Static and Dynamic Characteristics of Antiparallel Diode as Part of ...

ISSN 10683712, Russian Electrical Engineering, 2010, Vol. 81, No. 11, pp. 619–626. © Allerton Press, Inc., 2010. Original Russian Text © A.V. Gorbatyuk, I.V. Grekhov, D.V. Gusin, B.V. Ivanov, 2010, published in Elektrotekhnika, 2010, No. 11, pp. 53–61.

Static and Dynamic Characteristics of Antiparallel Diode as Part of Switching Power Module A. V. Gorbatyuka, I. V. Grekhova, D. V. Gusinb, and B. V. Ivanovc a

Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, Russia b St. Petersburg State Technical University, St. Petersburg, Russia cUl’yanov (Lenin) State University of Electrical Engineering, St. Petersburg, Russia Received October 18, 2010

Abstract—A numerical simulation of the operation of a fast antiparallel diode in the switching power module of an autonomous voltage inverter is carried out. The time of turning off the transient and the static and tran sient heat losses in the diode are calculated as a function of the electrophysical parameters of the semicon ductor diode structure. Keywords: voltage inverter, antiparallel diode, reverse recovery, carrier lifetime, injection efficiency. DOI: 10.3103/S106837121011009X

The development of new designs of switching power devices requires the improvement of the char acteristics of fast antiparallel diodes as part of power semiconductor modules. The main application of these modules in the inverter technology is the bridge circuit of a voltage inverter, which makes it possible to gradually adjust the frequency and amplitude of the alternating voltage applied across a load. The load, e.g., one of the windings of the stator of an induction motor with inductance Ll and active resistance Rl, is connected across the bridge diagonal of four modules

LI

(Fig. 1a, a singlephase version of the circuit). The cir cuit input is connected to a voltage supply U with an inparallel capacitive filter, i.e., a capacitor Cf. Para sitic inductances of the conductors from the supply rails to the power modules are included in the induc tance LI Ⰶ Ll; the inductances associated with ele ment connections in the modules are negligible com pared to LI. The power switches are controlled by the law of pulsewidth modulation (PWM). A sinusoidal voltage with a period T must be formed across the load. In one

(a)

(b)

1

2 LI

+ U

Cf

Ll

Rl

– Il + 4

3

I0

U –

Fig. 1. (a) Singlephase version of a voltage inverter circuit and (b) equivalent circuit for simulating the process of turning off a diode.

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of the halfperiods T/2, only the switches of modules 1 and 3 operate; the load current varies from the peak value –I0 to +I0. In the second halfperiod, switches 1 and 3 are turned off and the other pair of modules is used. The PWM period Tk is constant and equal to the sum of two intervals ton and toff. At the beginning of each such period, devices 1 and 3 are switched on by positivepolarity driving pulses with a width ton; the current, which increases with time, flows from the supply sequentially through the turnedon switches of the modules and the load Ll, Rl. After feeding driving pulses that turn off switches 1 and 3, the current flows through the diodes of modules 2 and 4 and the induc tive load in the same direction. The inductance Ll is now connected to the supply in the reverse polarity, and the current through it decreases. The duration of this phase is equal to toff. At the beginning of the next modulation period, switches 1 and 3 are switched on again, and the reverse recovery of diodes 2 and 4 occurs. Antiparallel diodes in an inverter operate under extreme conditions, meaning they must conduct the total operating current and they must recover at a high rate of rise of reverse current. The characteristics of the on state of the diode and the process of recovering its blocking ability depend heavily on the structural parameters of the device, including the thickness of the base, the concentration of the impurity and the diffusion length within it, and the injection efficiency of the anode pemitter). The selection of parameters of the power diode under development is based on the tradeoff relationship caused by the following three factors: (1) heat losses in the forward current flow Wst ≈ VonI0toff, where Von is the forward voltage drop across a turned on diode; (2) transient heat losses Woff equal to the time inte gral of V(t)I(t) in the recovery process; (3) limitations of the operating current and voltage with respect to the static and dynamic avalanche breakdown. In addition, a diode must exhibit soft recovery behavior, which is shown through the limitation in the allowable rate of reversecurrent decay. Exceeding this rate leads to highfrequency voltage fluctuations with large amplitudes due to the parasitic inductance of the circuit. In this work, using a numerical simulation, we dis cuss the effect of technological parameters of the structure on the characteristics of a diode designed for operating in a power module with a fieldcontrolled integrated thyristor [1] or an insulatedgate bipolar transistor. Similar studies were carried out in several works. For example, in [2], a onedimensional numer ical simulation is used to optimize the reverse recovery behavior by selecting the doping concentration and

the thickness of the p+ emitter of the diode. The vari ations in each of these technological parameters were accompanied by the choice of lifetime at a high injec tion level in the base in order to provide a constant voltage drop across the turnedon diode at a given cur rent density. Based on the simulation results, the reverse recovery time and the amplitude and rate of decay of the reverse current were found. It is pertinent to note that the effect of the above parameters of the device structure on the values of static and transient heat losses, which limit the allowable operating switching frequency of a power module, was not stud ied in [2]. In addition, the static breakdown voltage of the diode studied in the work was 1200 V. It is of cur rent concern to increase the operating voltage of fast recovery diodes taking into account the fundamental limitations due to avalanche breakdown [3]. NUMERICAL SIMULATION OF RECOVERY PROCESS According to the principle of operation of a PWM voltage inverter, the instantaneous value of load cur rent achieves the peak value +I0 when the ratio of pulses that drive switches 1 and 3 is 2. In this case, the diodes of modules 2 and 4 conduct forward current during the time Tk/2; after that, they recover in the reversebias mode with turning on switches 1 and 3. The value of Tk is low compared to T/2, e.g., for high voltage power switches, the operating frequency fk = 1 kHz, and the frequency of output voltage is 50 Hz. In turn, the reverse recovery of a diode usually occurs within a few microseconds, which is much less than Tk/2. In the studied period Tk, the load current change is negligible compared to I0. Therefore, to analyze the transient phenomenon in this PWM period, the inductive load can be replaced with a dc supply I0. The case Il ≈ I0 is the most critical for a diode. This case should be analyzed to determine the effect of the design parameters of the device on its static and dynamic characteristics and to reveal the limitations of operating voltage and load current of the inverter. To solve the formulated problem, we can pass from the original circuit of Fig. 1a to an equivalent circuit with one diode and a power switch (in fact, they enter the different modules, e.g., 2 and 3). Here, the para sitic inductance LI is included; the inductance Ll is replaced with a current supply I0; and the active load resistance is negligible. At time toff = Tk/2, a stationary distribution of injected carrier concentration settles in the diode base. Since the turnon time of fast semi conductor switches of any type (about 10–20 ns) is significantly shorter than other characteristic times in the circuit (ton, toff, and the diode reverse recovery time), a perfect switch, which switches on within 10 ns, is used instead of a particular power device in the equivalent circuit (Fig. 1b). Below, we shall use the example of a fast silicon diode designed for a limiting blocking voltage of 3500 V.

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To prepare a diode with this breakdown voltage, the n base must be formed of a material with a resistivity ρ ≈ 150 Ω cm, i.e., with a donor impurity concentration Nd ≈ 3.3 × 1013 cm–3. The thickness of the base w0 usu ally must be no less than the thickness of the space charge region at a maximum voltage; for this diode, we selected w0 = 350 μm. The cathode emitter, which is formed by thermal diffusion of a donor impurity, has a thickness of 3 μm with a surface impurity concentra tion n+ = 1 × 1020 cm–3. The p'–n junction depth d is 6 μm. The other parameters of the diode structure were selected in the following ranges: ⎯surface concentration p' of 5 × 1015 to 1 × 1019 cm–3; ⎯lifetime in the base at a high injection level τh = τn0 + τp0 = 4–15 μs. The static and dynamic characteristics of the diode were calculated by the numerical simulation of the on state and transient recovery of the diode using SILVACO ATLAS 5.14.0.R software [4]. This software package makes it possible to simulate manufacturing processes and the modes of operation of semiconductor devices based on various materials. The model used in the pro gram is based on the drift–diffusion approximation, which includes continuity equations for concentra tions of electrons and holes supplemented with the Poisson equation for electric field. All electrophysical processes in the material are taken into account, including the Shockley–Read recombination (with concentrationdependent lifetimes), Auger recombi nation, impact ionization in an electric field, and non linear dependences of carrier mobility on field and on dopant concentration. The simulation process involves the solution of a twodimensional problem for the real geometry and semiconductor structure of the device in combination with the equation for external circuits. At first, the stationary problem was solved as fol lows: in the initial state, the switch is turned on and the forward current I0 flows through the diode, which leads to the filling of the nbase of the diode with elec tron–hole plasma. The switching on of the power switch was simulated by a change in its resistance from 50 MΩ to 5 mΩ for 10 ns without delay. The operating temperature of the diode crystal T = 125°C. The parasitic inductance LI was assumed to be 5 μH (according to the known data [5], in most applications, the parasitic inductance is no less than 5 μH cm2). The working area of the diode was taken to be 1 cm2. The calculation results are time diagrams of current through the diode and voltage drops on it. These data make it possible to determine the switchoff time of the diode and transient heat losses in the semiconduc tor structure during the switchoff process. Static losses in the turnedon diode were estimated based on the forwardcurrent pulse width of 500 μs, which cor responds to the phase with a maximum load current I ≈ I0 at the operating frequency of power switches fk = 1 kHz. Losses (static Wst, transient Woff, and total WΣ) RUSSIAN ELECTRICAL ENGINEERING

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are given in millijoules per 1 cm2, which is the speci fied working area. In addition, the distributions of electric field and free carrier concentrations in the diode base are calculated for several fixed instants of time. A transient process must end in the settling of the blocking state of the diode. In addition, before the blocking of the source voltage U at the reverse voltage av across the diode Urev < U < U st , a dynamic avalanche breakdown (DAB) of the p'–n junction of the diode can occur due to increasing electric field intensity owning to the high charge density of free holes that are carried away from electron–hole plasma in the base region under reverse bias [3, 5, 6]. To prevent diode failure, it is necessary to limit the operating current I0 and voltage U and to prevent an increase in the electric field intensity near the p'–n junction above the thresh old of collision ionization (about 2 × 105 V/cm). As a rule, in the measurement of the dynamic characteris tics of a diode, as well as in operation, the operating source voltage U must be no higher than 50% of the av static breakdown voltage of the diode U st . RESULTS AND DISCUSSION In a highvoltage diode of standard design p+–n–n+, the surface doping concentrations of both the cathode n+ layer and the anode p+ layer are high (usually 1017 to 1019 cm–3). Therefore, at the stage of forward current flow, in the base of this diode, the distribution of injected carrier concentrations is formed with a mini mum deep in the base. A high accumulated excess charge at a high lifetime (τh = 15–30 μs) provides a low (1.5 V and less) voltage drop across the turnedon diode; however, the duration of the recovery process (Fig. 2, curve 2) is inacceptably long due to high com mutation heat losses (more than 100 mJ). The decrease in the lifetime in the base down to a few microseconds leads to a lower excess charge in the n base of the switchedon diode; however, the pattern of the plasma concentration distribution remains the same. In this case, the current limitation is too fast, which does not meet the requirement of soft recovery and leads to highfrequency voltage fluctuations on the diode and parasitic inductances of the circuit (Fig. 2, curve 1). In highvoltage fast diodes designed for operation in voltage inverters, the technology must provide a special pattern of distribution of injected carrier con centrations with a minimum at the anode boundary of the nbase. Thus, in the process of switching off, a rel atively rapid decrease in the carrier concentration will occur directly near the p+–n junction and, from the side of the cathode, the residual electron–hole plasma will remain unchanged for some time and enable the arrival of holes into the expanding spacecharge region [2]. Thus, it is possible to obtain a tradeoff relation

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Urev, kV 1

2.5

U 2

2.0

0

1.5 –40 U 1.0 I 1

2

–80 0.5

–120 0

0.4

0.8

1.2

1.6

t, µs

2.0

0

Fig. 2. Process of turning off a standard diode with high (15 μs) and decreased (4 μs) lifetimes in the base: (1) τh = 4 μs, Von = 1.54 V, Wst = 19.25 mJ, Woff = 51.85 mJ, and WΣ = 71.10 mJ) and (2) τh = 15 μs, Von = 0.91 V, Wst = 11.38 mJ, Woff = 116.14 mJ, and WΣ = 127.52 mJ.

ship between the recovery time and the current decay rate, i.e., fast yet soft recovery. The required pattern of concentration distribution can be obtained in several ways, in particular, through a decrease in the injection efficiency of the anode emitter or a decrease in the carrier lifetime in the base near the p+–n junction via exposing the structure to a proton beam with the penetration depth being somewhat greater than the junction depth. In this work, we discuss the technological control of the injection efficiency of a p+emitter. An obvious way to decrease the injection efficiency is the decrease in the doping level of the p+(p')layer from the typical values of 1019 to 1016 cm–3 (surface concentrations of acceptors in the preparation of a p'–n junction by the thermal diffusion of acceptor impurity). In this case, the minimum distribution of injected plasma concentration is shifted toward the anode emitter. The characteristics of transient recovery also depend on the lifetime in the base; therefore, it must be selected along with the concentration of doping of the p'layer. The results of calculations for various concentrations of p' and lifetimes τh. As a basic version, we selected a diode with the surface concentration in the p'emitter decreased to 1 × 1016 cm–3 and with a short lifetime τh = τn0 + τp0 = 6 μs (we assume that τn0 =

4 μs and τp0 = 2 μs). Taking into account the estimate of the breakdown voltage of this diode, we set decreased operating voltage U = 1200 V and current I0 = 25 A (current density J0= 25 A/cm2). The transient process was calculated in the range 0 ≤ t ≤ 1.5 μs. The results are shown in Fig. 3 (current and reverse voltage across the diode as a function of time, curve 2) and Fig. 4 (distri bution of concentrations of free holes and electric field in the nbase). The forward current flowing through the diode changes its direction within 100 ns after the arrival (at t = 0) of a driving pulse to turn off the switch. The reverse current is maximal (44.3 A) at t = 344 ns, and the reverse recovery time of the diode is about 500 ns. The maximum field intensity near the p'–n junction Emax = 1.89 × 105 V/cm takes place at t = 425 ns (Fig. 4). This field intensity is close to the threshold of avalanche breakdown of the diode. The presence of the impact ionization current component is shown, in particular through the increase in the concentrations of holes in the base toward the anode boundary (Fig. 4, curves p(x) for t = 350 and 400 ns). The effect of the doping level of the p'emitter on the turnoff duration, as well as static and dynamic

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623 Urev, kV

3

2

5

4

20

2.0

U

1

1.5

0 1 I

–20

2 3

–40

1.0 4 5

0.5

–60 0

0.2

0.4

0.8

0.6

1.0

1.2

t, µs

0

Fig. 3. Process of turning off diodes with different doping of the anode emitter: (1) p' = 5 × 1015 cm–3, Von = 2.29 V, Wst = 28.63 mJ, Woff = 18.86 mJ, and WΣ = 47.49 mJ; (2) p' = 1 × 1016 cm–3, Von = 2.07 V, Wst = 25.88 mJ, Woff = 21.10 mJ, and WΣ = 46.98 mJ; (3) p' = 2 × 1016 cm–3, Von = 1.87 V, Wst = 23.38 mJ, Woff = 24.20 mJ, and WΣ = 47.58 mJ; (4) p' = 5 × 1016 cm–3, Von = 1.63 V, Wst = 20.37 mJ, Woff = 30.36 mJ, and WΣ = 50.73 mJ; and (5) p' = 1 × 1017 cm–3, Von = 1.50 V, Wst = 18.75 mJ, Woff = 36.92 mJ, and WΣ = 55.67 mJ.

losses, is illustrated by time diagrams of the transient process (Fig. 3). We searched through the following values of surface concentration of p': 5 × 1015, 1 × 1016, 2 × 1016, 5 × 1016, and 1 × 1017 cm–3 so that, in all cases, the p'–n junction depth was 6 μm. The lifetime at a high injection level is 6 μs. In all these versions, the turnon current I0 = 25 A and the forward current decay rate (it is governed by the parasitic inductance value rather than by the device structure parameters) are the same. The highest transient losses (36.9 mJ, Fig. 3) and turnoff time are found in the diode with the doping level p' = 1 × 1017 cm–3. At the same time, in this case, the forward voltage drop is minimal; the static losses are equal to onehalf of the dynamic losses. The minimum doping of the emitter of 5 × 1015 cm–3 yields the fastest recovery of the diode at minimal transient losses (18.9 mJ). In addition, in this case, due the decreased concentration of injected carriers near the anode, the turnoff process occurs at a lower reverse current and hence at a lower field intensity Emax = 1.85 × 105 V/cm. Finally, the total heat losses are mini mal (about 47 mJ) for the diode with an emitter doping of 1 × 1016 cm–3 and almost the same for 5 × 1015 cm–3. The effect of lifetime τh on the characteristics of the diodes is shown in Fig. 5. We studied the processes of RUSSIAN ELECTRICAL ENGINEERING

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recovery of the diodes with the same injection efficien cies of emitters (i.e., at one value p' = 5 × 1015 cm–3) yet with the different lifetimes in the base (4, 6, 9, and 12 μs). An increase in τh leads to the following changes in the diode characteristics: ⎯an increase in the recovery time of the diode approximately from 400 to 650 ns; ⎯an increase in the dynamic losses from 12.9 to 39.0 mJ; ⎯a decrease in the static losses from 40.6 to 18.9 mJ. A decrease in the lifetime even to 4 μs leads to the residual voltage Von = 3.25 V, i.e., the static losses Wst ≈ 41 mJ > 3Woff. The calculations for long lifetimes show an appreciable decrease in Wst and increase in Woff (by a factor of 2.2 and 3, respectively, in the tran sition from τh = 4 μs to τh = 12 μs). The recovery time also increases (650–700 ns for the longest lifetime). The total value of heat losses is minimal for the device with τh = 6 μs. Thus, the lowest total heat losses are achieved at a doping level of the anode emitter of 5 × 1015 to 1 × 1016 cm–3 and a lifetime of 6 μs. In this case, the version with the minimal emitter doping is more advantageous, because, at a close value of total

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A n

n0

p'

(a)

p, cm–3

1016 0 ns

1015

7 6 5

4

3

2

1

1014

1013

0 E, 105 V/cm

50

100

150

(b)

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300

x, µm w0

2.0 Emax – 1.89 × 105 V/cm 1.8 1.6 1.4 1.2 1.0 3

0.8

2

1

4 0.6 5 0.4

6

0.2 0

7 50

100

150

200

250

300

x, µm w0

Fig. 4. Distributions of concentrations of (a) holes and (b) electric field in nbase at fixed instants of time for a diode with the parameters p = 1 × 1016 cm–3 and τh = 6 μs (diode structure is shown schematically at the top): (a) (1) 200, (2) 300, (3) 350, (4) 400, (5) 450, (6) 500, and (7) 550 ns) and (b) (1) 300, (2) 350, (3) 400, (4) 425, (5) 475, (6) 500, and (7) 550 ns. RUSSIAN ELECTRICAL ENGINEERING

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625 Urev, kV

2 3

1

4

20

2.0 U

0

1.5 1

2

–20

1.0 3

I

4 –40

0.5

–60 0

0.2

0.4

0.6

1.0

0.8

1.2

t, µs

0

Fig. 5. Process of turning off diodes with different lifetimes in the base: (1) τh = 4 μs, Von = 3.25 V, Wst = 40.63 mJ, Woff = 12.91 mJ, and WΣ = 53.54 mJ; (2) τh = 6 μs, Von = 2.29 V, Wst = 28.63 mJ, Woff = 18.86 mJ, and WΣ = 47.49 mJ; (3) τh = 9 μs, Von = 1.75 V, Wst = 21.88 mJ, Woff = 28.44 mJ, and WΣ = 50.32 mJ; and (4) τh = 12 μs, Von = 1.51 V, Wst = 18.87 mJ, Woff = 39.02 mJ, and WΣ = 57.89 mJ.

losses (47.5 mJ), it is characterized by the shortest recovery time and the lowest reverse current. It is per tinent to note that, in the version with the minimum injection efficiency of the emitter, the total losses increase with increasing τh > 6 μs; at τh ≥ 9 μs, the con tribution of dynamic losses becomes dominant (Fig. 5). In a diode of standard design with a high emitter doping, the total losses (Fig. 2) significantly exceed the values obtained for diodes with a decreased doping level and optimized lifetime. CONCLUSIONS (1) Using a fullscale numerical simulation, we studied the isothermal static and dynamic characteris tics of a fast diode designed for operating in conjunc tion with a powerful switching type device (an inte grated thyristor or an insulatedgate bipolar transistor) in a voltage inverter. (2) Based on given values of operating voltage and current taking into account the estimated value of par asitic inductance in a real circuit of 5 μH, we carried out the simulation of transient processes in the circuit operation phase that is most critical for a diode. RUSSIAN ELECTRICAL ENGINEERING

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(3) Quantitative data are obtained to characterize the dependences of characteristics of a diode (forward voltage drop, static heat losses, reverse recovery time, and turnoff transient losses) on the technological parameters of its structure (p'emitter doping and life time in the base at a high injection level). (4) It is shown that a diode with a high doping of anode and cathode emitters cannot be used in a PWM voltage inverter circuit due to the long recovery time (for long lifetimes) and an abrupt break of current (for short lifetimes). (5) It is proven that the injected plasma concentra tion near the anode must be decreased, e.g., by decreasing the injection efficiency of the anode emit ter (the doping level of the p'layer near the structure surface must be 5 × 1015 to 1 × 1016 cm–3). To provide a soft recovery process, the fastest operation, and mini mum total losses, the lifetime for a high level of injec tion in the base can be selected in a range of 6–9 μs. The maximum allowable operating voltage of a circuit with this diode is limited by the effect of breakdown under reverse recovery conditions. At a source voltage of 1200 V, the maximum field in the plane of the p–n junc tion is approximately 1.9 × 105 V/cm; i.e., it is fairly close to the threshold value for incipient dynamic

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breakdown. In the presence of any technological het erogeneities of the device structure, even the initial stage of breakdown can be accompanied by current localization and lead to the device failure. ACKNOWLEDGMENTS This work was supported by the Federal Agency for Science and Innovations (state contract no. 02.526.12.6016) under the Federal Program “Research and Development in Priority Directions of the ScientificTechnological Complex for 2007– 2012” and in part by the Russian Foundation for Basic Research (project no. 070800689). REFERENCES 1. Grekhov, I.V., Kostina, L.S., Rozhkov, A.V., et al., Research of Statistical Characteristics and Features of Switching Process of Integral Thyristor with External Field Control, Zh. Tekhn. Fiz., 2008, vol. 78, issue 12, pp. 78–84.

2. Rahimo, M.T. and Shammas, N.Y.A., Optimization of the Reverse Recovery Behavior of Fast Power Diodes Using Injection Efficiency and Lifetime Control Tech niques, Proc. 7th Europ. Conf. on Power Electronics and Applications (EPE’97), Trondheim, Sept. 1997, pp. 2.99–2.104. 3. Gorbatyuk, A.V., Grekhov, I.V., and Gusin, D.V., Bipolar Switchers with Distributed Microshutters. Conditions for Entering into Dynamical Fault under Turn up, Zh. Tekhn. Fiz., 2009, vol. 79, issue 10, pp. 80–88. 4. ATLAS. User’s Manual, Santa Clara: SILVACO Inc., 2010. 5. Ogura, T., Ninomiya, H., Sugiyama, K., and Inoue, T., Turn–off Switching Analysis Considering Dynamic Avalanche Effect for Low Turn–off Loss High–Voltage IGBTs, IEEE Trans. Electron. Devices, 2004, vol. 51, no. 4 pp. 629–635. 6. Domeij, M., Lutz, J., and Silber, D., On the Destruc tion Limit of Si Power Diodes during Reverse Recovery with Dynamic Avalanche, IEEE Trans. Electron. Devices, 2003, vol. 50, no. 2, pp. 486–493.

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