ring and induced n-layer is reverse biased (Fig. 1). In this case the possible way for leakage current flow under the surface is the p-type silicon layer between the induced n-type layer and the depletion region. The conductivity of this p-layer is determined by its thickness and the concentration of free holes. The model proposed here to explain experimental results is very close to the construction of junction field effect transistor (JFET). The inner ring of the MGR structure biased negatively from
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
201
Fig. 4. Current—voltage characteristics for a detector with MGR on the ohmic side when the current is measured at pad contact with inner p>-ring (ring A) grounded to collect the surface leakage current. A fragment shows the same characteristics in a smaller current range.
Fig. 5. Potential distribution between the rings in MGR structure versus the reverse bias (all rings are active).
VI. GaAs AND SILICON DEVICES
202
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Fig. 6. Potential distribution between the rings in MGR structure with reduced number of active p>-rings versus the reverse bias: (a) ring A connected with pad (3 active rings), (b) ring A and B connected with pad (2 active rings), (c) ring A—C connected with pad (1 active ring), and (d) ring A—D connected with pad (no active rings).
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
203
Fig. 6. (Continued).
the outer ring can be considered as the “drain” of a p-channel JFET. Consequently the outer p>-ring can be considered as the source. The conductivity of the p-channel can be modulated by the potential
applied between the source (outer ring) and the n>-side of the detector which acts as a gate. According to this model the MGR structure operates as five JFETs connected in series.
VI. GaAs AND SILICON DEVICES
204
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Fig. 7. Current—voltage characteristics for a detector with MGR on the ohmic side when the current is measured from the pad contact with different number of active p>-rings: (a) four active rings, (b) ring A connected with pad (3 active rings), (c) ring A and B connected with pad (2 active rings), and (d) ring A—C connected with pad (1 active ting).
To investigate the influence of the number of the rings in MGR structure on the detector characteristics, some of inner rings were connected with the p>-pad. So the pad and the connected rings can be considered as a single p>-contact. The potential distribution among other active rings is presented in Fig. 6. In each case the potential is divided linearly between the rings in the certain range of operation bias. The highest bias of the operation range decreases with the reduction of the number of active p>-rings. The averaged potential difference between the neighboring rings, which does not change the properties of MGR structure, is about 150 V per gap. Finally, the I—» characteristics for the detector with different number of active p>-rings in MGR on the ohmic side are presented in Fig. 7. The maximum operation bias increases with the increase number of the active p>-rings used. Each additional ring in MGR increases the range of bias to 150 V approximately, which is in a good agreement with results of the potential distribution between the rings. These properties of MGR on the
ohmic side allow one to predict the I—» characteristics of the detector with any number of the rings, or estimate the necessary number of rings for a given maximum reverse bias.
5. Summary The investigation of n>/p/p> silicon planar detector with MGR on the ohmic (p>) side shows that this approach for n>-p junction protection (MGR on the ohmic (p>) side) can be successfully applied for detector with operation bias up to 1000 V. The MGR structure on the ohmic side operates as a linear potential divider between the rings in the range of operation biases, which makes the potential distribution easily predictable. The same conclusion is valid for the maximum operation bias of such a detector because each additional ring in MGR structure increases it to a predictable value. This study is the first attempt to investigate electrical and physical properties of MGR structures. More investigation is needed to explain the physics of
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
inter-ring gap as an adjustable resistor in the potential divider and the physical limitation for the absolute maximum potential that can be applied to MGR on the ohmic side. Acknowledgements The authors would like to express their sincere gratitude to E. Verbitskaya for useful discussions and encouragement. References [1] C.B. Goud, K.N. Bhat, IEEE Trans. Electron Devices ED-38 (1991) 1497.
205
[2] D. Jaume, G. Charitat, J.M. Reynes, P. Rossel, IEEE Trans. Electron Devices ED-38 (1991) 1681. [3] A. Bischoff, N. Findeis, D. Hauff, P. Holl, J. Kemmer, P. Klein, P. Lechner, G. Lutz, R.H. Richter, L. Struder, Nucl. Instr. and Meth. A 326 (1993) 27. [4] K.P. Brieger, W. Gerlach, J. Pelka, Solid-State Electron. 26(8) (1983) 739. [5] R. Stengl, U. Gosele, C. Fellinger, M. Beyer, S. Walesch, IEEE Trans. Electron Devices ED-33 (1986) 426. [6] L. Eversen, A. Hanneborg, B.S. Avset, M. Nese, Nucl. Instr. and Meth. A 337 (1993) 44. [7] V. Eremin, E. Verbitskaya, Z. Li, A. Sidorov, E. Fretwirst, G. Lindstroem, Nucl. Instr. and Meth. A 388 (1997) 350. [8] M. Da Rold, A. Paccagnella, A. Da Re, N. Bacchetta, A. Candelori, D. Bisello, G. Verzellesi, G.-F. Dalla Betta, G. Soncini, R. Wheadon, Radiation effects in breakdown characteristics of multiguarded devices, Proc. IEEE Nucl. Sci. Symp., Anaheim 3—9 November 1996.
VI. GaAs AND SILICON DEVICES
Operation of guard rings on the ohmic side of n>—p—p> diodes夽 N. Egorov , V. Eremin *, S. Golubkov , K. Konkov , Yu. Kozlov , Z. Li, A. Sidorov Russian Institute of Material Sciences and Technology, Zelenograd, Russia Ioffe Physico-Technical Institute RAS, St-Petersburg, Russia Brookhaven National Laboratory, Upton, NY, USA
Abstract Detectors from high-resistivity p-type silicon with multi guard rings structure located on the ohmic side have been investigated. The processed detectors were constructed with four p>-rings surrounding the central p>-pad. The n>—p junction opposite to the contact with rings had not any protection and was cut-through by chip scribing. Investigation of the potential distribution between the floating p>-rings has shown that the potential difference arises just after the depletion of the detector bulk and is accompanied by the detector leakage current saturation. The saturated current is mainly a leakage current flowing in a scribed periphery. Grounding of one of the p>-rings to collect the detector leakage current allows the separation of the bulk and the surface components of the current, which reduces the current of a central p>-pad contact down to tens nA/cm. A model based on the field effect in the gap between the neighboring p>-rings is proposed. The experiments carried out in this study show that construction of the ohmic side of Si detectors with multi guard rings is a perspective approach to design the detectors operational at high biases. Detectors processed in this way allow one to apply biases up to 600 V. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Detectors; MGR structures; n>—p—p> diodes
1. Introduction The main efforts in the development of semiconductor detectors with p—n junction were concentrated on the elaboration of a design that avoids p—n junction breakdown at operational biases and reaches a stable I—» characteristic with minimum leakage current. This kind of design focuses on the
夽
This investigation was supported in part by the U.S. Department of Energy: Contract No: DE-AC02-98CH10886. * Corresponding author.
reduction of the maximum value of a local electric field, which appears near the border of p—n junction and can lead to impact ionization and avalanche breakdown. There are two main approaches in the literature to minimize this electric field: (1) metallization of the junction area, which overlaps the SiO passivating layer (field plate) [1—3] and (2) surrounding of the junction by multi guard ring (MGR) termination structure [4—6]. The first approach allows the reduction of the electric field at the border of heavily doped junction area that is in contact with the accumulated n-layer under SiO induced by the fixed oxide charge. The
0168-9002/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 4 9 2 - 2 VI. GaAs AND SILICON DEVICES
198
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
second approach makes it possible to redistribute the potential around a junction over a wide area covered by MGR, which reduces the electric field component that is parallel to the detector surface. Both approaches are successfully used in the design of all type of Si planar detectors including pad and strip ones, and drift detectors. The physics of MGR structure operation on the junction side of detectors has been investigated in detail in the literature. The properties of these structures can be summarized as: (1) the rings are floating; (2) the potential spreads from a central ring to the outer ones; (3) the potential in a ring appears when the space charge region touches it (pouch-through) and (4) the potential drop between the neighboring rings depends on the gap between them. The configuration of MGR on a junction side can be optimized by a computer simulation of the electric field distribution at the p>—n junction taking into account the resistivity of silicon used and characteristics arisen from planar technology, such as the doping profile and fixed charge density in the SiO passivating layer between rings [6]. In this paper we investigate an alternative approach for the field termination in planar detectors, which deals with the backside of the detector with ohmic, non-injecting contact (ohmic side). This approach was not considered in detail until now, although some papers in the literature contain data confirming that this approach is realistic enough. As an example, it can be found in the study of neutron irradiated Si detectors processed from the n-type silicon and designed with MGR on the p>contact [7,8]. For high resistivity (*4 k) cm) Si detectors, at fluences over 10 n/cm the sign of a space charge in depletion region, initially positive (concentration of ionized donors is larger than that of acceptors), changes to negative (space charge sign inversion, SCSI, or simply inversion). Conse-
quently the junction of the detector moves to the back side and is formed now by the n>-contact and the acceptor-type space charge region in the bulk silicon. In this situation the p>-contact with MGR operates as non-injecting ohmic contact. Investigation of the reverse current of these detectors has shown that no breakdown is observed at bias of hundreds of volts in spite of n>—p junction heavily damaged by scribing. One of the possible causes can be attributed to MGR effect on the ohmic side. Below the detailed investigation on operation of MGR on the ohmic side is presented.
2. Device and experimental technique Detectors studied here are diodes processed from p-type silicon wafers with a resistivity of about 10 k) cm and 400 lm in thickness. A cross-section of the diode is shown in Fig. 1 and geometrical parameters of MGR structure are listed in Table 1. The n>-contact on the junction side was processed by phosphorus implantation into the entire wafer surface followed by a metallization using Al. On the ohmic side the p>-contact was formed by boron implantation through a thin SiO layer. The central p>-pad contact had an area of 10;10 mm and
Fig. 1. Cross-section of the detector with MGR.
Table 1 Parameters for detectors used in this study Distance between (mkm)
PAD — Ring A 25
Rings A—B 25
Rings B—C 35
Rings C—D 55
Ring C-cut 110
Width of (mkm)
Ring A 40
Ring B 40
Ring C 40
Ring D 45
CUT 400
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
was surrounded by 4 square-shaped p>-rings. The p>-pad area and p>-rings were metallized. The gap on the ohmic side between the neighboring detectors used for detector chip scribing was boron implanted and metallized as well. Processed wafers were scribed by a diamond saw. No additional treatment of the scribed periphery of individual detector chips was used. Diced detectors were then attached to the supporting board by conducting glue on the n>-side, and all p>-contacts (pad, 4 rings and metallized area at the border of a chip) were wire bonded to the contacts on the board. All measurements were performed in air at room temperature with no humidity control.
3. Experimental results In Fig. 2a, the schematics used for current—voltage characteristic measurements are shown. The ohmic side was at low potential and the junction side was positively biased. This allows one to measure the I—» characteristics at different modes of p>-rings grounding or floating, and to separate the two components of the current — the current flowing through a pad contact and that through
Fig. 2. Schematics of the detector biasing schemes: (a) for I—» measurements, and (b) for measurements of potentials on the MGR on the ohmic side.
199
one of the rings. The amount of working p>-rings in MGR structure can be varied by parallel connection of pad electrode with some neighboring rings. This mode was used to simulate the operation of detector with reduced number of rings. In the extreme case, all rings and the p>-contact (the latter named here as CUT) were connected together to the ground leaving the detector with no protection against low voltage breakdown at all. In Fig. 3, the I—» characteristic of the detector with all p>-rings active (not grounded and/or connected with pad contact) and CUT floating is shown. At a bias below 100 V the reverse current increases near linearly with bias, which corresponds to a leakage resistance of about 1.6;10 ). At higher biases the current saturates at a value of about 50 lA and remains nearly constant in the bias range of 100 to 600 V. The current measured from the CUT contact (with the pad and all rings are floating) shows the same value and the same dependence on the bias in the range of 0 to 100 V. Comparison of these two characteristics leads to the conclusion that the origin of the high leakage current is the damaged layer on the chip border caused by scribing. For the explanation of the current saturation, the full depletion voltage (» ) of the detector was measured by C—» method. The value of » , about 97 V, is very close to the begin ning of the current saturation. Hence, the effect of current saturation is due to the MGR, which become active after the full depletion of the detector. This is in agreement with I—» characteristic of pad current with the inner p>-ring grounded (Fig. 4). After full depletion the surface leakage current is separated from the pad current, and the residual low value of 7 nA (see fragment in the Fig. 5) is the bulk generation current component. The potential distribution between the p>-rings (Fig. 5) has been measured using a scheme shown in Fig. 2b. At biases (» no potential difference between the pad and any ring was detected as all p>-rings were effectively connected with pad contact through non depleted p-type bulk material. At higher biases rings become activated in a narrow voltage range (100—105 V) when the front of space charge region approaches the ohmic side of the detector. At over depletion the MGR on the ohmic side act as a linear divider of potential between the
VI. GaAs AND SILICON DEVICES
200
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Fig. 3. Current—voltage characteristics for a detector with MGR on the ohmic side when the current is measured from the pad contact with active MGR structure (curve “PAD current”), and from the CUT contact with MGR and pad contact floating (curve “CUT current”).
p>-rings (the divider coefficient is not affected by the bias applied to the detector) in the wide range of over depletion bias.
4. Discussion Results described above show that MGR structure on the ohmic side is a main factor of the stable I—» characteristic of these detectors. The MGR adjust their equivalent resistor under the bias applied to the detector to maintain the stable leakage current in the wide range of bias. Phenomenological MGR structure can be considered as a sequence of resistors distributed between the pad electrode and the detector periphery, each of them is associated with gaps between neighboring p>-rings. The properties of the individual resistor are determined by the structure of the inter-ring gap. The simplified model of the gap includes the following factors: (1) SiO passivating layer containing the positive fixed charge;
(2) n-type silicon layer, which is induced by the positive fixed charge of SiO layer; (3) non depleted layer of neutral p-type silicon (starting material) and (4) the space charge region originating from the main, reverse-biased n>-p junction. It is easy to see that the electron accumulation layer cannot conduct the current between the p>-neighboring rings because the potential of the inner p>-ring is more positive than the outer one, so that the left junction in each gap between the p>-doped ring and induced n-layer is reverse biased (Fig. 1). In this case the possible way for leakage current flow under the surface is the p-type silicon layer between the induced n-type layer and the depletion region. The conductivity of this p-layer is determined by its thickness and the concentration of free holes. The model proposed here to explain experimental results is very close to the construction of junction field effect transistor (JFET). The inner ring of the MGR structure biased negatively from
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
201
Fig. 4. Current—voltage characteristics for a detector with MGR on the ohmic side when the current is measured at pad contact with inner p>-ring (ring A) grounded to collect the surface leakage current. A fragment shows the same characteristics in a smaller current range.
Fig. 5. Potential distribution between the rings in MGR structure versus the reverse bias (all rings are active).
VI. GaAs AND SILICON DEVICES
202
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Fig. 6. Potential distribution between the rings in MGR structure with reduced number of active p>-rings versus the reverse bias: (a) ring A connected with pad (3 active rings), (b) ring A and B connected with pad (2 active rings), (c) ring A—C connected with pad (1 active ring), and (d) ring A—D connected with pad (no active rings).
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
203
Fig. 6. (Continued).
the outer ring can be considered as the “drain” of a p-channel JFET. Consequently the outer p>-ring can be considered as the source. The conductivity of the p-channel can be modulated by the potential
applied between the source (outer ring) and the n>-side of the detector which acts as a gate. According to this model the MGR structure operates as five JFETs connected in series.
VI. GaAs AND SILICON DEVICES
204
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
Fig. 7. Current—voltage characteristics for a detector with MGR on the ohmic side when the current is measured from the pad contact with different number of active p>-rings: (a) four active rings, (b) ring A connected with pad (3 active rings), (c) ring A and B connected with pad (2 active rings), and (d) ring A—C connected with pad (1 active ting).
To investigate the influence of the number of the rings in MGR structure on the detector characteristics, some of inner rings were connected with the p>-pad. So the pad and the connected rings can be considered as a single p>-contact. The potential distribution among other active rings is presented in Fig. 6. In each case the potential is divided linearly between the rings in the certain range of operation bias. The highest bias of the operation range decreases with the reduction of the number of active p>-rings. The averaged potential difference between the neighboring rings, which does not change the properties of MGR structure, is about 150 V per gap. Finally, the I—» characteristics for the detector with different number of active p>-rings in MGR on the ohmic side are presented in Fig. 7. The maximum operation bias increases with the increase number of the active p>-rings used. Each additional ring in MGR increases the range of bias to 150 V approximately, which is in a good agreement with results of the potential distribution between the rings. These properties of MGR on the
ohmic side allow one to predict the I—» characteristics of the detector with any number of the rings, or estimate the necessary number of rings for a given maximum reverse bias.
5. Summary The investigation of n>/p/p> silicon planar detector with MGR on the ohmic (p>) side shows that this approach for n>-p junction protection (MGR on the ohmic (p>) side) can be successfully applied for detector with operation bias up to 1000 V. The MGR structure on the ohmic side operates as a linear potential divider between the rings in the range of operation biases, which makes the potential distribution easily predictable. The same conclusion is valid for the maximum operation bias of such a detector because each additional ring in MGR structure increases it to a predictable value. This study is the first attempt to investigate electrical and physical properties of MGR structures. More investigation is needed to explain the physics of
N. Egorov et al. / Nuclear Instruments and Methods in Physics Research A 426 (1999) 197—205
inter-ring gap as an adjustable resistor in the potential divider and the physical limitation for the absolute maximum potential that can be applied to MGR on the ohmic side. Acknowledgements The authors would like to express their sincere gratitude to E. Verbitskaya for useful discussions and encouragement. References [1] C.B. Goud, K.N. Bhat, IEEE Trans. Electron Devices ED-38 (1991) 1497.
205
[2] D. Jaume, G. Charitat, J.M. Reynes, P. Rossel, IEEE Trans. Electron Devices ED-38 (1991) 1681. [3] A. Bischoff, N. Findeis, D. Hauff, P. Holl, J. Kemmer, P. Klein, P. Lechner, G. Lutz, R.H. Richter, L. Struder, Nucl. Instr. and Meth. A 326 (1993) 27. [4] K.P. Brieger, W. Gerlach, J. Pelka, Solid-State Electron. 26(8) (1983) 739. [5] R. Stengl, U. Gosele, C. Fellinger, M. Beyer, S. Walesch, IEEE Trans. Electron Devices ED-33 (1986) 426. [6] L. Eversen, A. Hanneborg, B.S. Avset, M. Nese, Nucl. Instr. and Meth. A 337 (1993) 44. [7] V. Eremin, E. Verbitskaya, Z. Li, A. Sidorov, E. Fretwirst, G. Lindstroem, Nucl. Instr. and Meth. A 388 (1997) 350. [8] M. Da Rold, A. Paccagnella, A. Da Re, N. Bacchetta, A. Candelori, D. Bisello, G. Verzellesi, G.-F. Dalla Betta, G. Soncini, R. Wheadon, Radiation effects in breakdown characteristics of multiguarded devices, Proc. IEEE Nucl. Sci. Symp., Anaheim 3—9 November 1996.
VI. GaAs AND SILICON DEVICES