power than the aforementioned transistors without
excessive heating or deviating from its nominal
impedance. The design of this element is rather simple:
it is made as a conducting thin film deposited on top
of a dielectric substrate, and the energy is dissipated
as joule heating in the area covered by the resistor
(Figure 4). At low frequency, the temperature rise
in the resistor can be easily kept under control, e.g.
below 125ºC, by increasing its size, and thus reducing
the power density. However, at higher frequencies, the
resistor behavior is more complicated, shifting from a
purely resistive impedance to a complex impedance in
which a parasitic capacitance and inductance appear
(Figure 4), rapidly deviating the impedance from the
nominal impedance of the resistor when the frequency
scales into the GHz range. Since the parasitic
capacitance scales with the size of the resistor, the
resistor needs to be scaled down to operate at higher
This results in an increase of the heat density, and to
keep the resistor temperature under control dielectric
substrates with a high thermal conductivity are needed.
When operating high-power resistors (>100W)
below 5 GHz, this is feasible by using high thermal
conductivity dielectrics like AlN and BeO as
substrates. Above that frequency, the size of the
device needs to be reduced to the limit where only
CVD diamond substrates are capable of dealing
with the power density dissipated in the resistor.
High-power resistors using CVD diamond substrates
can operate at up to five times higher frequency
than any other resistor, reaching frequencies well
above the X-band ( 12 GHz). This makes of them the
perfect match with GaN-on-diamond transistors to
enable simplifying the design of next-gen high-power
electronics working above 5 GHz.
Figure 4. Top: Sketch of a thin film resistor and its lumped representation for high
frequency. Bottom: Maximum frequency at which the deviation from an ideal 100 W
resistor reaches 1.25 VSWR.
Figure 3. 120-140 mm CVD diamond wafers of different thermal grades. A metallized
heat spreader ready to be attached to an electronic device is also shown (not to scale).