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GaAs heterostructure RT-SCR model
ARTICLE IN PRESS Microelectronics Journal 39 (2008) 1504– 1508
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Characteristics of AlGaAs/GaAs heterostructure RT-SCR model B.D. Barkana Electrical Engineering Department, University of Bridgeport, 221 University Avenue, Bridgeport, CT 06604, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 February 2008 Accepted 23 April 2008 Available online 16 June 2008
Electrical properties of a resonant-tunneling-semiconductor-controlled rectifier (RT-SCR) model have been presented. The current, temperature, gain, doping concentration, and layer size versus voltage relationships have been numerically obtained. The RT-SCR device requires smaller turn-on voltage than a comparable traditional device for the same gate current. This indicates that, in comparison with the traditional thyristor, a smaller control current may be used to turn on the device at a particular voltage. Characteristics of the device are affected by p1, n1, and p2 regions. It is showed that higher doping concentrations cause lower turn-on voltages and an increase in the region width results in higher turnon voltages for p1 and p2 regions. Changing the doping concentration and width in n1 region affects the characteristics of the structure differently from that of the p1 and p2 regions. & 2008 Elsevier Ltd. All rights reserved.
Keywords: RT-SCR Thyristors Semiconductor devices
1. Introduction
2. Resonant-tunneling-semiconductor-controlled rectifier
The pnpn thyristor structure has been around and used as a semiconductor switch to handle large amounts of currents. Traditionally, the pnpn thyristor structure is modeled with two transistors: a pnp transistor and an npn transistor—the base of each transistor is connected to the collector of the other transistor. For a resonant-tunneling-semiconductor-controlled rectifier (RT-SCR) device, the npn transistor is replaced with a resonanttunneling transistor (RTT) with an anticipation of reducing the turn-on current. The simulation results indicate that the RT-SCR is turned on at a lower voltage than the turn-on voltage of the traditional SCR for the same gate current [1]. Because tunneling is inherently a very fast phenomenon, the structure has a high potential to be used as a fast turn-on switch. Although resonant tunneling has some major problems, it has many advantages for high-speed devices [2,3]. The RTD is considered among the fastest devices ever made [4]. Like RTD, RTTs have many advantages such as faster switching time for high-speed operation devices; the RT-SCR is expected to have a fast switching response [1]. In this work, the tunneling characteristics of a RT-SCR device are outlined, and electrical properties of the RT-SCR model are presented.
A pnpnpn AlGaAs/GaAs heterostructure has been used in the simulation of the RT-SCR device. The circuit given in Fig. 1 is used to model the RT-SCR structure. It is the modified two-transistor model, which has T1 as a traditional pnp bipolar transistor and T2 as an RTT [5]. 2.1. The current– voltage equations The anode current of RT-SCR can be expressed as follows: IART ¼
a2RTT Ig þ ICORTT þ ICO1 1 ða1 þ a2RTT Þ
(1)
The common-base current gain a2RTT for the resonant-tunneling structure will be as follows: a2RTT ¼
ICRTT IERTT
(2)
The voltage distribution in the model for the RT-SCR is shown in Fig. 1. The anode–cathode voltage is as follows: V AK ¼ V BE1 þ V BC þ V BERTT
(3)
2.2. The physical structure
Tel.: +1 203 576 4577.
E-mail address: [email protected] 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.04.014
For the model, a physical RTT structure reported by Wu et al. [6] is adopted. The structure for the RT-SCR is shown by splitting the structure and adding necessary regions as shown in Fig. 2.
ARTICLE IN PRESS B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
In the model, the tunneling takes place between the gate and the cathode. The physical properties of p1, p2, n1, and n2 regions are given in Table 1.
3. Simulation results
1505
Table 1 Physical properties of the four layers in the RT-SCR structure Region
Material
Doping (cm3)
Thickness (mm)
p1 n1 p2 n2
GaAs GaAs GaAs AlGaAs
5 1016 5 1016 5 1016 8 1017
0.3 0.3 0.3 0.15
MATLAB has been used to obtain electrical properties of the device. The results are presented in a graphical form in Figs. 3–13.
0.12 Ig=1 mA Ig=5 mA Ig=10 mA
Current (IAK) (A)
0.1
0.08
0.06
0.04
0.02
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Voltage (VAK) (V) Fig. 1. Currents and voltage distributions in a two-transistor model for the RT-SCR.
Fig. 3. Current–voltage characteristic of RT-SCR for different gate currents at 300 K.
Fig. 2. Cross-sectional drawing of the two-transistor RT-SCR model.
ARTICLE IN PRESS 1506
B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
1.3 Ig=1 mA
1.2
1.4 1.2
1
1
0.9
Total current Gain
Total current Gain
1.1
T=300K T=310K T=320K T=330K Ig=1 mA
0.8 0.7 0.6
0.8
0.6
0.4
0.5 0.4 0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0.2
2
Voltage (VAK) (V)
0 0
Fig. 4. (a1+a2)—voltage characteristics of RT-SCR model.
0.5
1
1.5
2
2.5
Voltage (VAK) (V) Fig. 6. (a1+a2)—Voltage characteristics of RT-SCR model for different temperatures.
0.012 T=300 K T=310 K T=320 K Ig=1 mA
0.018 0.016
0.008
0.014
Current (IAK) (A)
Current (IAK) (A)
0.01
0.006
0.004
0.012 0.01 0.008 0.006
0.002 0.004
0 0
0.5
1
1.5
2
2.5
Voltage (VAK) (V)
0.002 0 0
Fig. 5. Current–voltage characteristic of RT-SCR for different temperatures.
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Voltage (VAK) (V) Fig. 7. Current–voltage characteristics of RT-SCR for different p1 region widths.
Fig. 3 shows the current–voltage characteristics of the device for 1, 5, and 10 mA of the gate current. In the figure, the current increases sharply for VAK around 1.6 V, which means the switch is turned on. As the gate current increases, the turn-on voltage of the device decreases. In Fig. 4, the sum of the common-base current gains (a1+a2) of the device—T1 and T2 transistors model—is plotted against the switch voltage. Figs. 5 and 6 show the current–voltage and total gain–voltage characteristics of the device. As the temperature increases, the turn-on voltage of the device increases. After the temperature has crossed 320 K, the turn-on voltage starts decreasing. This phenomenon can be seen in Fig. 6. RT-SCR device behaves very sensitively to temperature. Figs. 7 and 8 show the characteristics of RT-SCR for different p1 region widths. The turn-on voltage of the device decreases as
the width of the p1 region increases. If we look at the total gain–voltage characteristic of the device in Fig. 8, the sum of the common-base current gain for RT-SCR increases almost exponentially with voltage. In Figs. 9 and 10, the characteristics of RT-SCR for different n1 region widths are presented. The effects of n1 region’s width is very dramatic. If we continue to increase the width of n1 region, turn-on voltage of the device immediately drops. In Fig. 10, the total gain of the T1 and T2 transistors for a two-transistor model (a1+a2) versus voltage characteristics of the RT-SCR show the same fact as Fig. 9. Figs. 11–13 show the effects of the p1, n1, and p2 regions’ doping concentrations on the device. As the doping concentration
ARTICLE IN PRESS
1.3
1.4
1.2
1.2
1.1
1
1
Current (IAK) (A)
Total current Gain
B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
0.9 0.8
1507
0.8 0.6 0.4
0.7 0.2
0.6 0
0.5 -0.2
0.4 0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0
2
1
2
3
4
5
6
Voltage (VAK) (V)
Voltage (VAK) (V) Fig. 8. (a1+a2)—Voltage characteristics of RT-SCR model for different p1 region widths.
Fig. 10. (a1+a2)—Voltage characteristics of RT-SCR model for different n1 region widths.
0.012
0.012
p1=1∗1016 (cm-3) p1=3∗1016 (cm-3)
0.01
0.008
Current (IAK) (A)
Current (IAK) (A)
0.01
0.006
0.004
p1=5∗1016 (cm-3) p1=7∗1016 (cm-3) T=300K Ig=1 mA
0.008
0.006
0.004
0.002
0.002
0
0 0
0.5
1 1.5 Voltage (VAK) (V)
2
2.5
0
0.2
0.4
0.6 0.8 1 1.2 Voltage (VAK) (V)
1.4
1.6
1.8
Fig. 9. Current–voltage characteristics of RT-SCR for different n1 region widths.
Fig. 11. Current–voltage characteristics of RT-SCR for different p1 region doping concentrations.
increases, the turn-on voltage of the device decreases for p1 and p2 regions. RT-SCR structure conducts at lower voltages for higher doping concentrations. N1 region behaves differently from the others. As the doping concentration increases, the turn-on voltage of the device increases for n1 region.
switching time for high-speed operation devices. The previous study showed that RT-SCR starts to conduct much faster than traditional SCR [1]. The RT-SCR structure attains high switching speed in spite of traditional SCR. It is presented in a more detailed study on the current–voltage and total current gain–voltage characteristics for different regions’ widths and doping concentrations to prove that RT-SCRs have similar advantages as RTDs and RTBTs presented here. It is shown that higher doping concentrations cause lower turn-on voltages and an increase in the region width results in higher turn-on voltages for p1 and p2 regions. Changing doping concentration and width in n1 region affects the characteristics of the structure differently from that of the p1 and p2 regions. All calculations for simulation results are found using Matlab.
4. Conclusion In this study, electrical characteristics of the RT-SCR model have been proposed. The model includes two transistors: one of them is traditional, the other is RTBT. Although resonant tunneling has some major problems, it has many advantages for high-speed devices. RTTs have many advantages such as faster
ARTICLE IN PRESS 1508
B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
0.018
[3] H.J. Pan, S.C. Feng, W.C. Wang, K.W. Lin, K.H. Yu, C.Z. Wu, L.W. Laih, W.C. Liu, Investigation of an InGaP/GaAs resonant tunneling heterojunction bipolar transistor, Solid State Electron. (45) (2001) 489–494. [4] A. Qingmin Liu Seabaugh, P. Chalal, F.J. Morris, Unified AC model for the resonant tunneling diode, IEEE Trans. Electron. Devices 51 (5) (2004) 653–657. [5] B.D. Barkana, H.H. Erkaya, A Model for Resonant Tunneling Bipolar Transistors, Innovative Algorithms and Techniques in Automation, Industrial Electronics and Telecommunications, Springer, 2006, p. 75–78. [6] J.S. Wu, C.Y. Chang, C.P. Lee, K.H. Chanh, D.G. Liu, D.C. Liou, Characterization of improved AlGaAs/GaAs resonant tunneling heterostructure bipolar transistors, Jpn. J. Appl. Phys. 30 (2A) (1991) L160–LL62.
n1=2∗1016 (cm-3) n1=4∗1016 (cm-3)
0.016
n1=6∗1016 (cm-3) n1=8∗1016 (cm-3) T=300K Ig=1 mA
Current (IAK) (A)
0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 0
0.5
1
1.5
2
2.5
Voltage (VAK) (V) Fig. 12. Current–voltage characteristics of RT-SCR for different n1 region doping concentrations.
0.03
Current (IAK) (A)
0.025
0.02
0.015
0.01
0.005
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Voltage (VAK) (V) Fig. 13. Current–voltage characteristics of RT-SCR for different p2 region doping concentrations.
References [1] B.D. Barkana, H.H. Erkaya, A model for the resonant tunneling semiconductorcontrolled rectifier, Microelectron. J. 38 (2007) 871–876. [2] R. Lacomb, F. Jain, A self-consistent model to simulate large-signal electrical characteristics of resonant tunneling bipolar transistors, Solid State Electron. 39 (11) (1996) 1621–1627.
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Characteristics of AlGaAs/GaAs heterostructure RT-SCR model B.D. Barkana Electrical Engineering Department, University of Bridgeport, 221 University Avenue, Bridgeport, CT 06604, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 February 2008 Accepted 23 April 2008 Available online 16 June 2008
Electrical properties of a resonant-tunneling-semiconductor-controlled rectifier (RT-SCR) model have been presented. The current, temperature, gain, doping concentration, and layer size versus voltage relationships have been numerically obtained. The RT-SCR device requires smaller turn-on voltage than a comparable traditional device for the same gate current. This indicates that, in comparison with the traditional thyristor, a smaller control current may be used to turn on the device at a particular voltage. Characteristics of the device are affected by p1, n1, and p2 regions. It is showed that higher doping concentrations cause lower turn-on voltages and an increase in the region width results in higher turnon voltages for p1 and p2 regions. Changing the doping concentration and width in n1 region affects the characteristics of the structure differently from that of the p1 and p2 regions. & 2008 Elsevier Ltd. All rights reserved.
Keywords: RT-SCR Thyristors Semiconductor devices
1. Introduction
2. Resonant-tunneling-semiconductor-controlled rectifier
The pnpn thyristor structure has been around and used as a semiconductor switch to handle large amounts of currents. Traditionally, the pnpn thyristor structure is modeled with two transistors: a pnp transistor and an npn transistor—the base of each transistor is connected to the collector of the other transistor. For a resonant-tunneling-semiconductor-controlled rectifier (RT-SCR) device, the npn transistor is replaced with a resonanttunneling transistor (RTT) with an anticipation of reducing the turn-on current. The simulation results indicate that the RT-SCR is turned on at a lower voltage than the turn-on voltage of the traditional SCR for the same gate current [1]. Because tunneling is inherently a very fast phenomenon, the structure has a high potential to be used as a fast turn-on switch. Although resonant tunneling has some major problems, it has many advantages for high-speed devices [2,3]. The RTD is considered among the fastest devices ever made [4]. Like RTD, RTTs have many advantages such as faster switching time for high-speed operation devices; the RT-SCR is expected to have a fast switching response [1]. In this work, the tunneling characteristics of a RT-SCR device are outlined, and electrical properties of the RT-SCR model are presented.
A pnpnpn AlGaAs/GaAs heterostructure has been used in the simulation of the RT-SCR device. The circuit given in Fig. 1 is used to model the RT-SCR structure. It is the modified two-transistor model, which has T1 as a traditional pnp bipolar transistor and T2 as an RTT [5]. 2.1. The current– voltage equations The anode current of RT-SCR can be expressed as follows: IART ¼
a2RTT Ig þ ICORTT þ ICO1 1 ða1 þ a2RTT Þ
(1)
The common-base current gain a2RTT for the resonant-tunneling structure will be as follows: a2RTT ¼
ICRTT IERTT
(2)
The voltage distribution in the model for the RT-SCR is shown in Fig. 1. The anode–cathode voltage is as follows: V AK ¼ V BE1 þ V BC þ V BERTT
(3)
2.2. The physical structure
Tel.: +1 203 576 4577.
E-mail address: [email protected] 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.04.014
For the model, a physical RTT structure reported by Wu et al. [6] is adopted. The structure for the RT-SCR is shown by splitting the structure and adding necessary regions as shown in Fig. 2.
ARTICLE IN PRESS B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
In the model, the tunneling takes place between the gate and the cathode. The physical properties of p1, p2, n1, and n2 regions are given in Table 1.
3. Simulation results
1505
Table 1 Physical properties of the four layers in the RT-SCR structure Region
Material
Doping (cm3)
Thickness (mm)
p1 n1 p2 n2
GaAs GaAs GaAs AlGaAs
5 1016 5 1016 5 1016 8 1017
0.3 0.3 0.3 0.15
MATLAB has been used to obtain electrical properties of the device. The results are presented in a graphical form in Figs. 3–13.
0.12 Ig=1 mA Ig=5 mA Ig=10 mA
Current (IAK) (A)
0.1
0.08
0.06
0.04
0.02
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Voltage (VAK) (V) Fig. 1. Currents and voltage distributions in a two-transistor model for the RT-SCR.
Fig. 3. Current–voltage characteristic of RT-SCR for different gate currents at 300 K.
Fig. 2. Cross-sectional drawing of the two-transistor RT-SCR model.
ARTICLE IN PRESS 1506
B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
1.3 Ig=1 mA
1.2
1.4 1.2
1
1
0.9
Total current Gain
Total current Gain
1.1
T=300K T=310K T=320K T=330K Ig=1 mA
0.8 0.7 0.6
0.8
0.6
0.4
0.5 0.4 0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0.2
2
Voltage (VAK) (V)
0 0
Fig. 4. (a1+a2)—voltage characteristics of RT-SCR model.
0.5
1
1.5
2
2.5
Voltage (VAK) (V) Fig. 6. (a1+a2)—Voltage characteristics of RT-SCR model for different temperatures.
0.012 T=300 K T=310 K T=320 K Ig=1 mA
0.018 0.016
0.008
0.014
Current (IAK) (A)
Current (IAK) (A)
0.01
0.006
0.004
0.012 0.01 0.008 0.006
0.002 0.004
0 0
0.5
1
1.5
2
2.5
Voltage (VAK) (V)
0.002 0 0
Fig. 5. Current–voltage characteristic of RT-SCR for different temperatures.
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Voltage (VAK) (V) Fig. 7. Current–voltage characteristics of RT-SCR for different p1 region widths.
Fig. 3 shows the current–voltage characteristics of the device for 1, 5, and 10 mA of the gate current. In the figure, the current increases sharply for VAK around 1.6 V, which means the switch is turned on. As the gate current increases, the turn-on voltage of the device decreases. In Fig. 4, the sum of the common-base current gains (a1+a2) of the device—T1 and T2 transistors model—is plotted against the switch voltage. Figs. 5 and 6 show the current–voltage and total gain–voltage characteristics of the device. As the temperature increases, the turn-on voltage of the device increases. After the temperature has crossed 320 K, the turn-on voltage starts decreasing. This phenomenon can be seen in Fig. 6. RT-SCR device behaves very sensitively to temperature. Figs. 7 and 8 show the characteristics of RT-SCR for different p1 region widths. The turn-on voltage of the device decreases as
the width of the p1 region increases. If we look at the total gain–voltage characteristic of the device in Fig. 8, the sum of the common-base current gain for RT-SCR increases almost exponentially with voltage. In Figs. 9 and 10, the characteristics of RT-SCR for different n1 region widths are presented. The effects of n1 region’s width is very dramatic. If we continue to increase the width of n1 region, turn-on voltage of the device immediately drops. In Fig. 10, the total gain of the T1 and T2 transistors for a two-transistor model (a1+a2) versus voltage characteristics of the RT-SCR show the same fact as Fig. 9. Figs. 11–13 show the effects of the p1, n1, and p2 regions’ doping concentrations on the device. As the doping concentration
ARTICLE IN PRESS
1.3
1.4
1.2
1.2
1.1
1
1
Current (IAK) (A)
Total current Gain
B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
0.9 0.8
1507
0.8 0.6 0.4
0.7 0.2
0.6 0
0.5 -0.2
0.4 0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0
2
1
2
3
4
5
6
Voltage (VAK) (V)
Voltage (VAK) (V) Fig. 8. (a1+a2)—Voltage characteristics of RT-SCR model for different p1 region widths.
Fig. 10. (a1+a2)—Voltage characteristics of RT-SCR model for different n1 region widths.
0.012
0.012
p1=1∗1016 (cm-3) p1=3∗1016 (cm-3)
0.01
0.008
Current (IAK) (A)
Current (IAK) (A)
0.01
0.006
0.004
p1=5∗1016 (cm-3) p1=7∗1016 (cm-3) T=300K Ig=1 mA
0.008
0.006
0.004
0.002
0.002
0
0 0
0.5
1 1.5 Voltage (VAK) (V)
2
2.5
0
0.2
0.4
0.6 0.8 1 1.2 Voltage (VAK) (V)
1.4
1.6
1.8
Fig. 9. Current–voltage characteristics of RT-SCR for different n1 region widths.
Fig. 11. Current–voltage characteristics of RT-SCR for different p1 region doping concentrations.
increases, the turn-on voltage of the device decreases for p1 and p2 regions. RT-SCR structure conducts at lower voltages for higher doping concentrations. N1 region behaves differently from the others. As the doping concentration increases, the turn-on voltage of the device increases for n1 region.
switching time for high-speed operation devices. The previous study showed that RT-SCR starts to conduct much faster than traditional SCR [1]. The RT-SCR structure attains high switching speed in spite of traditional SCR. It is presented in a more detailed study on the current–voltage and total current gain–voltage characteristics for different regions’ widths and doping concentrations to prove that RT-SCRs have similar advantages as RTDs and RTBTs presented here. It is shown that higher doping concentrations cause lower turn-on voltages and an increase in the region width results in higher turn-on voltages for p1 and p2 regions. Changing doping concentration and width in n1 region affects the characteristics of the structure differently from that of the p1 and p2 regions. All calculations for simulation results are found using Matlab.
4. Conclusion In this study, electrical characteristics of the RT-SCR model have been proposed. The model includes two transistors: one of them is traditional, the other is RTBT. Although resonant tunneling has some major problems, it has many advantages for high-speed devices. RTTs have many advantages such as faster
ARTICLE IN PRESS 1508
B.D. Barkana / Microelectronics Journal 39 (2008) 1504–1508
0.018
[3] H.J. Pan, S.C. Feng, W.C. Wang, K.W. Lin, K.H. Yu, C.Z. Wu, L.W. Laih, W.C. Liu, Investigation of an InGaP/GaAs resonant tunneling heterojunction bipolar transistor, Solid State Electron. (45) (2001) 489–494. [4] A. Qingmin Liu Seabaugh, P. Chalal, F.J. Morris, Unified AC model for the resonant tunneling diode, IEEE Trans. Electron. Devices 51 (5) (2004) 653–657. [5] B.D. Barkana, H.H. Erkaya, A Model for Resonant Tunneling Bipolar Transistors, Innovative Algorithms and Techniques in Automation, Industrial Electronics and Telecommunications, Springer, 2006, p. 75–78. [6] J.S. Wu, C.Y. Chang, C.P. Lee, K.H. Chanh, D.G. Liu, D.C. Liou, Characterization of improved AlGaAs/GaAs resonant tunneling heterostructure bipolar transistors, Jpn. J. Appl. Phys. 30 (2A) (1991) L160–LL62.
n1=2∗1016 (cm-3) n1=4∗1016 (cm-3)
0.016
n1=6∗1016 (cm-3) n1=8∗1016 (cm-3) T=300K Ig=1 mA
Current (IAK) (A)
0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 0
0.5
1
1.5
2
2.5
Voltage (VAK) (V) Fig. 12. Current–voltage characteristics of RT-SCR for different n1 region doping concentrations.
0.03
Current (IAK) (A)
0.025
0.02
0.015
0.01
0.005
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Voltage (VAK) (V) Fig. 13. Current–voltage characteristics of RT-SCR for different p2 region doping concentrations.
References [1] B.D. Barkana, H.H. Erkaya, A model for the resonant tunneling semiconductorcontrolled rectifier, Microelectron. J. 38 (2007) 871–876. [2] R. Lacomb, F. Jain, A self-consistent model to simulate large-signal electrical characteristics of resonant tunneling bipolar transistors, Solid State Electron. 39 (11) (1996) 1621–1627.