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R. Singh et al.
In the upper THz frequency band, THz imaging and spectroscopy systems need
higher photon densities to overcome diffraction, absorption, and the large beam
diameter issue. UWBG semiconductor devices with high output power and highfrequency operation provide promising features for implementing such devices [11].
Here, we discuss key technological development of GaN and β-Ga 2 O 3 based
HEMTs, quantitative valuation of corresponding parameters reported to date in terms
of their performance for RF and DC high power applications, and their possible role
in imaging and spectroscopy of THz systems.
2 GaN-Based HEMTs
AlGaN/GaN HEMTs having spontaneous and piezoelectric polarization induced
2DEG channel of carrier density ~10
13 cm
−2 with high electron mobility up to
~2000 cm
2 V
−1 s
−1 (epi-layer) facilitates a low resistive channel, resulting in exceptionally low device on-resistance. The reasonable high saturation velocity ~2 × 10
7
cm s
−1 enabled high current densities—high P OUT , and efficient RF power operation. Higher value of E g supported high critical electric field of GaN, which is ten
times higher than Si. AlGaN/GaN HEMTs possesses large blocking voltage and high
drain voltage resulting high P OUT density with high output impedance. This facilitates
easier matching between multistage amplifiers with lower reflective losses.
2.1 GaN Power Amplifier
Excellent progress in material growth techniques such as epitaxy (MBE) and chemical vapor deposition (MOCVD) enabled high electron mobility in GaN-based
HEMTs. Moreover, high breakdown voltage-empowered GaN HEMTs find applications as power amplifiers (PAs) with sufficient power added efficiency (PAE), also
show higher output power and significant power density over materials such as Si,
GaAs, or InP. For better thermal conductivity, SiC substrate is preferred, which made
this technology to be commercialized for various applications: pulsed radars, cable
television (CATV) amplifier modules, and mobile base stations [8]. After the demonstration of first GaN-based HEMT by Asif Khan et al. [12] in year 1994, in a couple of
years, Wu et al. [13] demonstrated first RF measurement on an AlGaN/GaN HEMT
with P OUT = 1.1 W/mm at 2 GHz with a PAE = 18.6%. Although due to ‘dispersion’
microwave output power of earlier GaN devices were severely affected, primarily
because of trap related phenomena [1, 14, 15]. Subsequent research addressed the
issue by the use Si 3 N 4 passivation [16] of undoped AlGaN/GaN HEMTs, which pave
the way for increased output power up to 32.2 W/mm [17–29], and 40 W/mm [24]
at 4 GHz using field plate (FP).
R. Singh et al.
In the upper THz frequency band, THz imaging and spectroscopy systems need
higher photon densities to overcome diffraction, absorption, and the large beam
diameter issue. UWBG semiconductor devices with high output power and highfrequency operation provide promising features for implementing such devices [11].
Here, we discuss key technological development of GaN and β-Ga 2 O 3 based
HEMTs, quantitative valuation of corresponding parameters reported to date in terms
of their performance for RF and DC high power applications, and their possible role
in imaging and spectroscopy of THz systems.
2 GaN-Based HEMTs
AlGaN/GaN HEMTs having spontaneous and piezoelectric polarization induced
2DEG channel of carrier density ~10
13 cm
−2 with high electron mobility up to
~2000 cm
2 V
−1 s
−1 (epi-layer) facilitates a low resistive channel, resulting in exceptionally low device on-resistance. The reasonable high saturation velocity ~2 × 10
7
cm s
−1 enabled high current densities—high P OUT , and efficient RF power operation. Higher value of E g supported high critical electric field of GaN, which is ten
times higher than Si. AlGaN/GaN HEMTs possesses large blocking voltage and high
drain voltage resulting high P OUT density with high output impedance. This facilitates
easier matching between multistage amplifiers with lower reflective losses.
2.1 GaN Power Amplifier
Excellent progress in material growth techniques such as epitaxy (MBE) and chemical vapor deposition (MOCVD) enabled high electron mobility in GaN-based
HEMTs. Moreover, high breakdown voltage-empowered GaN HEMTs find applications as power amplifiers (PAs) with sufficient power added efficiency (PAE), also
show higher output power and significant power density over materials such as Si,
GaAs, or InP. For better thermal conductivity, SiC substrate is preferred, which made
this technology to be commercialized for various applications: pulsed radars, cable
television (CATV) amplifier modules, and mobile base stations [8]. After the demonstration of first GaN-based HEMT by Asif Khan et al. [12] in year 1994, in a couple of
years, Wu et al. [13] demonstrated first RF measurement on an AlGaN/GaN HEMT
with P OUT = 1.1 W/mm at 2 GHz with a PAE = 18.6%. Although due to ‘dispersion’
microwave output power of earlier GaN devices were severely affected, primarily
because of trap related phenomena [1, 14, 15]. Subsequent research addressed the
issue by the use Si 3 N 4 passivation [16] of undoped AlGaN/GaN HEMTs, which pave
the way for increased output power up to 32.2 W/mm [17–29], and 40 W/mm [24]
at 4 GHz using field plate (FP).