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Posted on 25 June 2019

# Freewheeling Diodes for High Performance Inverter Systems

The engineers at Proton-Electrotex developed freewheeling diodes to operate at the reverse recovery process with high change rate of reverse recovery current (dIrrm/dt), and high power supply reverse voltage (VR) by means of proton irradiation technology, sintering technology and implementation of n’-buffer. The diodes are designed to be used as reverse diodes for high voltage stacks assembled using IGBT and IGCT.

By A.A. Pisarev, A.M. Surma, A.A. Chernikov, Proton-Electrotex JSC, Russia

Complex circuits of multilevel voltage inverters assembled using power press-pack IGCT and IGBT require corresponding complementary diodes. Conventional fast recovery diodes have dynamic limitations at operating in similar circuits. They decrease inverter efficiency and reliability making cost higher.

To increase inverters efficiency by means of lowering power losses of IGBT at turn-on it is necessary to execute its commutation at high rate of rise of on-state current (dI/dt), which for modern IGBT reaches 10 000 A/μs. However complementary diodes are required to have same change rate of reverse recovery current (dIrrm/dt), which for conventional fast recovery diodes equals 500 A/μs.

That is why to get all the advantages of the modern IGBTs and IGCTs, which ensure increased inverters efficiency, it is necessary for the complementary diodes to have the following requirements:

• increased change rate of reverse recovery current (dIrrm/dt);
• low reverse recovery current (Irrm);
• improved reverse recovery softness (s);
• possibility to operate without snubber RC circuits;
• resistivity to snappy effect and dynamic avalanche breakdown;
• high operating temperature.

During the process of reverse recovery of the diodes in such modes there can occur the following problems:

• snappy-effect of the excess electrons and holes charge in n-base of the diode during reverse recovery process. During the second phase of the reverse recovery diode has a very high density of reverse current in the n-base, which is being supported by means of recombination of residual charge of excess electrons and holes accumulated in the n-base earlier during direct current flow. Since voltage is very high, then rate of rise of reverse voltage in the second phase is high as well, that is why shifting process of the excess holes is accompanied by rapid increase of the thickness of space charge region of р-n-junction. Space charge region boarder moves inside the base, the holes are being captured by the field of р-n-junction and shifted into p+-emitter. For conventional diodes, as a rule, a similar process of appearance and expansion of high field domain adjacent to n+-layer is carried out, and this pseudospace charge region by expanding captures the electrons out of the n-base. If both these areas contact a snappy effect of excess electrons and holes occurs in the n-base, which is being accompanied by extremely rapid reverse recovery current drop what leads to voltage outburst, which, as a rule, exceeds avalanche breakdown voltage of the diode. Even if the diode has resistivity to avalanche breakdown and is still operating, such mode leads to appearance of extremely high frequency electromagnetic noise, which is not acceptable for the majority of the modern equipment, because it generates the conditions for failure of drivers;
• dynamic avalanche breakdown. In the second phase of recovery through space charge region of p-n-junction there is a high hole current flow. Holes transmission speed is limited and equals about 0.8*107 cm/s that is why holes current flow is accompanied by generation of additional positive charge in space charge region (mobile holes). For example if current density equals 30 A/cm2 (i.e. current 600A for the diode with diameter of silicon element 56mm and 1200A for the diode with diameter of silicon element 80mm) additional concentration will be 2,34*1013 cm-3, which is similar to concentration of ionized donors for the diode on silicon wafer with electrical resistivity 200 Ohm*cm. Presence of this moving additional charge in space charge region of p-n-junction leads to the decrease in avalanche breakdown voltage. The problem is not that there is a physical process of avalanche in p-n-junction, but that in this mode the large area diode is biased to reverse recovery current filamentation, which leads to its failure.

That is why this is a relevant objective to develop high voltage soft recovery diodes with expanded safe operating area during reverse recovery adapted to operate with high power IGCT and IGBT.

It is well-known that [1, 2] there is a novel technology, which allows developing soft fast recovery diodes that are resistant to snappy effect and applicable for operation in wide range of reverse voltage values (VR) and change rate of reverse recovery current (dIrrm/dt). It is fabrication of axial inhomogeneous distribution of carrier lifetime (τ) in semiconductor element. For conventional diodes with p-n-n+-element it is necessary to lower lifetime value in lightly doped layers close to p-n-junction and increase in the area of n-layer adjacent to n+-layer.

Despite the huge number of successful applications of such approach to produce soft high voltage diodes the question of detailed type of ideal axial inhomogeneous distribution of carrier lifetime (τ) is still open [3 - 6]. There are two main reasons for that:

1. Technical level of high power fast recovery diode can be characterized by repetitive peak reverse voltage (VRRM), voltage forward drop (VFM), reverse recovery charge (Qrr), reverse recovery energy losses (ERQ), reverse recovery current (Irrm), reverse recovery softness (s). If VRRM and VFM are fixed as some constant values then the technical level of fast recovery diode will be the higher the less Qrr, ERQ, Irrm and the higher s. Experience has shown that type of axial inhomogeneous distribution of carrier lifetime (τ), ideal from the minimization of Qrr and ERQ point of view, differs from axial inhomogeneous distribution of carrier lifetime (τ) ideal from minimum Irrm point of view and maximum s. For example, fabrication of axial inhomogeneous distribution of carrier lifetime (τ) of step-like type with very low value of τ in p-n-junction area and very high value of τ in the remaining area of n-layer allows having high softness (s) and low values of reverse recovery current (Irrm), however Qrr and ERQ values are far from ideal due to the presence of long tail of reverse recovery current in the process of reverse recovery. In such a way, form of ideal distribution of carrier lifetime (τ) is determined by certain requirements set for combination of Qrr, ERQ, Irrm and s of power diode.

2. Limitations of usage of one of the fabrication technologies of axial inhomogeneous distribution of carrier lifetime (τ). To have an inhomogeneous distribution of carrier lifetime (τ) atoms diffusion of noble metals (gold, platinum) is usually used, or irradiation with light high energy ions (hydrogen, helium etc.). In case of usage of atoms doping of noble metals possibilities of carrier lifetime distribution form variations are limited. Irradiation with light ions has more options to change this distribution. It is known that at the end of light ion path in semiconductor a local area with extremely low carrier lifetime appears. Therefore varying ion energy spectrum during irradiation it might seem easy to have carrier lifetime distribution of any form. However, as proven by detailed investigations [7, 8], space distribution of carrier lifetime (τ) generated during irradiation with mono-energetic ions significantly differs from δ–function or narrow Gauss distribution as a result of radiation damage of smaller layers of the semiconductor than ion path depth as well as due to diffusion of ion energy spectrum during passing through semiconductor layers. As a result, these effects set certain limits on possibilities to fabrication required distributions of carrier lifetime (τ).

Further the investigation results of characteristics combination optimization of VFM, Qrr, ERQ, IrrM, S of high voltage fast recovery diodes by means proton irradiation technology for fabrication of inhomogeneous distribution of carrier lifetime are being described [8].

### Design and Process Features of High Power Freewheeling Diodes

High power freewheeling diodes produced by Proton-Electrotex are designed for current from several hundred up to several thousand amperes and voltage from 1000 up to 4500 volt. A round semiconductor element has a diameter from 24 up to 100 mm for different types of the diodes.

To ensure electric thermal cycling resistivity of the large area diodes the following features were taken:

• semiconductor element is joined with molybdenum disc;
• semiconductor element connected with molybdenum disc is encapsulated in press-pack metal-ceramic case.

The described design allows simultaneously ensure high electric thermal cycling resistivity, low thermal resistance, high energy ratio to accumulate energy loss in pulse and emergency modes.

Adjustment of Qrr, ERQ, IrrM, S characteristics is done with the help of proton irradiation. There was used a proton beam with initial energy of 24 MeV. The beam was streamed in the air and with the help of special baffles was partially dissipated to generate the working area enough for irradiation of large area semiconductor elements. Final adjustment of the proton path depth in the semiconductor element was done with the help of plug-in aluminum baffles with various thickness. The described technology allows irradiating of semiconductor elements with diameter up to 125 mm at a depth of 1000 μm with 15 μm adjustment interval for the proton path depth. In the figure 1 there is a diagram of typical distribution of carrier lifetime after proton irradiation.

Conventional production technology means joining the semiconductor element with the molybdenum disc by means of alloying – high temperature vacuum soldering using Al-Si-discs. To get a reliable ohmic contact it is necessary to join molybdenum disc with the surface of p-type connectivity (anode surface), because this connection partially dissipate the surface layer of silicon with consecutive generation of p+-type connectivity after cooling of the recrystallized surface layer.

After joining high thickness of the molybdenum disc do not let irradiation process to be done from the side of anode surface. Proton irradiation of the semiconductor element before joining with molybdenum disc is impossible as well, since the joining process is undergoing with high temperature of about 700°С.

In such a way the diodes with conventional design can be irradiated with the protons only from the side of cathode surface, which is quite a serious limitation from the point of possibility of optimization carrier lifetime distribution form. During fabrication the layer with decreased carrier lifetime value near the p-n-junction the protons have to preliminary go through all n-type conductivity layers, i.e. through the major part of the semiconductor element. After that there is quite a smooth carrier lifetime distribution and its values near the p-n-junction cannot be decreased lower than 0,2-0,3 of the value close to the n+-layer border. In the figure 2(a) there is a typical carrier lifetime distribution during proton irradiation through the cathode surface of the semiconductor element.

In order to expand variations of carrier lifetime distribution form the technology of joining semiconductor element with molybdenum disc was changed using silver paste [9]. The joining process using this technology is carried out at temperature about 250°С, which allows proton irradiation before joining with molybdenum disc, as well from the anode side of the semiconductor element surface. Furthermore, the protons do not get into the layers of the semiconductor element close to the cathode surface, that is why generation of carrier lifetime with higher asymmetry comparing to the conventional semiconductor elements technology. In the figure 2(b) there is a typical carrier lifetime distribution during proton irradiation through the anode surface of the semiconductor element.

### Experimental Samples

To evaluate the offered constrictive technological solutions, which are planned to be used for mass production, the experimental diode samples were produced with semiconductor element 56 mm in diameter designed for repetitive reverse voltage 4500 V and average current 800 A. The diodes were produced with the wafers of neutron transmutation doped silicon with electrical resistivity 250 Ohm*cm and thickness 640 μm.

1. The diodes with conventional design were produced using technology of alloying the silicon wafer with molybdenum disc. The proton irradiation was done after alloying from the side of cathode surface. The proton irradiation was done with two various doses in order to ensure the most complete base to compare with the experimental diodes. The carrier lifetime distribution form (τ) in the semiconductor element corresponded to the figure 2(a).

2. The experimental diodes were produced using proton irradiation from the side of anode surface, after which the joining of silicon wafer with molybdenum disc was done by means of sintering technology using silver paste. The carrier lifetime distribution form (τ) in the semiconductor element corresponded to the figure 2(b). Experimental Results The obtained electrical characteristics of the diodes are shown in the table 1.

Analyzing these characteristics it is, first of all, necessary to note that the experimental diodes have significantly lower values of Irrm and Qrr (second phase of reverse recovery – reverse current drop – chord approximation through the points 0,9*Irrm and 0,25*Irrm). In the figures 3(a) and 3(b) correlation of VFM/Irrm and VFM/Qrr for the experimental diodes are shown in comparison with the diodes produced using conventional technology.

However, correlation of VFM and integral reverse recovery charge (Qrr-i) for both groups of the diodes is almost the same, as shown in the figure 4.

In the figure 5 there is a circuit of experimental equipment, which was used to study the reverse recovery characteristics at high change rate of reverse recovery current (dIrrm/dt) and high power supply reverse voltage (VR).

Direct current pulse was formed during discharge of oscillating circuit L1-C1, commuted by the thyristor VS1. The current pulse with adjustable amplitude (0 – 2000 A) and 1ms period is being applied to the tested diode through the pulse transformer. At the moment when direct current reaches its maximum thyristor VS2 commutes the charge in low inductance contour C2-Ls (Ls ~ 0.5 μH), which ensures reverse recovery of the tested diode. Reverse recovery of the tested diode is done in snubberless mode, reverse voltage (VRDС) was adjusted in range of 500 – 3000 V. As a result, changing VRDС it was possible to adjust the change rate of reverse recovery current in range of 1000 – 5000 A/μs.

Typical oscillograph records of anode current and voltage during reverse recovery produced using conventional technology at various reverse voltage values VRDC are shown in the figure 6(a).

It is worth mentioning that the experimental diodes were stable and fail-safe operating till the reverse voltage value VRDC ~ 3200 V. Reverse recovery softness value was over 1 (figure 7(b)).

Moreover, total reverse voltage value (Vrrm) at all modes of reverse recovery was not exceeding 4000 V (figure 8), i.e. the experimental diodes are fit for operation in snubberless circuits in complementary pair with IGCT and IGBT calculated for the repetitive pulse voltage of 4500 V.

There are the formulas 1,2 and 3, which were used to calculate the reverse recovery softness values:

Conclusion

Based on the above presented design and process approaches and made investigations the range of the freewheeling diodes was developed adapted to operate in complementary pair with the commercial IGBT and IGCT. Main electrical and thermal characteristics of such diodes are shown in the table 2.

Such diodes allow getting new advantages to engineering of inverters for induction heating systems and motor drives systems due to compliance with all necessary requirements for reliable operation with the modern presspack IGCT and IGBT:

• increased change rate of reverse recovery current (dIrrm/dt);
• low reverse recovery current (Irrm);
• improved reverse recovery softness (over 1);
• possibility to operate without snubber RC circuits;
• resistivity to snappy effect and dynamic avalanche breakdown;

References
1. V. Benda. Design Considerations for Fast Soft Reverse Recovery Diodes.- 5th EPE Conference Brighton, U.K. Sept. 1993 pp 288-292.
2. M. T. Rahimo, N. A. Shammas. “Optimization of the Reverse Recovery Behaviour of Fast Power Diodes using Injection Efficiency Techniques and Lifetime Control Techniques. EPE’97, Trondheim, Norway. Sept.-97 pp 2.99 - 2.104
3. B. J. Baliga, Y. S. Sun. Comparison of Gold, Platinum, and Electron Irradiation for Controlling Lifetime in Power Rectifiers. IEEE Trans. on Electron Devices, 1977 pp 685-688.
4. J. Lutz, U. Scheuermann. Advantages of the new Controlled Axial Lifetime Diode. – PCIM’94 Proceedings, 1994, p. 163 – 169.
5. J. Vobecky, P. Hazdra. Future trends in local lifetime control.- ISPSD ‘96 Proceedings, 1996 pp. 161-164.
6. J. Lutz. The Freewheeling diode - No Longer the Weak Component.- PCIM’97, June Proceeding 1997, pp 259 - 265.
7. V.A. Potaptchouk et al. Distinctions of Lifetime Damage in Silicon Diode Layers at Various Radiation Processing: Influence on Power Losses and Softness of Reverse Recovery Characteristic - PCIM’2002 Proceedings, 2002, pp. 293-299.
8. V. N. Gubarev, A. Y. Semenov, A. M. Surma, V. S. Stolbunov. Applying Proton Irradiation for Performance Improvement of Power Semiconductors. - Power Electronics Europe, 2011, Issue 3, p. 35 – 38.
9. A.A. Chernikov, A.V. Stavtsev, A.M. Surma. Features of wafer - molybdenum joining by sintering of silver paste for large area silicon devices. – 15th EPE Conference Lille, France. Sept. 2013 pp 1 – 7.

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