Using wide bandgap devices for switched-mode power supplies

Introduction

• Idea:
• Required: 180 W \text{ } 19.5 \text{V DC} \text{ } 1.2 \frac{\text{W}}{\text{cm}^3} \text{ } 94 \text{\% efficiency}
• Can new wide bandgap devices be used to fulfill this requirements?

Material table

Wide Bandgap

Impact ionization

• Collision of moving carriers with crystal atoms causes new electron-hole-pairs
• Reason for avalanche breakdown
• A moving carrier creates $\alpha_{n/p} = a_{n/p} \cdot e^{-\frac{b_{n/p}}{E}}$ electron-hole-pairs per meter
• Temperature dependent!
• Different for electrons and holes!
• Wide Bandgap: More energy required to push an electron into the conduction band

Voltage blocking capability

• Avalanche depends on drift length and field strength (gradient)
• 
• Assume $\alpha_n = \alpha_p = \alpha$
• An electron-hole-pair causes $\int_0^L \alpha(x) \mathrm{d}x$ impact ionizations
• Avalanche occurs, if this integral is $\ge 1$
• Homogeneous field: $V_{BR} = \frac{b \cdot L}{ln(L \cdot a)}$
• Linear field: solve $\int_0^L a \cdot e^{- \frac{b \cdot L^2}{2 \cdot V_{BR} \cdot x}} \mathrm{d}x - 1 = 0$
• Necessary dopant density: $N_D = \frac{2 \cdot \epsilon_0 \cdot \epsilon_r \cdot V_{BR}}{q_0 \cdot L^2}$
• $\alpha_n \ne \alpha_p$:
• Calc with $\alpha = \alpha_n$
• Calc with $\alpha = \alpha_p$
• Take the maximum/minimum!

On-Resistance

• Specific on-resistance:
  $\rho = \frac{1}{q_0 \cdot \mu_n \cdot N_D}$ \text{ } $R_{DS, on} \cdot A = \rho \cdot L = \frac{L^3}{2 \cdot \epsilon_0 \cdot \epsilon_r \cdot \mu_n \cdot V_{BR}}$
• Simplification:
• Assume critical maximum field strength as avalanche condition
• $R_{DS, on} \cdot A = \frac{4 \cdot V_{BR}^2}{\epsilon_0 \cdot \epsilon_r \cdot \mu_n \cdot E_{crit}^3}$
• $BFoM = \epsilon_r \cdot \mu_n \cdot E_{crit}^3$
• N-drift zone resistance of GaN by a factor of 2000-8000 lower than Si
• Channel- and packaging resistance dominate for low voltages!

Linear field in a MOSFET

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Temperature stability

• High temperature stability:
  
• No benefit for laptop SMPS!

Problems with application of GaN

• High PN-threshold voltage, short carrier lifetime
• Wide bandgap is unattractive for plasma devices (BJT, IGBT)
• No natural gate oxide
• Gate dielectric has to be built with aluminium oxide or silicon nitride
• High production effort
• Difficult crystal growth
• GaN bulk too expensive
• No vertical GaN devices
• Grow GaN on Si-wavers
• Only thin, lateral devices possible
• Gate driver and power transistor on same chip

HEMT

• An other FET device type for GaN:
  
• 
• Difficult to built a normally-off, insulated gate HEMT

Questions?