Advanced power semiconductors for power electronic systems

Advances in power electronics systems over the last three decades have been marked by five major inventions: light-triggered thyristors in the top-end power range, GTOs in the high-end power range, IGBTs in the mid-range power band, power MOSFETs in the low-end power range, and SMART power ICs for monolithic system integration in the low-end power range.

LIGHT-TRIGGERED THYRISTORS
Since its introduction in the 1950s, thyristor technology has steadily advanced (Figure 1). Improvement in silicon material quality and manufacturing technology have resulted in such high-power devices that, for example, a single device with a blocking capability of 8kV can switch periodic currents of 6000A. For most applications—even in high-voltage dc (HVDC) transmission lines in the power range up to 3.5GW, or in energy management such as reactive power compensation in the power range up to several 100MVA—such high current capability is sufficient thus absolving the need for thyristors to be connected in parallel. However, to achieve high voltages of up to several hundred kV, many thyristors have to be connected in series. Since each thyristor requires trigger circuitry and electronic protection circuitry, current developments in thyristor switches are aimed at reducing both the number of thyristors, and the number of components in the trigger and protection circuits.
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The trigger circuitry can be simplified by integrating a lightsensitive region into the thyristor, where the photo-generated current is used to trigger the thyristor. With regard to the protection circuitry, there are three important failure events the thyristors must be protected against: damage by overvoltage when avalanche breakdown occurs, displacement currents which are generated when a voltage pulse is applied to the device, and current filamentation during forward recovery time when the charge carriers are extracted from the blocking junction.

IGBTS FOR THE HIGH, MEDIUM POWER APPS
Since the 1980s, the IGBT family of components has become the ‘motor’ for innovation in electrical drive technology. Today’s generation of IGBTs covers the entire power spectrum for industrial drive engineering, drive technology in the field of transport, and usage in home appliances, with currents ranging from 1 to 4000A, and with voltages that range from 600V to 6.5kV. Major development objectives have been to reduce dynamic and static power losses, improve robustness and reliability, raise the limit load characteristics, and maintain a safe operating area, while at the same time drastically reducing the production costs.

Main enablers for these improvements were the implementation of modern IC technology into the IGBT process, e.g. small planar transistor cells as well as trench transistor cells, and evolutional steps of the vertical structure from PT (punch through) to NPT (non punch through) to FS (field stop) concept (Figure 2) by using ultra-thin wafer technology.

UNIPOLAR MOSCONTROLLED POWER SWITCH
System miniaturization is an ongoing trend in electronics. Compared to functionality, power supply is often a primary concern when it comes to miniaturization, and together with robustness, efficiency is also of primary improtance.

Although this power semiconductor device was introduced towards the end of the 1970s, continuous advances and improvements have been achieved. Improvements targeted lower RDson, lower self-capacitances, and higher ruggedness. In case of the low voltage power MOSFET with increased cell density, the ON-state resistor and self capacitances could be reduced significantly simultaneously. The Epi-Resistor in the drift region dominated the high voltage power MOSFET performance.

A breakthrough was achieved 20 years after the introduction of the power MOSFET by applying superjunction technology.

The success story of high voltage devices based on the revolutionary compensation principle started with the introduction of CoolMOS in 1998. The superior performance of compensation devices results from the drastic reduction of the area specific on-resistance. A relatively high doping area achieves this. The adverse effect of this high doping level on the breakdown voltage is compensated by additional adjacent p-doped areas in the drift zone being arranged in a closed packed manner with the current conducting nregions. In the blocking state these adjacent n- and p-areas deplete each other from mobile carriers and act combined as a nearly intrinsic layer.

SIC DEVICES SHOW EXCELLENT PERFORMANCE
Since the introduction of three superjunction technology several development steps have been applied (Figure 3). In the first step, a revolutionary improvement was achieved in terms of RDson reduction and the improvement of the input capacitors. In the second step, a poly gate structure was developed to contact every single cell on the die with the same small interval gate resistor. An improvement of the overload capability followed. The final development was targeted to improve the FOM (figure of merit) number substantially.

Certain types of SiC-materials show excellent electrical and thermal properties when appied to power semiconductors components.

Silicon Carbide (SiC) Schottky diodes as unipolar devices offer unique ultra fast switching behavior making them extremely attractive for applications requiring blocking voltages ranging from 300 to 3000V, and switching frequencies higher than 50kHz. For blocking voltages less than 300V there is a choice of other unipolar diodes like Si or GaAs Schottky diodes. Above 3000V, most likely SiC-bipolar diodes will be the option with the best overall performance due to their significantly lower leakage current together with a weaker temperature dependence of this property compared to Schottky diodes.

SUMMARY
Over the last 20 years, power semiconductor development determined power electronic systems. In the future, the overall system integration will be the driving force, and will include total system tailoring, matching passive components with novel assemble technology, and outstanding performance of the Si/SiC devices. These factors stand for system miniaturization, improvement in electrical/thermal behavior, lifetime reliability, and cost reduction.