Operational frequency degradation induced trapping in scaled GaN HEMTs
Introduction
A huge potential of gallium nitride (GaN) high electron mobility transistors (HEMTs) in future radio frequency (RF) and power applications has been fuelling their research and development in the recent years. Material properties of wide-band-gap nitrides such as spontaneous (SP) and piezoelectric (PZ) polarizations allow creation of a two-dimensional electron gas (2DEG) with sheet densities above 1012 cm− 2, a high electron mobility (up to 2000 cm2V− 1s− 1 in 2DEG), a large energy band gap (3.4 eV), a good thermal conductivity (160 WK− 1m− 1) ensuring a good heat dissipation, and a very high breakdown field (3.3 MV cm− 1) make it an ideal candidate for all devices requiring fast carrier transport with high breakdown voltage [1], [2]. However, the maximum RF power output is not always reproducible due to the existence of defects/traps in the GaN device structure inhibiting the wide commercial use of these devices. The trap related phenomena, affecting the reliability of device, may result in a reduction of the drain current, virtual gate formation, transconductance frequency dispersion [3], light sensitivity [4] and restricted microwave output power [5]. These effects are largely due to the formation of a non-equilibrium charge distribution in the device. Various proposals have been made regarding the nature, location, and effects of traps in GaN devices [6] such as a correlation between RF output power degradation and bulk traps [7], and a high electrical stress induced traps [6] opposing the traditional explanation by a self-heating [8].
Since experimental techniques are insufficient to provide a detailed understanding of carrier transport process in GaN based devices, physically-based simulations are required to uncover complex transport phenomena [9]. Since the GaN HEMTs for power and RF applications have typically microscale (power/RF) or sub-microscale dimensions (RF), drift-diffusion and hydrodynamics transport models [1] are suitable for fast turn-out modelling while ensemble Monte Carlo simulations are used for in-depth modelling into details of hot-carrier non-equilibrium transport [10]. A compromise is a combination of the Fermi kinetic transport model with Maxwell equations to efficiently capture the essential physics of hot-carriers to model the electromagnetic wave effects in the HEMTs [11].
This paper studies the effects of electric field induced bulk traps on the DC and RF characteristics of GaN HEMTs. The investigations are carried out using Atlas simulation toolbox by Silvaco [12] using the drift-diffusion transport model for all simulations. The study is based on careful calibration of device I-V characteristics using a well-developed methodology [1]. The RF parameters are calculated from the small signal conductance and capacitance values during small signal analysis. The admittance parameters (Y) are obtained directly from the small signal parameters and then converted to scattering parameters (S) using matrix transformation [12].
Experimentally reported values for the cut-off frequency for GaN HEMTs have a fT × LG product of 13 GHz μm [13], although this can be improved by increasing the gate recess depth [14] and has been shown to depend on the substrate material [15]. We will show that results from our simulations are in a good agreement with the experimentally reported ones [16], [17]. We give an in-sight, firstly, on the variation of drain current and cut-off frequency with scaling down the device. Thereafter, traps are placed in the device according to Ref. [6] in order to predict their effect on the DC and RF performance of the device. The induced traps are predicted to cause a positive shift in the threshold voltage and a general reduction in the cut-off frequency.
Section snippets
Device structure and simulation methodology
Fig. 1 illustrates the schematic cross-section of an investigated asymmetrical 1 μm gate length GaN HEMT. It consists of a 2 nm GaN cap, 20 nm AlGaN barrier, 1 nm AlN spacer, and 3 μm GaN buffer, all grown on a SiC substrate with source-to-gate and gate-to-drain separations of LSG = 2 μm and LGD = 3 μm, respectively. Fig. 2 shows the conduction band profile overlapped with electron current density showing the role of the thin aluminium nitride (AlN) layer in additional confinement of the 2DEG.
The
Lateral and vertical scaling of the device
The maximum drain current for the investigated 1 μm gate length GaN HEMT is IDmax = 516.7 mA/mm (see Fig. 3) while the extracted cut-off frequency is f = 12.1 GHz which closely agrees with the experimental results of 11.5 GHz (1 μm gate length Al0.22Ga0.78N/GaN HEMT on sapphire) reported in Ref. [17]. We have then scaled down the gate length from LG = 1 μm to LG = 0.5 μm, LG = 0.25 μm, LG = 0.125 μm and, correspondingly, the source-to-gate and gate-to-drain distances in order to predict possible improvements in the
Trapping effects on DC and RF performance
Although degradation mechanisms in GaN HEMTs have yet to be fully understood, a number of explanations have been advanced as possible mechanisms leading to degradation in these devices. Some of these include: (i) virtual gate formation as a result of tunnelling from the gate [25]; (ii) Poole-Frenkel gate leakage surface conduction [26]; (iii) hot electron trapping and interface state creation [27]; (iv) hot electron trapping at the surface and AlGaN barrier [28], [29]; (v) trap generation in
Trapping effects on a laterally scaled device
In order to gain some insight into how device behaviour in a laterally scaled device is affected by the traps, we have scaled down the traps generation region in proportion to the various device dimensions. Assuming a relatively small value of the trap density up to 1017 cm− 3 for smaller device dimensions may not, in reality, represent accurately the effects of traps as smaller devices would experience higher electric fields under similar biases. We observe in Fig. 9 that the current is
Conclusions
The detrimental effect of traps generated by a high electric field in the stressed GaN HEMTs has been studied in scaled devices. The study is based on accurate calibration of the TCAD model to experimental transfer and output characteristics of 1 μm gate length GaN HEMT with fT = 12.1 GHz obtained using commercial simulation software Atlas by Silvaco.
The scaled device exhibits improvement in the RF performance by increase in the cut-off frequency (fT = 12.1 GHz) by 118% (fT = 26.4 GHz), 330% (fT = 52.1 GHz)
Acknowledgements
This research is funded by the Sêr Cymru National Research Network in Advanced Engineering and Materials [Grant code: NRN081]. We thank Edward Wasige and Abdullah Al-Khalidi of the University of Glasgow for providing us with the experimental data.
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