[EN]In recent years, the development of GaN technology has made significant inroads into
high-power and high-frequency applications with respect to other semiconductor
competitors such as GaAs or InP.
In this dissertation, by means of an in-house Monte Carlo tool, we study the possibility
of generating Gunn oscillations through vertical n+nn+ (without notch) and n+n-nn+
(notched) diodes based on InP and GaN with different lengths of the active region and
two doping profiles. In general, when the notch accomplishes its role of fixing the onset
of charge accumulation near the cathode, the oscillations are of lower frequency and
power. For InP-based diodes, the fundamental frequency reaches 140 GHz (notched,
L=1.2 μm) and 400 GHz (without notch, L=0.75 μm). For the GaN-based diode with an
active length L=1.5 μm, despite the fact that the fundamental harmonic is around
100 GHz, the power spectral density for the 10th harmonic ( 1 THz) is still significant. InP
diodes with L=0.9 μm offer an efficiency (η) of up to 5.5 % for frequencies around
225 GHz. The generation bands in GaN diodes appear at higher frequencies (up to
675 GHz with η=0.1 %) than in InP.
When circuits work at high powers, thermal models become essential to determine
temperature limits with a view to preventing device failure, thus reducing manufacturing
costs. In order to include thermal effects in our Monte Carlo code, two techniques have
been implemented: (i) a thermal resistance method (TRM), and (ii) an advanced electrothermal
model that solves the steady-state heat diffusion equation, called HDEM. We
calibrate/validate our simulator by comparison with experimental measurements of an
AlGaN/GaN diode. For the TRM, several thermal resistance values are employed, and for
the HDEM different substrates (polycrystalline diamond, diamond, carbide silicon, silicon
and sapphire), thicknesses and die lengths are tested. In addition, we include the effect
of thermal boundary resistance. Using temperature-independent thermal conductivity
in the HDEM allows us to extract an equivalent thermal resistance, Rth, for each geometry
and substrate material. The extracted Rth values are constant with the dissipated power,
Pdiss. However, when a more real temperature-dependent thermal conductivity is
employed, Rth exhibits a strong dependence on Pdiss. As final test device, we analyse for
an HEMT, the effect of (i) the heat-sink temperature and (ii) gate-length, through electrothermal
simulations.
