RF Performance of Ultra-wide Band Gap HEMTs
53
In most of the RF power applications, high values of PAs and PAE are required;
reduced power consumption of the high power devices have less cooling requirement, which facilitates reduced device size and density of the system [8]. More
output power was achieved using longer FP (up to a certain length), although gateconnected FP causes additional gate-to-drain capacitance (C gd ) which deteriorates
current gain and power-gain cut-off frequencies [6]. Recently, Romanczyk et al. [30]
demonstrated N-polar GaN HEMTs which maintains the same output power density
at different frequencies: 10, 30, and 94 GHz, with peak power density 8 W/mm (total
output power = 600 mW, power density × gate width ~0.7 μm). Further, improved
power performance was shown by POC HEMT [31] with peak PAE of 71.6% and
power density 10.4 W/mm at 5 GHz. High-efficiency RF devices with high P OUT are
vital to fully exploit the superior qualities of GaN material [8]. For high-frequency
RF applications, higher current gain cut-off frequency (f T ) and maximum powergain oscillation frequency (f MAX ) are required for operation beyond Ka-band (26–
40 GHz). Device scaling has successfully increased f T and f MAX of GaN HEMTs
but at the cost of lower breakdown voltage [8]. AlGaN/GaN HEMT were grown by
metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates with different
Al concentrations (20, 27, and 35%) showed f T (24.9, 34.6, and 50 GHz) and f MAX
(54.9, 61.8, and 100.9 GHz) [32]. Additionally, high-frequency GaN HEMTs for RF
MMIC application-based devices [29, 33–36] such as: enhancement-mode (E-mode)
GaN HEMT with ultrashort gate length (160 nm) achieved f T of 85 GHz and f MAX of
150 GHz [33], 60-nm-gate-length HEMT with recessed AlGaN barrier layer exhibited f MAX of 300 GHz [34], and using tall-stem T-gate N-polar GaN/InAlN HEMT
achieved f MAX of 400 GHz [35]. Figure 2 shows small signal RF performance of
GaN-based HEMTs, Refs. [34, 35].
To improve the performance of GaN HEMTs for mm-wave applications,
researchers have developed new process and device technologies—non-alloyed
ohmic contact [37, 38] using ion-implantation to get smooth surface and so reduced
gate-drain as well as gate-source spacing which results in further lowering the
f max = 300 GHz
f T = 70 GHz
f max = 400 GHz
f T = 142 GHz
(a)
(b)
Fig. 2 Small signal RF performance of GaN HEMTs. a L G = 60 nm, W = 50 μm, V DS = 16 V.
b L G = 100 nm, W = 25 μm, V DS = 8 V
53
In most of the RF power applications, high values of PAs and PAE are required;
reduced power consumption of the high power devices have less cooling requirement, which facilitates reduced device size and density of the system [8]. More
output power was achieved using longer FP (up to a certain length), although gateconnected FP causes additional gate-to-drain capacitance (C gd ) which deteriorates
current gain and power-gain cut-off frequencies [6]. Recently, Romanczyk et al. [30]
demonstrated N-polar GaN HEMTs which maintains the same output power density
at different frequencies: 10, 30, and 94 GHz, with peak power density 8 W/mm (total
output power = 600 mW, power density × gate width ~0.7 μm). Further, improved
power performance was shown by POC HEMT [31] with peak PAE of 71.6% and
power density 10.4 W/mm at 5 GHz. High-efficiency RF devices with high P OUT are
vital to fully exploit the superior qualities of GaN material [8]. For high-frequency
RF applications, higher current gain cut-off frequency (f T ) and maximum powergain oscillation frequency (f MAX ) are required for operation beyond Ka-band (26–
40 GHz). Device scaling has successfully increased f T and f MAX of GaN HEMTs
but at the cost of lower breakdown voltage [8]. AlGaN/GaN HEMT were grown by
metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates with different
Al concentrations (20, 27, and 35%) showed f T (24.9, 34.6, and 50 GHz) and f MAX
(54.9, 61.8, and 100.9 GHz) [32]. Additionally, high-frequency GaN HEMTs for RF
MMIC application-based devices [29, 33–36] such as: enhancement-mode (E-mode)
GaN HEMT with ultrashort gate length (160 nm) achieved f T of 85 GHz and f MAX of
150 GHz [33], 60-nm-gate-length HEMT with recessed AlGaN barrier layer exhibited f MAX of 300 GHz [34], and using tall-stem T-gate N-polar GaN/InAlN HEMT
achieved f MAX of 400 GHz [35]. Figure 2 shows small signal RF performance of
GaN-based HEMTs, Refs. [34, 35].
To improve the performance of GaN HEMTs for mm-wave applications,
researchers have developed new process and device technologies—non-alloyed
ohmic contact [37, 38] using ion-implantation to get smooth surface and so reduced
gate-drain as well as gate-source spacing which results in further lowering the
f max = 300 GHz
f T = 70 GHz
f max = 400 GHz
f T = 142 GHz
(a)
(b)
Fig. 2 Small signal RF performance of GaN HEMTs. a L G = 60 nm, W = 50 μm, V DS = 16 V.
b L G = 100 nm, W = 25 μm, V DS = 8 V