SiC Schottky Diode

1. Theory

Definition: A SiC Schottky diode is a vertical metal-semiconductor power rectifier built on silicon carbide (almost always the 4H polytype).
Wide bandgap: 4H-SiC’s bandgap ( $\approx 3.26$ eV) gives a critical field of about 2.2–3.0 MV/cm, roughly eight to ten times that of silicon.
Thin drift region: The high critical field lets a thin, relatively heavily doped drift layer block 600 V to several kV with low on-resistance.
Native substrates: Large, low-defect SiC wafers allow true vertical devices with a well-controlled drift layer, which is why SiC power diodes are a mature product.
High thermal conductivity: SiC conducts heat better than Si or GaN, supporting high current density and high junction temperatures.
Merged structures: Most commercial devices are junction-barrier Schottky (JBS) or merged PiN-Schottky (MPS) diodes, with embedded $P^{+}$ regions that shield the Schottky contact.

2. Working Principle

Forward bias: Majority electrons cross the metal-SiC barrier by thermionic emission, with a forward drop of about 1–1.5 V.
Reverse blocking: The depletion region spreads through the drift layer; in JBS/MPS designs, the $P^{+}$ grid pinches off and screens the Schottky interface, cutting barrier-lowering leakage.
Surge handling: In MPS diodes the embedded PN junctions inject carriers at high surge current, lowering the forward drop during surges.
Near-zero reverse recovery: No minority-carrier storage, so turn-off loss is only capacitive and is almost independent of temperature.
High-temperature operation: The wide bandgap keeps intrinsic carrier density and leakage low at high junction temperatures.

3. Applications

Power factor correction (PFC): Boost diodes in server, telecom and industrial supplies at 600–650 V.
EV chargers and traction inverters: Low-loss rectifiers and freewheeling diodes at 650–1200 V.
Solar inverters: High-efficiency boost and freewheeling stages.
Antiparallel diodes for IGBTs and SiC MOSFETs: Cut reverse-recovery loss in high-voltage motor drives.

Name	Formula	Related Formulae	Variables/Constants
Breakdown voltage (one-sided drift region)	$B V \approx \frac{ε _{s} E _{cr i t}^{2}}{2 q N _{D}}$	$(see PN Junction Diode / Avalanche Diode / GaN Power Diode pages)$	$B V : b r e ak d o w n v o l t a g e$ $E_{cr i t} : cr i t i c a l f i e l d (\approx 2.2 - 3.0 MV/cm f or 4 H - S i C, v s \approx 3.3 MV/cm f or G a N, v s \approx 0.3 MV/cm f or S i)$ $N_{D} : d r i f t r e g i o n d o p in g$
Unipolar specific on-resistance limit (Baliga’s limit)	$R_{o n, s p} = \frac{4 B V ^{2}}{ε _{s} μ _{n} E _{cr i t}^{3}}$	$(see GaN Power Diode page for full derivation)$	$R_{o n, s p} : a r e a - n or ma l i z e d o n - r es i s t an ce$ $μ_{n} : e l ec t r o n m o bi l i t y (\approx 900 - 1000 c m^{2} /V \cdot s f or b u l k 4 H - S i C)$
Baliga figure of merit (power)	$B F O M = ε_{s} μ_{n} E_{cr i t}^{3}$	$R_{o n, s p} = \frac{4 B V ^{2}}{B F O M} (see GaN Power Diode page)$	$B F O M : ma t er ia l f i g u r e o f m er i t f or s t a t i c co n d u c t i o n l oss in u ni p o l a r p o w er d e v i ces; S i C^{'} s B F O M i s l o w er t han G a N^{'} s (d r i v e n main l y b y t h e E_{cr i t}^{3} t er m) b u t s t i l l v a s tl y hi g h er t han S i^{'} s$
Baliga high-frequency figure of merit (switching)	$B H F F O M = μ_{n} E_{cr i t}^{2}$		$B H F F O M : f i g u r e o f m er i t f or s w i t c hin g - l oss (f r e q u e n cy) p er f or man ce, a s o pp ose d t o s t a t i c co n d u c t i o n l oss$
Thermal resistance (conduction)	$R_{t h} = \frac{t}{κ A}$		$R_{t h} : t h er ma l r es i s t an ce o f t h e co n d u c t in g p a t h f r o m j u n c t i o n t o h e a t s ink$ $t : t hi c k n ess o f t h e co n d u c t in g l a y er$ $κ : t h er ma l co n d u c t i v i t y (\approx 3.7 - 4.9 W/cm \cdot K f or S i C, n o t ab l y hi g h er t han G a N or S i)$
Reverse leakage current (barrier-lowering enhanced)	$I_{R} (V) = A A^{*} T^{2} exp (- \frac{q [ ϕ _{B} - Δ ϕ _{B} ( V )]}{k _{B} T})$	$(see Schottky Diode / Metal-Semiconductor Contacts pages)$	$I_{R} : r e v er se l e ak a g e c u r r e n t, w hi c h g r o w s w i t h ∣ V ∣ r a t h er t han s a t u r a t in g$ $k_{B} : B o l t z mann co n s t an t$

Important Questions

Ques: Why is SiC the dominant choice for mature vertical power devices, while most commercial GaN power devices remain lateral?

SiC can be grown as large, relatively low-defect-density bulk single-crystal boules and sliced into native substrates, allowing true vertical devices to be built with a thick, well-controlled drift region grown directly on a SiC wafer. GaN, by contrast, lacks a practical, large-scale, low-cost native bulk substrate, and is instead almost always grown heteroepitaxially on a foreign substrate (commonly Si, as discussed on the GaN Power Diode and GaN HEMT pages), which introduces lattice-mismatch-related defects and has historically confined most commercial GaN power devices to lateral, HEMT-derived structures rather than classical vertical drift-region devices. This substrate availability difference, not any fundamental material-physics limitation, is the main practical reason SiC vertical power diodes are mature, widely available commercial products today while vertical GaN devices remain comparatively early-stage. Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Ques: Why does thermal conductivity matter as much as the electrical figures of merit when choosing a power diode material?

Power diodes dissipate real heat during operation, both from conduction loss ( $I^{2} R_{o n, s p}$ ) and from switching loss, and a material’s thermal conductivity directly determines how effectively that heat can be extracted from the junction to the package and heat sink without the device overheating. SiC’s thermal conductivity is notably higher than both GaN’s and silicon’s, giving it a genuine practical advantage for sustaining high current and power density reliably, even in cases where GaN’s higher Baliga figure of merit would suggest a theoretically lower on-resistance. This is a major reason SiC remains the preferred choice for the highest raw-power applications (industrial motor drives, EV traction inverters), while GaN tends to dominate lower-power, higher-switching-frequency niches where its thermal disadvantage matters less. Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Ques: Why might GaN be preferred over SiC for very high switching frequency applications despite SiC’s advantages in raw power handling?

The Baliga high-frequency figure of merit $B H F F O M = μ_{n} E_{cr i t}^{2}$ weighs carrier mobility more heavily relative to critical field than the static power figure of merit does, and GaN’s combination of very high channel mobility (particularly in 2DEG-based HEMT structures, see the GaN HEMT page) with a higher critical field than SiC gives it a substantially higher $B H F F O M$ . A higher value here translates to faster achievable switching with lower frequency-dependent losses, which is why GaN devices are generally favored for compact, high-switching-frequency power conversion applications, while SiC’s advantage lies more in sustained high-power, high-voltage conduction where its superior thermal conductivity and mature vertical device technology matter more than raw switching speed. Reference: B.J. Baliga, “Power semiconductor device figure of merit for high-frequency applications,” IEEE Electron Device Letters 10, 455 (1989).

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