IEEE Power Electronics Magazine - December 2019 - 23

a thermal interface material (TIM) and through the
top-side heatsink to ambient. Absent a heatsink,
only a small portion of heat can flow through the
top of the die and by means of radiation and convection only.
Figure 2 illustrates the thermal resistances for the
chip-scale packaged GaN transistor for the two examples given in Figure 1. The GaN transistor is mounted
with the active device region facing the PCB and separated by the solder bumps/bars that form the device terminals. R iJC is defined as the thermal resistance from
the active area (junction) of the die to the case and R iJB
as the thermal resistance from junction to the board,
which includes the solder bumps/bars to the copper
traces on the PCB.
These thermal resistances, R iJB and R iJC, for a chipscale package are analogous to the thermal resistances,
R iJC (bottom) and R iJC (top), for a lead-frame package, respectively. However, in contrast to a lead-frame package, both
the top and bottom sides of a chip-scale GaN transistor are
effective cooling paths.

case-to-ambient thermal resistances are very low, then
the minimum possible junction-to-ambient thermal
resistance is equivalent to R iJC in parallel with R iJB .
Figure 3(a) shows the thermal resistances for some
example Si MOSFETs and chip-scale GaN transistors.
As shown, the bottom-cooling thermal resistance, R iJB,
follows an inverse trend with the device size. However,
the top-cooling thermal resistance, R iJC, is much lower
for GaN transistors due to the chip-scale package.
Figure 3(b) demonstrates that a chip-scale GaN transistor can achieve much lower R iJA with an effective

Rθ SA
Heatsink
TIM

Rθ CA
Rθ JC

Si Substrate

Rθ JC

Active GaN Device Region

Junction-to-Ambient Thermal Resistance
The overall thermal resistance between the junction
and ambient, R iJA, is often listed in device data sheets
standardized to a specific PCB area (typically 1 in 2)
with no heatsink or forced air flow. This includes the
PCB-to-ambient thermal resistance, R iBA, as well as
some direct case-to-ambient thermal resistance, R iCA .
However, the actual thermal resistance between junction and ambient will depend on the design of the converter, including PCB size and layout, air flow, and
attachment of a heatsink. If the board-to-ambient and

Rθ CS

Rθ JB

Solder Bars

Copper Traces
PCB

Rθ JB

PCB

(a)

(b)

FIG 1 The cross section of a chip-scale packaged GaN transistor, highlighting the dominant heat flow paths from the top
and bottom of the die. The diagrams illustrate both (a) without
a heatsink and (b) with a heatsink attached to the top of the
transistor. (Source: EPC; used with permission.)

Rθ SA
Heatsink Temp (TS)
Rθ CS

Rθ CA
Case Temp (TC)

Case Temp (TC)

Rθ JC

Rθ JC
Junction Temp (TJ)

Power
Loss

Rθ JB

Junction Temp (TJ)

Power
Loss

Rθ JB

Board Temp (TB)

Board Temp (TB)
Rθ BA

Rθ BA
Ambient Temp (TA)
(a)

Ambient Temp (TA)
(b)

FIG 2 The steady-state thermal resistance schematic models for the physical device structures given in Figure 1. (a) Without a heatsink, and (b) with a heatsink attached to the top of the device. (Source: EPC; used with permission.)

December 2019

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IEEE Power Electronics Magazine - December 2019

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