Inductive switching of 4H-SiC gate turn-off thyristors

The high-temperature operation of a silicon carbide gate turn-off thyristor is evaluated for use in inductively loaded switching circuits. Compared to purely resistive load elements, inductive loads subject the switching device to higher internal power dissipation. The ability of silicon carbide components to operate at elevated temperatures and high power dissipations are important factors for their use in future power conversion/control systems. In this work, a maximum current density of 540 A/cm² at 600 V was switched at a frequency of 2 kHz and at several case temperatures up to 150°C. The turn-off and turn-on characteristics of the thyristor are discussed     

High current density 800-V 4H-SiC gate turn-off thyristors

4H-SiC gate turn-off thyristors (GTOs) were fabricated using the recently developed inductively-coupled plasma (ICP) dry etching technique. DC and ac characterisation have been done to evaluate forward blocking voltage, leakage current, on-state voltage drop and switching performance. GTOs over 800 V dc blocking capability has been demonstrated with a blocking layer thickness of 7 μm. The dc on-state voltage drops of a typical device at 25 and 300°C were 4.5 and 3.6 V, respectively, for a current density of 1000 A/cm². The devices can be reliably turned on and turned off under an anode current density of 5000 A/cm² without observable degradation     

700-V asymmetrical 4H-SiC gate turn-off thyristors (GTO's)

Silicon Carbide (4H-SiC), asymmetrical gate turn-off thyristors (GTO's) were fabricated and tested with respect to forward voltage drop (V_F), forward blocking voltage, and turn-off characteristics. Devices were tested from room temperature to 350°C in the dc mode. Forward blocking voltages ranged from 600-800 V at room temperature for the devices tested. V_F of a typical device at 350°C was 4.8 V at a current density of 500 A/cm². Turn-off time was less than 1 μs. Although no beveling or advanced edge termination techniques were used, the blocking voltage represented approximately 50% of the theoretical value when tested in an air ambient. Also, four GTO cells were connected in parallel to demonstrate 600-V, 1.4 A (800 A/cm ²) performance     

Simulated turn-off of 4H-SiC gate turn-off thyristors with gateelectrodes on the p-base or the n-base

Turn-off simulations of a 4H-SiC GTO thyristor structure having a gated p-base and p-type substrate are compared with that having a gated n-base and n-type substrate. Two gate drive circuits are considered, one with a voltage source and resistor between the gate and adjacent emitter region, and the other with a voltage source and resistor between the gate and farthest emitter region. The gated n-base thyristor's substrate current increases atypically before the device turns off. Also, the gated n-base structure turns off when the gate circuit is connected directly to the emitter region furthest from the gate region, but the gated p-base structure does not. Furthermore, turn-off gain is lower for the gated n-base structure due to mobility differences as demonstrated by current-voltage (I-V) and current versus time (I-t) curves     

3100 V, asymmetrical, gate turn-off (GTO) thyristors in 4H-SiC

A 2-mm×2-mm, 4H-SiC, asymmetrical npnp gate turn-off (GTO) thyristor with a blocking voltage of 3100 V and a forward current of 12 A is reported. This is the highest reported power handling capability of 37 kW for a single device in SiC. The 5-epilayer structure utilized a blocking layer that was 50 μm thick, p-type, doped at about 7-9×10¹⁴ cm^-3. The devices were terminated with a single zone junction termination extension (JTE) region formed by ion-implantation of nitrogen at 650°C. The device was able to reliably turn-on and turn-off 20 A (500 A/cm²) of anode current with a turn-on gain (I_K/I_{G, on}) of 20 and a turn-off gain (I_K/I_{G, off}) of 3.3     

Ultrahigh-voltage high-current gate turn-off thyristors

High-power GTO's with ratings of 2500 V . 2000 A have been developed, and a 4500 V . 2000 A GTO was trial fabricated and performance tested, for use in traction motor control equipment. Their low ON-state voltage was attained by applying a unique anode emitter shorting structure which does not require doping of a lifetime killer such as gold to obtain suitable GTO characteristics. Their high interrupt current was obtained by introducing a ring-shaped gate structure which has uniform operation between many segments in the devices during turn-off process.     

Turn-on process in 4H-SiC thyristors

Detailed turn-on measurements of 4H-Silicon Carbide (SiC) npnp thyristors are presented for a wide range of operating conditions. Comparisons with similarly-rated silicon and Gallium Arsenide thyristors show a superior rise time and pulsed turn-on performance of SiC thyristors. Rise time for a 400 V blocking voltage, 4 V forward drop (2.8×10³ A/cm²) SiC thyristor has been found to be of the order of 3-5 ns. Pulsed turn on measurements show a residual voltage of only 50 V when a current density of 10⁵ A/cm² (35 A) was achieved in 20 ns     

Gate turn-off capability of depletion-mode thyristors

Experimental results are described which demonstrate the ability to switch a thyristor from its ON state to its OFF state by using a depletion layer formed by the application of gate bias to a trench-gate MOSFET integrated within the thyristor structure. The maximum controllable current is found to be a function of the gate bias voltage, the trench depth, and the ambient temperature. The maximum controllable current can be increased by increasing the trench depth and decreasing the p-base sheet resistance. The maximum controllable current decreases at high temperatures, as in the case of other MOS-bipolar devices, but is significantly better than for previous devices. The absolute values of the maximum turnoff current are well above 1000 A/cm² at room temperature and 500 A/cm² at 200°C     

Gate-assisted turn-off effect in TIL-type thyristors

The research reported in this work was focused on the turnoff performance of trial high-power gate-assisted turn-off thyristors (GATT's) based upon the novel double-interdigitated or two interdigitation levels (TIL) gate-cathode concept. The experiments were performed on high-voltage (2000 V) devices which were driven up to an anode current ofi_{T} = 500A. The test TIL GATT's were both gold-doped and normal (nongold,diffused). The investigations have shown that the application of a negative gate current of only 4 A leads to a reduction of the turn-off time iqby a factor of 3 and 4-4.5 in gold-doped and normal devices, respectively, at Tj= 100°C. At constant gate current, the dependence of tqon the anode current level tTwas found relatively weak. The additional experimental data provided in this work show clearly that sought-for benefits could be achieved by implementing the TIL pattern in power GATTs.     

Induction cooking unit using a gate turn-off thyristor

This paper deals with the design and construction of a 500W 60kHz induction cooking power supply. The novel aspect of the unit is its use of a gate turn-off thyristor (GTO) in a resonant circuit topology, i.e. a Class E circuit.     

Numerical modeling of gate turn-off thyristor using SICOS

A numerical model of gate turn-off thyristors (GTOs) is presented. The concept of a controlled switch realized by a controlled current source is first introduced. Using this basic model, an equivalent circuit for the GTO is given. Using the GTO characteristics given by manufacturers, the equations connected with all the parameters of the equivalent circuit are deduced, and all of the parameters are determined. A sample study is presented. Simulation of this numerical model with the SICOS program gave results in accordance with the experiment     

Analysis of insulated gate transistor turn-off characteristics

A model based upon a MOSFET driving a wide-base p-n-p transistor is presented for analysis of the turn-off behavior of n-channel insulated gate transistors. This model is found to provide a very good quantitative explanation of the shape of the collector current waveform during turn-off. Verification was accomplished using insulated gate transistors (IGT's) fabricated with two voltage ratings and a variety of radiation doses. This analysis allows the separation of the channel (electron) and minority carrier (hole) current flow in the IGT for the first time.     

High-power dual amplifying gate light triggered thyristors

The feasibility of directly light triggering a high power phase control thyristor is investigated. Work is described on an optically triggered gated 53-mm diameter 2600-V 1000-A thyristor which is similar to an electrically gated production version. Test results describing the response of this thyristor to various optical signals are presented. Our work has shown that this cell can be directly triggered by light at an equivalent gate current which is a factor of three below its present dynamic gate requirements and still largely retain all its blocking and dynamic characteristics. This improvement is obtained by the use of a second very sensitive amplifying gate stage which is responsive to light. All wafer processing of the light sensitive thyristor was carried out on standard production lines. Tests made on static dV/dt, di/dt, blocking voltage, and leakage current on light sensitive devices all closely match parameters of the standard electrically fired equivalent cell.     

Transient thermal response of amplifying gate thyristors

A computer-aided investigation of the transient thermal behaviour of amplifying gate thyristors (a.g.t.s.) has been performed by taking into consideration the nonlinear properties of silicon over a wide range of temperature excursions.     

Numerical analysis of turn-off characteristics for a gate turn-off thyristor with a shorted anode emitter

Turn-off current waveform for a gate turn-off thyristor (GTO) with a shorted anode emitter has been calculated numerically by solving the semiconductor basic equations in an equivalent one-dimensional model device. This model is derived from the analysis of current and carrier distributions obtained by a two-dimensional calculation of the on-state of GTO. A calculated turn-off current waveform agrees well with the experimental waveform. The computational time of one case is about 2 min. It is shown that this one-dimensional analysis method is useful for the calculation of the turn-off time. Using this one-dimensional model during the turn-off process and the two-dimensional model in the on-state, the relation between turn-off time and the forward voltage drop can be obtained in relation to the shorted emitter structure. It is shown that the shorted emitter structure is useful to improve this tradeoff relation.     

Turn-off operation of a MOS-gate 2.6 kV 4H–SiC gate turn-off thyristor

The turn-off operation of a 4H–SiC gate turn-off thyristor (GTO) with 2.6 kV breakover voltage has been investigated using an external Si-MOSFET as a gate-to-emitter shunt (MOS-gate mode), in the temperature interval 293–496 K. The maximum cathode current density j_cmax that can be turned off in such a mode decreases from 1850 A/cm² at 400 K to 700 A/cm² at 496 K. The room temperature j_cmax value is estimated to be about 3700 A/cm². The above j_cmax values are essentially higher than those observed when turning this thyristor off in the conventional GTO mode. Turn-off transients in the MOS-gate mode have been studied in both quasi-static and pulse regimes. Temperature dependencies of the turn-on and turn-off times, as well as those of the turn-on and turn-off energy losses have been measured. The upper switching frequency of the GTO is estimated to be about 700 kHz.     

2500-V 600-a gate turn-off thyristor (GTO)

GTO self-turn-off capability provides an advantage over an ordinary thyistor, because of forced commutation circuit removal upon inverter and chopper application, thus substantially reducing equipment size, weight, and mechanical noise. A series of high-power GTO's has been developed, with the present 2500-V-600-A unit as its peak. The most essential design problem for this unit is to establish a principle for increasing maximum gate turn-off current (IATO), while keeping overall thyristor characteristics in reasonable balance. High IATOwas attained by decreasing p-base sheet resistance, as well as decreasing n-emitter finger width. Excellent thyristor characteristics were obtained by adopting a low acceptor concentration near the cathode-gate junction. From a device process point of view, introducing a phosphorus redeposition annealing increased carrier lifetime in the p base to a sufficiently high level. This process contributed most strikingly to improving the off-state voltage.     

Shorter turn-off times in insulated gate transistors by proton implantation

Proton implantation has been used to shorten the turnoff time of insulated gate transistors. A narrow region of low carrier lifetime was created at 100-µm depth by implanting 3.1-MeV protons. As with other techniques of lifetime control, an increase in forward voltage drop with decreasing turn-off time was observed. The use of localized lifetime control provides the opportunity of improving the trade-off relationship between turn-off time and forward voltage drop at the device's operating current. A definite improvement in trade-off curves was observed when the new technique of proton implantation was compared to the method of electron irradiation.     

Effects of rapid thermal annealing on nitrided gate oxide grown on 4H-SiC

The electrical characteristics of thermally nitrided gate oxides on n-type 4H-SiC, with and without rapid thermal annealing processes, have been investigated and compared in this paper. The effects of annealing time (isothermal annealing) and annealing temperature (isochronal annealing) on the gate oxide quality have also been systematically investigated. After rapid isothermal and isochronal annealings, there has been a significant increase in positive oxide-charge density and in oxide-breakdown time. A correlation between the density of the positive oxide charge and the oxide breakdown reliability has been established. We proposed that the improvement in the oxide-breakdown reliability, tested at electric field of 11 MV/cm, is attributed to trapping of injected electron by the positive oxide charge and not solely due to reduction of SiC-SiO₂ interface-trap density.