Michael E. Levinshtein

Paradoxes of carrier lifetime measurements in high-voltage SiC diodes

For Silicon Carbide (SiC) high-voltage rectifier diodes, contradictions appear when the most important parameter of the diodes, the minority carrier lifetime, is measured by different techniques. A qualitative analysis and a computer simulation have been carried out to clarify the origin of these contradictions. For 4H-SiC p/sup +/n diodes with 6 kV blocking capability, data on residual voltage drop at high current densities, switch-on time, reverse current recovery, and post-injection voltage decay are analyzed. It is shown that the whole set of experimental data can be explained by the existence of a thin (l/spl sim/0.1 /spl mu/m) layer near the metallurgical boundary of the p/sup +/n junction with very small carrier lifetime /spl tau//sub l/ that is essentially smaller than the carrier lifetime /spl tau/ across the remaining part of the 50-/spl mu/m n-base. It is emphasized that the existence of such a layer allows, under certain conditions, the combination of a relatively low residual forward voltage drop and very fast reverse recovery. Approaches to minority carrier lifetime measurements are discussed.

Low-frequency noise in AlGaN/GaN heterojunction field effect transistors on SiC and sapphire substrates

Low frequency noise has been investigated in gallium nitride/gallium–aluminum nitride GaN/GaAlN field effect transistors grown on sapphire and silicon carbide (SiC) substrates under identical conditions. GaN/AlGaN heterostructures grown on SiC substrate have a lower level of 1/f noise and a higher electron mobility compared to samples grown on sapphire. The noise of the gate leakage current Ig can give the main contribution to the output noise of the drain current Id even at Ig/Id ratios as small as 10−4–10−5. For the structures grown on SiC, a very weak temperature dependence of the low frequency noise was found in the temperature range 300 320u200aK. The effect of band-to-band illumination on the low-frequency noise is similar to that for silicon and gallium arsenide (GaAs) based transistors. The Hooge parameter α for the wafers grown on...

Turn-on process in 4H-SiC thyristors

Detailed turn-on measurements of 4H-Silicon Carbide (SiC) npnp thyristors are presented for a wide range of operating conditions. Comparisons with similarly-rated silicon and Gallium Arsenide thyristors show a superior rise time and pulsed turn-on performance of SiC thyristors. Rise time for a 400 V blocking voltage, 4 V forward drop (2.8/spl times/10/sup 3/ A/cm/sup 2/) SiC thyristor has been found to be of the order of 3-5 ns. Pulsed turn on measurements show a residual voltage of only 50 V when a current density of 10/sup 5/ A/cm/sup 2/ (35 A) was achieved in 20 ns.

MATERIALS PROPERTIES OF NITRIDES: SUMMARY

The interest in III-N materials (stimulated by pioneering work of Pankove, Akasaki, Nakamura, and many others) dates back to 1970s. High-power microwave/millimeter wave and optoelectronic applications of nitrides have emerged, with nitride-based visible LEDs already accounting for billion dollar markets. These applications demand the improved materials quality and better device design, which in turn require the knowledge of nitride materials parameters. Our recent book published in 2001 1 has listed important nitride parameters. However, intensive research on properties and device applications is continuing albeit with a saturated rate of growth (see Fig. 1.). The new results require revisions of the important parameters for these materials (the energy gap of InN being a dramatic example .) The goal of this Chapter is to provide such an update.

Failure mechanisms in blue InGaN/GaN LEDs for high power operation

Unpredictable fast failure of blue power InGaN/GaN LEDs is caused by redistribution of In under action of injection currents between nano-scale regions of InGaN alloy with non-equilibrium composition. Unreliable LEDs can be recognized by the increase in forward current values at U < 2 V which is not accompanied by simultaneous reversed current increase during short aging tests (less than 100 h).

Generation-recombination noise in forward biased 4H‐SiC p‐n diodes

Low frequency noise has been studied in forward biased 4H‐SiC p+‐n diodes at current densities from 10−4to10A∕cm2. At small current densities j⩽10−3A∕cm2, the spectral noise density SI follows the law SI∝1∕f3∕2. At 10−3A∕cm2 <j<10−2A∕cm2, the generation-recombination (GR) noise predominates. The amplitude of this GR noise nonmonotonically depends on current. At j⩾10−2A∕cm2, the 1∕f (flicker noise) dominates. It has been shown that the recombination time in the space charge region of the p+‐n junction, τR, is about 70ns. This value is approximately one order of magnitude larger than that reported earlier for SiC p‐n structures. A model of GR noise in forward biased p‐n junctions has been proposed. The model links the GR noise with fluctuations of the charge state of a trap in the space charge region.

Impact of high energy electron irradiation on high voltage Ni/4H-SiC Schottky diodes

We report the results of the high energy (0.9u2009MeV) electron irradiation impact on the electrical properties of high voltage Ni/4H-SiC Schottky diodes. Within the range of the irradiation dose from 0.2u2009×u20091016u2009cm−2 to 7u2009×u20091016u2009cm−2, electron irradiation led to 6 orders of magnitude increase in the base resistance, appearance of slow relaxation processes at pico-ampere current range, and increase in the ideality factor.

Generation-recombination noise in GaN and GaN-based devices

Generation-recombination (GR) noise in GaN and AlGaN thin films, GaN based Metal Semiconductor Field Effect Transistors (MESFETs), Heterostructure Field Effect Transistors (HFETs) and Schottky diode photodetectors was investigated. AlGaN thin films, AlGaN/GaN HFETs and Schottky barrier Al0.4Ga0.6N diodes exhibited GR noise with activation energies of 0.8 - 1 eV. AlGaN/GaN HFETs also presented GR noise with activation energies of 1 - 3 meV and 0.24 eV at cryogenic temperatures. No such noise was observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN/InGaN/GaN Double Heterostructure Field Effect Transistors (DHFETs). We conclude that the local levels responsible for the observed noise in HFETs and DHFETs could be located in AlGaN barrier layers.

Specific features of the switch-on gate current and different switch-on modes in silicon carbide thyristors

The specific features of the temperature and bias dependences of the switch-on gate current in SiC thyristors are examined analytically for two possible switching mechanisms. The so-called ?-mechanism, which is highly typical of the conventional Si thyristors, is characterized by very weak temperature and bias dependences. By contrast, the so-called ?-mechanism, which is very characteristic of SiC thyristors, is highly sensitive to changes in temperature and bias. If the thyristor is switched on by the ?-mechanism, the switch-on gate current density decreases very steeply with increasing temperature. As a result, the thyristor can lose its working capacity at elevated temperatures due to the instability against even very weak impacts. With decreasing the bias voltage Ua, the gate switch-on current increases very steeply, which can make switching the thyristor on difficult. The unintentional shunting, which is apparently present in high-voltage SiC thyristors, causes the transition from the ?- to the ?-mechanism at elevated temperatures and high biases. It can be supposed that introduction of a controllable technological shunting of the emitter?thin base junction allows stabilization of the temperature and bias parameters of SiC thyristors. The analytical results are confirmed by computer simulations performed in wide temperature and bias ranges for a 4H-SiC thyristor of the 18 kV class.

Low Frequency Noise in Gallium Nitride Field Effect Transistors

We report on experimental study of the low frequency noise in GaN-based Field Effect Transistors. In both GaN Metal Semiconductor Field Effect Transistors (MESFETs) and AlGaN/GaN Heterostructure Field Effect Transistors (HFETs), the main noise sources are located in the channel. Gate voltage dependence of noise in MESFETs complies with the Hooge formula and indicates the bulk origin of noise. The dependencies of the Hooge parameter, α, on sheet electron concentration ns in HFETs are extracted from measured drain current fluctuations taking into account the contact resistance, and the resistance of the ungated regions of the transistors. At low channel concentrations α is inversely proportional to ns (α ~ 1/ns). This dependence as well as the temperature dependence of noise might be explained by electron tunneling from the 2D gas into the traps in the bulk GaN or AlGaN.