Zaffar H. Zaidi

Highly Sensitive UV Detection Mechanism in AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors are demonstrated with a very high dc responsivity ( ~ 4.3×107 A/W at ~ 5×10-11 W) to UV light. The gain mechanisms in the device are shown to be due to a photo voltage effect in both the AlGaN barrier layer and the GaN buffer layer. In the case of absorption in the barrier layer, trapped surface electrons, which are a characteristic of unpassivated devices, are neutralized by the movement of holes toward the surface, creating a net positive-gate bias. The photovoltage generated in the GaN buffer region is due to photocarrier separation and acts as a positive back-gate bias. The dc photocurrent response and responsivity are very strong functions of intensity that are explained by the proposed mechanisms, with absorption in the GaN buffer region dominating.

Enhancement-mode metal–insulator–semiconductor GaN/AlInN/GaN heterostructure field-effect transistors on Si with a threshold voltage of +3.0 V and blocking voltage above 1000 V

Enhancement-mode AlInN/GaN metal–insulator–semiconductor heterostructure field-effect transistors on silicon are reported. A fluorine-based plasma treatment and gate dielectric are employed, and the devices exhibit a threshold voltage of +3 V. A drain current density of 295 mA/mm for a gate bias of +10 V is measured. An excellent off-state blocking voltage capability of 630 V for a leakage current of 1 µA/mm, and over 1000 V for 10 µA/mm are achieved on a 20-µm-gate–drain separation device at gate bias of 0 V. The dynamic on-resistance is ~2.2 times the DC on-resistance when pulsing from an off-state drain bias of 500 V.

Enhancement mode operation in AlInN/GaN (MIS)HEMTs on Si substrates using a fluorine implant

The authors acknowledge financial support from the Engineering and Physics Sciences Research Council (EPSRC) under EP/K014471/1 (Silicon Compatible GaN Power Electronics).

Impact of buffer charge on the reliability of carbon doped AlGaN/GaN-on-Si HEMTs

Charge trapping and transport in the carbon doped GaN buffer of an AlGaN/GaN-on-Si high electron mobility transistor (HEMT) have been investigated. Back-gating and dynamic Ron experiments show that a high vertical leakage current results in significant long-term negative charge trapping in the buffer leading to current collapse under standard device operating conditions. Controlling current-collapse requires control of not only the layer structures and its doping, but also the precise balance of leakage in each layer.

Sulfuric acid and hydrogen peroxide surface passivation effects on AlGaN/GaN high electron mobility transistors

In this work, we have compared SiNx passivation, hydrogen peroxide, and sulfuric acid treatment on AlGaN/GaN HEMTs surface after full device fabrication on Si substrate. Both the chemical treatments resulted in the suppression of device pinch-off gate leakage current below 1 μA/mm, which is much lower than that for SiNx passivation. The greatest suppression over the range of devices is observed with the sulfuric acid treatment. The device on/off current ratio is improved (from 104–105 to 107) and a reduction in the device sub-threshold (S.S.) slope (from ∼215 to 90 mV/decade) is achieved. The sulfuric acid is believed to work by oxidizing the surface which has a strong passivating effect on the gate leakage current. The interface trap charge density (Dit) is reduced (from 4.86 to 0.90 × 1012 cm−2 eV−1), calculated from the change in the device S.S. The gate surface leakage current mechanism is explained by combined Mott hopping conduction and Poole Frenkel models for both untreated and sulfuric acid treat...

Nanoscale structural and chemical analysis of F-implanted enhancement-mode InAlN/GaN heterostructure field effect transistors

We investigate the impact of a fluorine plasma treatment used to obtain enhancement-mode operation on the structure and chemistry at the nanometer and atomic scales of an InAlN/GaN field effect transistor. The fluorine plasma treatment is successful in that enhancement mode operation is achieved with a +2.8 V threshold voltage. However, the InAlN barrier layers are observed to have been damaged by the fluorine treatment with their thickness being reduced by up to 50%. The treatment also led to oxygen incorporation within the InAlN barrier layers. Furthermore, even in the as-grown structure, Ga was unintentionally incorporated during the growth of the InAlN barrier. The impact of both the reduced barrier thickness and the incorporated Ga within the barrier on the transistor properties has been evaluated theoretically and compared to the experimentally determined two-dimensional electron gas density and threshold voltage of the transistor. For devices without fluorine treatment, the two-dimensional electron...

Effects of surface plasma treatment on threshold voltage hysteresis and instability in metal-insulator-semiconductor (MIS) AlGaN/GaN heterostructure HEMTs

In a bid to understand the commonly observed hysteresis in the threshold voltage (VTH) in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors during forward gate bias stress, we have analyzed a series of measurements on devices with no surface treatment and with two different plasma treatments before the in-situ Al2O3 deposition. The observed changes between samples were quasi-equilibrium VTH, forward bias related VTH hysteresis, and electrical response to reverse bias stress. To explain these effects, a disorder induced gap state model, combined with a discrete level donor, at the dielectric/semiconductor interface was employed. Technology Computer-Aided Design modeling demonstrated the possible differences in the interface state distributions that could give a consistent explanation for the observations.

Vertical leakage mechanism in GaN on Si high electron mobility transistor buffer layers

Control of leakage currents in the buffer layers of GaN based transistors on Si substrates is vital for the demonstration of high performance devices. Here, we show that the growth conditions during the metal organic chemical vapour deposition growth of the graded AlGaN strain relief layers (SRLs) can significantly influence the vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron microscopy, we investigate the origins of leakage paths and show that they result from the preferential incorporation of oxygen impurities on the side wall facets of the inverted hexagonal pyramidal pits which can occur during the growth of the graded AlGaN SRL. We also show that when 2D growth of the AlGaN SRL is maintained a significant increase in the breakdown voltage can be achieved even in much thinner buffer layer structures. These results demonstrate the importance of controlling the morphology of the high electron mobility transistor buffer layer as even at a very low density the leakage paths identified would provide leakage paths in large area devices.

Characterization of p-GaN1−xAsx/n-GaN PN junction diodes

The structural properties and electrical conduction mechanisms of p-type amorphous GaN1−x As x /n-type crystalline GaN PN junction diodes are presented. A hole concentration of 8.5 × 1019 cm−3 is achieved which allows a specific contact resistance of 1.3 × 10−4 Ω cm2. An increased gallium beam equivalent pressure during growth produces reduced resistivity but can result in the formation of a polycrystalline structure. The conduction mechanism is found to be influenced by the crystallinity of the structure. Temperature dependent current voltage characteristics at low forward bias (<0.35 V) show that conduction is recombination dominated in the amorphous structure whereas a transition from tunneling to recombination is observed in the polycrystalline structure. At higher bias, the currents are space charge limited due to the low carrier density in the n-type region. In reverse bias, tunneling current dominates at low bias (<0.3 V) and recombination current becomes dominant at higher reverse bias.