Power semiconductor devices based on silicon (Si) are quickly approaching their limits, set by fundamental material properties. In order to address these limitations, new materials for use in devices must be investigated. Wide bandgap materials, such as silicon carbide (SiC) and gallium nitride (GaN) have suitable properties for power electronic applications; however, fabrication of practical devices from these materials may be challenging. SiC technology has matured to point of commercialized devices, whereas GaN requires further research to realize full material potential. This review covers fundamental material properties of GaN as they relate to Si and SiC. This is followed by a discussion of the contemporary issues involved with bulk GaN substrates and their fabrication and a brief overview of how devices are fabricated, both on native GaN substrate material and non-native substrate material. An overview of current device structures, which are being analyzed for use in power switching applications, is then provided; both vertical and lateral device structures are considered. Finally, a brief discussion of prototypes currently employing GaN devices is given.

GaN Technology for Power Electronic Applications: A Review

A survey of Gallium Nitride HEMT for RF and high power applications

A Decade of Nonpolar and Semipolar III-Nitrides: A Review of Successes and Challenges

This paper reviews the state of emerging transistor technologies capable of terahertz amplification, as well as the state of transistor modeling as required in terahertz electronic circuit research. Commercial terahertz radar sensors of today are being built using bulky and expensive technologies such as Schottky diode detectors and lasers, as well as using some emerging detection methods. Meanwhile, a considerable amount of research effort has recently been invested in process development and modeling of transistor technologies capable of amplifying in the terahertz band. Indium phosphide (InP) transistors have been able to reach maximum oscillation frequency (fmax) values of over 1 THz for around a decade already, while silicon-germanium bipolar complementary metal-oxide semiconductor (BiCMOS) compatible heterojunction bipolar transistors have only recently crossed the fmax = 0.7 THz mark. While it seems that the InP technology could be the ultimate terahertz technology, according to the fmax and related metrics, the BiCMOS technology has the added advantage of lower cost and supporting a wider set of integrated component types. BiCMOS can thus be seen as an enabling factor for re-engineering of complete terahertz radar systems, for the first time fabricated as miniaturized monolithic integrated circuits. Rapid commercial deployment of monolithic terahertz radar chips, furthermore, depends on the accuracy of transistor modeling at these frequencies. Considerations such as fabrication and modeling of passives and antennas, as well as packaging of complete systems, are closely related to the two main contributions of this paper and are also reviewed here. Finally, this paper probes active terahertz circuits that have already been reported and that have the potential to be deployed in a re-engineered terahertz radar sensor system and attempts to predict future directions in re-engineering of monolithic radar sensors.

https://www.mdpi.com/1424-8220/19/11/2454/pdf

Emerging Transistor Technologies Capable of Terahertz Amplification: A Way to Re-Engineer Terahertz Radar Sensors.

We report high-voltage regrown nonpolar ${m}$ -plane p-n diodes on freestanding GaN substrates. A high blocking voltage of 540 V at ~1 mA/cm $^{\textsf {2}}$ (corresponding to an electric field of E ~ 3.35 MV/cm), turn- ON voltages between 2.9 and 3.1 V, specific on-resistance of 1.7 $\text{m}\Omega \cdot \textsf {cm}^{\textsf {2}}$ at 300 A/cm $^{\textsf {2}}$ , and a minimum ideality factor of 1.7 were obtained for the regrown diodes. Our results suggest that Si, O, and C interfacial impurity levels up to $\textsf {2}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , $\textsf {8}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , and $\textsf {1}\times \textsf {10}^{\textsf {19}}$ cm $^{-\textsf {3}}$ , respectively, at the metallurgical junction of ${m}$ -plane, p-n diodes do not result in very early breakdown in the reverse bias although the off-state leakage current in the forward bias is affected. The impact of the growth interruption/regrowth on diode performance is also investigated.

High-Voltage Regrown Nonpolar ${m}$ -Plane Vertical p-n Diodes: A Step Toward Future Selective-Area-Doped Power Switches

A novel high breakdown voltage (BV) AlGaN/GaN high-electron mobility transistor (HEMT) with a high-K/low-K compound passivation layer is proposed. The compound passivation layer is formed by blocks of low-K dielectric (Si3N4) embedded in a high-K passivation layer (La2O3). Owing to their different dielectric constants, there is a discontinuity of the horizontal electrical field at the high-K/low-K interface, which can introduce a new electric field peak in the nearby channel in the semiconductor and can also modulate the distribution of the electric field along the channel. Hence, enhancement of BV can be achieved. Compared to the typical field-plate structure, high-K/low-K passivation introduces no parasitic capacitance. On the basis of the physical mechanism, several design principles for the high-K/low-K passivation layer are presented. Numerical simulation demonstrates a BV of 1400 V for the proposed device with four blocks of low-K dielectric embedded in a high-K passivation, compared to the BVs of 917 and 288 V for the device with high-K passivation and the device with low-K passivation, respectively.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/el.2014.3252

High breakdown voltage AlGaN/GaN HEMT with high-K/low-K compound passivation

DC and pulsed transfer characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) have been systematically investigated. A significant difference of transconductance linearity between DC and gate-pulsed measurements is clearly observed. The acceptor-like traps in the barrier layer under the gate is the main cause of non-linear behavior of AlGaN/GaN HEMTs transconductance. A physical model has been constructed to explain the phenomenon. In the modeling, an acceptor-like trap concentration of 1.2 × 1019 cm−3 with an energy level of 0.5 eV below the conduction band minimum shows the best fit to measurement results.

Study on transconductance non-linearity of AlGaN/GaN HEMTs considering acceptor-like traps in barrier layer under the gate

Design of high breakdown voltage GaN vertical HFETs with p-GaN buried buffer layers for power switching applications

In this paper, we report a new phenomenon in C-V measurement of different gate length MIS-HEMTs, which can be associated with traps character of the AlGaN/GaN interface. The analysis of DC measurement, frequency dependent capacitance-voltage measurements and simulation show that the stress from passivation layer may induce a decrease of drain output current I ds , an increase of on-resistance, serious nonlinearity of transconductance g m , and a new peak of C-V curve. The value of the peak is reduced to zero while the gate length and measure frequency are increasing to 21 μm and 1 MHz, respectively. By using conductance method, the SiN x /GaN interface traps with energy level of E C −0.42 eV to E C −0.45 eV and density of 3.2 × 10 12  ∼ 5.0 × 10 12  eV −1  cm −2 is obtained after passivation. According to the experimental and simulation results, formation of the acceptor-like traps with concentration of 3 × 10 11  cm −2 and energy level of E C −0.37 eV under the gate on AlGaN barrier side of AlGaN/GaN interface is the main reason for the degradation after the passivation.

Zhiyuan Bai

Papers

High breakdown voltage AlGaN/GaN HEMT with high-K/low-K compound passivation

Study on transconductance non-linearity of AlGaN/GaN HEMTs considering acceptor-like traps in barrier layer under the gate

Design of high breakdown voltage GaN vertical HFETs with p-GaN buried buffer layers for power switching applications

Study on the electrical degradation of AlGaN/GaN MIS-HEMTs induced by residual stress of SiN x passivation

Design of high breakdown voltage GaN-based vertical HFETs with p-GaN island structure for power applications