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Posted on 07 December 2020

Application Fields and Current Performance Limits for Power Semiconductors

 

The development of power semiconductors saw the onset of lasting success for power electronics across all fields of electrical engineering. Given the ever increasing call for resource conservation (e.g. energy saving agenda), the use of renewable energies (e.g.wind power and photovoltaics) and the need for alternatives to fossil fuels (e.g. electric and hybrid drives for vehicles), this success is gaining more and more momentum today.

 

This development is also largely driven by the interactions between system costs and market penetration, as well as the energy consumption required for production and the energy saving potential of products in operation. In addition to the general aim to expand the performance profile, the development aims "low materials consumption/ low costs" and "high efficiency" are gaining more and more importance.

Figure 1 shows maximum current and voltage values for controllable power semiconductors on the market today. Today, the use of parallel and series connections for power semiconductors, as well as power converters equipped with semiconductors, means that virtually any amount of electric power can be transformed, converted into another form of energy or "generated" from another type of energy.

Figure 1. Present current and voltage limits for controllable power semiconductors: a) shows common switching frequency ranges for various power semiconductors; b) illustrates the current key application fields and limits.

IGBTs ( Insulated Gate Bipolar Transistors) have become especially important for the "mass markets" of mains powered systems and equipment with a medium or high switching performance in the range of some kW and several MW, this is particularly true for potential free power modules.

Figure 2. a) Switching frequency ranges for various power semiconductors b) Current application areas and limits

Since the mid-eighties, these and other actively switchable power semiconductors such as power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), GTO (Gate Turn Off) thyristors and IGCT ( Integrated Gate Commutated Thyristor) have almost completely pushed back conventional thyristors to line commutated applications. Compared to other switchable power semiconductors, such as conventional GTO thyristors, IGBT and MOSFET have a number of application advantages, such as active turn-off even in the event of a short circuit, operation without snubbers, simple control, short switching times and, consequently, relatively low switching losses. The production of MOSFET and IGBT using technologies from the field of  microelectronics is comparatively simple and low priced.

Today, most applications for currents of some 10 A use power semiconductors with silicon chips integrated in potential free power modules. In 1975, it was SEMIKRON who launched them commercially for the first time. These modules often contain several silicon chips of identical or different components (e.g. IGBT and freewheeling diode, or thyristor and line rectifier diode), and more components (e.g. temperature and current sensors) or control and protective circuits ("intelligent power modules"/IPM), if required.

Despite the disadvantage of one side cooling only, for up to high power ranges, potential free power modules are gaining more ground than disk cells, even though the latter are able to dissipate about 30% more of the heat losses thanks to double sided cooling and are better suited to series connections from a mechanical point of view. The reason that modules are more popular than disk cells is that, apart from easy assembly, they boast "integrated", well proven electrical isolation between chip and heat sink, almost any combination of different components in one module and relatively low costs thanks to batch production.

Today, important areas of application for power MOSFET are power supply systems, low voltage switch applications in automotive electronics and applications featuring very high switching frequencies (50…500 kHz), where standardized power modules are of rather low importance.

The following chapters will detail the layout, function, characteristics and applications of line rectifier diodes and thyristors, power MOSFET and IGBT, and fast diodes required as freewheeling diodes, and outline development trends in these areas. Based on the requirements described above, the general aims and directions for the further development of power semiconductors can be summarized as follows:

The key aims for further development are as follows:

  • Increasing the switching performance (current, voltage)
  • Reducing losses in the semiconductors as well as in control and protective circuits
  • Expanding the operating temperature range
  • Improved service life, ruggedness and reliability
  • Reducing the amount of control and protection required; improving component behavior in the event of error / failure
  • Cost reduction

The development directions can be broken down as follows:

 Semiconductor materials

  • New semiconductor materials (e.g. wide bandgap materials)

 Chip technology

  • Higher permissible chip temperatures or current densities (reduction of chip area)
  • Finer structures (reduction of chip area)
  • New structures (improvement of chip characteristics)
  • Integration of functions on the chip (e.g. gate resistance, temperature measurement, monolithic system integration)
  • New monolithic components by combining functions (RC-IGBT, ESBT)
  • Improved stability of chip characteristics under different climatic conditions

 Packaging

  • Increase in thermal and power cycling capability
  • Improvement of heat dissipation (isolation substrate, base plate, heat sink)
  • Wider scope of application as regards climate conditions thanks to improvements in casing and potting materials or new packaging concepts
  • Optimisation of internal connections and connection layouts regarding parasitic elements
  • User friendly package optimisation to simplify device construction
  • Reduction of packaging costs and improvement of environmental compatibility in production, operation and recycling

Degree of integration

  • Increasing the complexity of power modules to reduce system costs
  • Integration of driver, monitoring and protective functions
  • System integration

Figure 3. Power Module Integration Levels

More complex technologies, smaller semiconductor structures and precise process control are inevitably driving the properties of modern power semiconductors towards the physical limits of silicon. For this reason, research into alternative semiconductor materials, which began as early as the 1950s, was pushed in recent years and has since resulted in the first mass products.

Today, the " wide bandgap materials " silicon carbide (SiC) and gallium nitride (GaN) are the main focus of this research. Compared to silicon, they display a far higher energetic gap between valence and conduction band, resulting in comparatively lower forward on-state losses and switching losses, higher permissible chip temperatures, and better heat conductivity than silicon.

Parameters     Si 4H-SiC GaN
Bandgap Energy Eg eV 1.12 3.26 3.39
Intrinsic Density ni cm-3 1.4 * 10-10 8.2 * 10-9 1.9 * 10-10
Breakdown field density Ec MV/cm 0.23 2.2 3.3
Electron mobility μn cm²/Vs 1400 950 1500
Drift Velocity vsat cm/s 107 2.7 * 107 2.5 * 107
Dielectric constant ε - 11.8 9.7 9.0
Heat Conductivity λ W/cmK 1.5 3.8 1.3

Table 1. Wide band gap semiconductor materials versus silicon: a comparison of material properties

Figure 4. Impact of different physical parameters of semiconductor materials

Today, the key to more widespread use of SiC, however, is to enable the cost efficient production of suitable monocrystalline chips that are sufficiently high in quality to eliminate crystal degradation (micropipes), and that are available in optimum wafer sizes for the power electronics industry. While Si is currently produced on 8“ wafers virtually defect free for € 0.10/cm², the defect density for SiC wafers with a diameter of 4“ is one order of magnitude higher, multiplying costs in comparison to Si. GaN, which displays slightly poorer properties than SiC, has been used mainly in optoelectronic components so far. The carrier material employed today is sapphire. Since this material is nonconductive, GaN components must have planar structures. The most common type of diode on the market today is the SiC Schottky diode.

Owing to the advanced development stage of Si power semiconductors, there is no technical need to introduce other semiconductor materials for MOSFET and IGBT in the voltage range < 1000 V. In this voltage range, wide bandgap semiconductor materials are more likely to be competitive in junction gate driven power semiconductors such as JFET (junction gate field effect transistors), bipolar transistors and thyristors, whereas MOS driven transistors clearly outplay silicon components when higher voltages are applied.

Owing to high material costs, power semiconductors made of "wide bandgap materials" are used first and foremost in applications where a particularly high efficiency ratio or minimum absolute losses are required, as well as in applications whose requirements – e.g. temperature, voltage or frequency – cannot be met with Si power semiconductors.

In order to to fully benefit from the main advantages that power semiconductors made of SiC or GaN have over conventional components, such as

  • low conduction and switching losses
  • higher blocking voltages
  • higher possible power densities
  • higher permissible operating temperatures
  • shorter switching times, higher switching frequencies,

it is vital for packaging to be further developed and improved on accordingly.

 

For more information, please read:

Power Electronics Packaging Technology

Pressure Contact Connection Technology

Sintering Technology

Wire Bonding Technology

 

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