Wide and narrow bandgap semiconductors for power electronics: A new valuation

Journal of Electronic Materials, Jun 2003 by Hudgins, Jerry L

An advantage for some wide bandgap materials that is often overlooked is that the thermal coefficient of expansion (CTE) is better matched to the ceramics in use for electronic-packaging technology. The optimal choice for unipolar devices is GaN and the associated material system of GaN/AlGaN. The future optimal choice for bipolar devices at all power levels is C (diamond). New expressions, [epsilon]^sub c^ = 1.73 x 10^sup 5^ (E^sub G^)^sup 2.5^ for direct-gap and [epsilon]^sub c^ = 2.38 x 10^sup 5^ (E^sub G^)^sup 2^ for indirect-gap semiconductors, relating the critical-electric field for breakdown in abrupt junctions to the material bandgap energy, and associated new expressions for specific on-resistance in power semiconductor devices is shown to further support the use of wide bandgap materials. Some low-voltage, power-electronics applications are shown to benefit by the use of Ge, C, and GaSb.*

Key words: Power electronics, wide bandgap, narrow bandgap, specific on-resistance, gallium nitride, diamond, critical-electric field

INTRODUCTION

Power-semiconductor devices made from materials with bandgap energies larger than in Si have been touted for many decades. The potential advantages of these wide bandgap devices include higher achievable junction temperatures and thinner drift regions (because of the associated higher critical-electric field values) that can result in much lower on-resistance than is possible in Si.1-3 There are, however, several disadvantages associated with the use of devices fabricated from wide bandgap materials. Among these is that the ratio of the electron-to-hole mobility values range much higher than in Si, and typically, the hole mobility is quite low, so that the use of wide bandgap semiconductors for bipolar devices is not desirable.

An advantage to the use of some wide bandgap materials that is often overlooked, however, is that the coefficient of thermal expansion (CTE) is better suited to the ceramics used today in packaging technology. In addition, the maturity and expense of material processing plays a role in the optimal choice of the semiconductor system best suited for power-semiconductor devices. It will be shown that the near-term optimal choice for unipolar devices is GaN. It will also be shown that the future optimal choice for unipolar and bipolar devices is C (diamond). In addition, a recently developed expression relating the critical-electric field for breakdown in abrupt junctions to the material bandgap energy is discussed along with newly derived expressions for specific on-resistance in power-semiconductor devices.4 A short discussion of the use of various materials in low-voltage, power-electronics applications is given and extended to include possible integrated-circuit design using complementary metal-oxide semiconductor (CMOS) or n metal-oxide semiconductor (NMOS) topologies.

OPTIMAL SEMICONDUCTOR MATERIALS FOR HIGH-VOLTAGE DEVICES

Thermomechanical Properties

Various semiconductor-material parameters are listed in Table I. The references for the material parameters in the table are denoted with capital letter superscripts and are listed separately in the reference section at the end of the paper. Table II lists some of the physical parameters of typical power-electronic package substrates, metals, and solder materials. From Tables I and II, it can be shown that the materials with a value of thermal conductivity close to or exceeding Si (above 100 W/m [middot] K) and with CTE values of 4-8 ppm/K (to closely match typical substrate materials) are as displayed in Fig. 1a and b. The resulting short list of semiconductors that have both a relatively high value of thermal conductivity and a close CTE match to package substrate materials becomes GaN, AlN, GaP, SiC (6H), and SiC (4H). Diamond and the cubic polytype of BN are included in the discussion because it can be argued that the extremely high, thermal-conductivity values of these materials make the CTE match less of an issue. In fact, it could be reasoned that diamond devices on an insulating (intrinsic or polycrystalline) diamond substrate would cause no CTE mismatch and provide low thermal resistance and high electrical isolation.

Electrical Properties

It has been shown that the traditionally used relationship18 between the critical-electric field (breakdown field) and energy bandgap of a semiconductor is not accurate.4 The traditionally used expression is

where q is the electron charge, [epsilon] is the permittivity of the semiconductor material, N^sub B^ is the impurity-doping concentration on the low-doped side of the junction, and EG is the bandgap energy. It should be noted that, in high-voltage power semiconductors, the impurity concentration on the low-doped side of the junction is typically as low as possible to allow for optimized voltage breakdown in the device (for Si in the range of 10^sup 13^-10^sup 14^ cm^sup -3^), resulting in a dependence of the critical field on the bandgap energy to the power of 0.75 and relatively independent of the exact value of N^sub B^. The expression in Eq. 1 has been used extensively in the literature to derive empirical dependencies between bandgap and critical field or breakdown voltage and to further derive forms of on-resistance, R^sub ON^, or specific on-resistance, R^sub ONsp^, as figures of merit for power-device performance.1-3,19