Technical Information

Semenov A. -  PROTON–ELECTROTEX
Stolbunov V. - THE INSTITUTE OF THEORETICAL AND EXPERIMENTAL PHYSICS
Surma A. - ALL-RUSSIAN ELECTROTECHNICAL INSTITUTE
Kovrov A. - ESTEL ELEKTROONICA OU

ABSTRACT
The new series of fast thyristors (current ratings range from 400A to 1250A, voltage ratings range from 300 to 3400V, turn-off time ratings range from 5 to 100 μs) with the guaranteed control of reverse recovery charge is submitted. Proton irradiation technology is applied to the control of the switching characteristics. The technological complex of proton irradiation allows to realize mass production of the mentioned thyristors as well as other high-voltage and fast devices including soft recovery diodes, IGBTs etc.

INTRODUCTION
The allowable current of a fast thyristor operating on a frequency higher than 1 kHz with the reverse blocking strongly depends on the energy loss during the reverse recovery. It is necessary to minimize the reverse recovery charge (Q_rr) for the reduction of this energy loss. The optimization of the relationship between the on-state voltage (V_TM), the turn-off time (t_q) and the reverse recovery charge can be reached by using non-uniform axial profile of lifetime (τ) in thyristor structure [1]. For this purpose the lifetime value close to the collector p-n junction should be higher, than close to the anode p-n junction, as shown in Fig.1. Proton irradiation is a suitable technology for the obtaining of such axial lifetime profile [2, 3].

Fig. 1. Typical axial lifetime profile in a silicon structure of thyristor after proton irradiation.

TECHNOLOGICAL COMPLEX OF PROTON IRRADIATIONIn collaboration with the Institute of Theoretical and Experimental Physics and All-Russian Electrotechnical Institute, PROTON-ELECTROTEX has developed a low-cost industrial technology for proton irradiation of semiconductor devices shown in Fig.2. The basis of the technological complex is a 24 MeV linear proton accelerator. The proton beam of this accelerator has the following general characteristics:- Peak beam current: up to 200 mA
- Average beam current: up to 5 μA
- Duration of current pulse: 2…30 μs
- Dose of protons, delivered into the working zone by one irradiation pulse: 1E9…5E11 proton/cm².

Thus the technological complex contains the box for placing cartridges with semiconductor structures before and after irradiation (4), the mechanical system of moving and positioning the irradiating structures (6), equipment for the control of irradiation dose and proton beam characteristics (7, 9) and mobile aluminium screens for control of proton path length in a semiconductor structure (8).
The special screen for the beam dissipation (11) in aggregate with the mechanical system of moving and positioning the irradiating structures ensure the irradiation of the wafer with diameter up to 125 mm.
The technological complex gives the following possibilities.
1. Continuous irradiation of the large device lots. It is possible to irradiate respectively up to 270 semiconductor structures with diameter of 95…105 mm, or up to 360 structures with diameter of 75…80 mm, or up to 450 structures with diameter of 40…60 mm, or up to 900 structures with diameter of 24…32 mm in a work cycle.
2. A short period of processing time. The duration of one work cycle is 4…5 hours, including the post-irradiating storage time necessary for reducing the radioactivity in semiconductor structures and technological cartridges up to the safe level.
3. The irradiation occurs in air environment, the vacuum is not required in the work zone.
4. Control of proton beam characteristics and irradiation dose. It is possible to control the distribution of current density and energy spectrum of protons within the working zone. These measurements are carried out by means of the mosaic current receiver (7) and system of mobile screens (8) at the testing of proton beam before a work cycle. During a work cycle the routine control of irradiation dose by means of the beam current receivers (9) is carried out.
5. Remote control the system of mobile screens (8) to alter proton path length in semiconductor layers of irradiating structures. The control of proton path length in semiconductor structure is achieved by change of the summary thickness of screens, through which proton beam penetrates before reaching the semiconductor surface. The proton path length in a silicon structure can be altered within 0…1200 μm with a step of 20 μm.
6. High level of radiation safety.

In the presented irradiating technology we use partially dissipated proton beam. The distribution of proton path length dR_p/dX in the semiconductor structure correlating with the spectrum of proton energy can be rather precisely described in this case by Gaussian distribution:
where X is the distance from surface of semiconductor structure, R_p^med is the median of proton path length distribution, σR_p is the standard deviation of proton path length.
The axial profile of lifetime damage factor K_τ in the semiconductor structure for this case has the characteristic form shown in Fig. 3.

Accordingly, the axial lifetime profile in semiconductor structure has the so-called extended layer of high recombination [4] with thickness about 2σR_p and with minimum lifetime in region of maximal K_τ values. For the presented technology the value of σR_p is about 75 μm, therefore the thickness of extended high recombination layer is about 150 μm (see Fig. 3).The experimental results show that the use of such lifetime profile is effective for minimizing the switching losses in silicon bipolar devices having the thickness of low-doped layer more than 100 μm. The blocking voltage of such devices usually exceeds 1000 V.Hence mass production of high-voltage and fast silicon devices, such as thyristors, soft recovery diodes, IGBTs etc. is possible through the presented technology.
THE NEW SERIES OF FAST THYRISTORS WITH CONTROLLABLE REVERSE RECOVERY CHARGE

The applying of above described technology has allowed putting into production the new series of fast thyristors with reduced reverse recovery charge. Such devices hold a number of the following key features.1. Lifetime control due to proton irradiation of the cathode side of thyristor structure. The thickness of preliminary aluminium screens is chosen so, that the region of proton path termination in silicon structure is located close to the anode p-n junction (see Fig.1). The lifetime close to the anode p-n junction (τ_a) can be in this case 2x to 3x less, than lifetime close to the collector p-n junction (τ_c). Such axial lifetime profile allows optimization of the relationship between V_TM and Q_rr: the 1.5x to 2x reduction of the Q_rr value at the same V_TM value is possible when using this axial profile instead of traditional uniform profile.2. The dense grid of cathode short elements. This cathode shorts are distributed within the emitter area, the next elements are located at the distance about 400 μm. Such a cathode short grid allows to obtain quite short turn-off time at rather large lifetime close to the collector p-n junction: the relation t_q/τ_c = 2÷3.3. Distributed amplifying gate (Fig. 4). The distributed gate together with rather high values of lifetime close to the collector p-n junction and in p base provide fast turn-on of all the thyristor area, reduce turn-on energy loss, increase repetitive di/dt-rate and operating frequency.


Fig. 4. Silicon structures of thyristors forming the new series
Table 1

The new series of fast thyristors is a result of joint work of PROTON-ELECTROTEX and ESTEL Elektroonika OU. The series contains 16 types of fast thyristors with symmetrical blocking characteristics in press-pack cases. Silicon structures of thyristors have diameters 32, 40, 56 mm (see Fig. 4). Average current ranges comprise the values from 400A to 1250A, blocking voltage ranges – up to 1000 V, up to 1500V, up to 2200V and up to 3400V.

The control of Q_rr and tq values is ensured for all thyristors. Maximum rating of Q_rr can be chosen from the values 100, 150, 200, 250, 300, 350, 400, 500, 600, 700, 800, 1000 μC and maximum rating of t_q - from the values 5, 6.3, 8, 10, 12.5, 16, 20, 25, 32, 40, 50, 63, 80, 100 μs. The relationship between allowable ranges of Q_rr and tq, blocking voltage (V_DRM, V_RRM), average current (I_TAV) and other parameters and characteristics of new thyristors are presented in Table 1.

Owing to the reduced Qrr and tq values, new thyristors can operate consequently in the frequency band up to 30kHz for 1000…1500V blocking voltage range, up to 10kHz for 2200V blocking voltage range and of 2…5 kHz for 3400V blocking voltage range. The topology of thyristor structures is adapted for high frequencies. New devices can reliably operate at repetitive di/dt’s of 800…1250 A/μs.Currently the developments are carried out connected with the thyristor structure diameter of 80 mm. In the nearest future 2 types of such thyristors with current ratings range from 1600A to 2000A, blocking voltage ratings range from 1500V to 2000V and turn-off time from 16 μs to 25 μs will be presented.
REFERENCES

[1] Bosterling W., Ehinger H., Sommer K. Einund Ausschaltverhalten optimiert – Elektrotechnik, 1981, 63, H.21, 6, S.16–23.[2] Sawko D.S., Bartko J. Production of fast switshing power thyristors by proton irradiation. – IEEE Trans. Nucl. Sci., 1983, V. N9-30, N 2, pp. 1756-1758.[3] Prikhodko A., Surma A. Proton irradiated 6kV GTO with full pressure contacts. - Conf. Proc. of EPE'97, Trondheim, 1997, pp.1.507-1.512.[4] Potaptchouk V.A. 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.
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