New Generation of High – Power Semiconductor Closing Switches for Pulsed Power Applications
I. Introduction
Solid state semiconductor switches are very inviting to use at pulsed power systems because these switches have high reliability, long lifetime, low costs during using, and environmental safety due to mercury and lead are absent. Semiconductor switches are able to work in any position, so, it is possible to design systems as for stationary laboratory using, and for mobile using. Therefore these switches are frequently regarded as replacement of gas-discharge devices – ignitrons, thyratrons, spark gaps and vacuum switches that generally use now in high-power electrophysical systems including power lasers.
Traditional thyristors (SCR) are semiconductor switches mostly using for pulse devices. SCR has small value of forward voltage drop at switch-on state, it has high overload capacity for current, and at last it has relatively low cost value due to the simple bipolar technology. Disadvantage of SCR is observed at switching of current pulses with very high peak value and short duration. Reason of this disadvantage is sufficiently slow process of switch-on state expansion from triggering electrode to external border of p-n junction after triggering pulse applying. This SCR feature is defined SCR using into millisecond range of current switching. Improvement of SCR pulse characteristics can be reached by using of the distributed gate design. This is allowed to decrease the time of total switch-on and greatly improve SCR switching capacity. Thus, ABB company is expanded the semiconductor switch using up to microsecond range by design of special pulse asymmetric thyristors (ASCR). These devices have distributing gate structure like a GTO. This thyristor design and forced triggering mode are obtained the high switching capacity of thyristor (=150kA, =50μs, di/dt = 18kA/μs, single pulse). However, in this design gate structure is covered large active area of thyristor (more than 50%) that decrease the efficiency of Si using and increase cost of device.
Si-thyristors and IGBT have demonstrated high switching characteristics at repetitive mode. However, such devices do not intend for switching of high pulse currents (tens of kiloamperes and more) because of well-known physical limits are existed such as low doping of emitters, short lifetime of minority carriers, small sizes of chips etc.
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Our investigation have obtained that switches based on reverse  – switched dinistors are more perspective solid-state switches to switch super high powers at microsecond and submillisecond ranges. Reverse – switched dinistors (RSD) is two-electrode analogue of reverse conducting thyristor with monolithical integrated freewheeling diode in Si. This diode is connected in parallel and in the back direction to the thyristor part of RSD. Triggering of RSD is provided by short pulse of trigger current at brief applying of reversal voltage to RSD. Design of RSD is made thus that triggering current passes through diode areas of RSD quasiaxially and uniformly along the Si structure area. This current produces the oncoming injection of charge carriers from both emitter junctions to base regions and initiates the regenerative process of switch-on for RSD thyristor areas. Such method of triggering for this special design of Si plate is provided total and uniform switchi
ng of RSD along all active area in the very short time like as diode switch-on. The freewheeling diode integrated into the RSD structure could be used as damping diode at fault mode in the discharge circuit. This fault mode such as breakdown of cable lines can lead to oscillating current through switch..
It has been experimentally obtained in that semiconductor switches based on RSD can work successfully in the pulsed power systems to drive flash lamps pumping high-power neodymium lasers. It was shown in that RSD-switches based on RSD wafer diameter of 63 mm (switch type KRD-25-100) and RSD-switches based on RSD wafer diameter of 76 mm (switch type KRD-25-180) can switch the current pulses with submillisecond duration and peak value of 120 kA and 180 kA respectively. Three switches (switch type KRD – 25-180) connected in parallel were successfully tested under the following mode: operating voltage = 25 kV, operating current Ip = 470 kA, and transferred charge Q = 145 Coulombs.
During 2000 – 2001, the capacitor bank for neodymium laser of facility LUCH was built at
RFNC-VNIIEF. This bank including 18 switches type KRD-25-100 operates successfully during 5 years without any failures of switches.
This report is submitted results of development of new generation of solid state switches having low losses of power and high-current switching capacity.
II. Development of RSD’s next generation
The technology of fabrication of new RSD structure has been developed to increase the switching capacity. This new structure is SPT (Soft Punch Through)-structure - with “soft” closing of space-charge region into buffer n'-layer.
Decreasing of n-base thickness and also improving of RSD switch-on uniformity by good spreading of charge carriers on the n'-layer at voltage inversion are provided decreasing of all components of losses energy such as losses at triggering, losses at transient process of switch-on, and losses at state-on. Our preliminary estimation was shown that such structure must provide the increasing of operating peak current through RSD approximately in 1.5 times.
Investigations were carried out for RSD with blocking voltage of 2.4 kV and Si waferdiameters of 63, 76, and 100 mm by special test station. The main goal of these investigations is definition of maximum permissible level of peak current passing through single RSD with given area. Current passing through RSD and voltage drop on RSD structure during current passing are measured at testing. In Fig.1 waveforms of peak currents and voltage drops is shown for RSD with size of 76 mm and blocking voltage of 2.4 kV.
Fig.1. Waveforms of pulse current (a) and voltage drop (b) for RSD with wafer size of 76 mm and blocking voltage of 2.4 kV
In according with study program current was slowly increased until maximum permissible level Ipm. When this level was reached the sharp rise of voltage and than the same sharp decay of voltage for curve U(t) was observed. Reason of voltage rise is strong decreasing of carrier mobility at high temperature, and reason of voltage decay is quick modulation of channel conductivity by thermal generated plasma that is appeared in accordance with sharp exponential dependence for own concentration of initial silicon into base areas of RSD at temperature of 400 –C.

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