Showing papers by "Michael Mikulla published in 2015"

Integrated reverse-diodes for GaN-HEMT structures

Abstract: This work introduces an enhanced GaN-HEMT structure that uses separated Schottky contacts as integrated free-wheeling diodes for the reverse operation. The principle is investigated and compared to other integrated reverse-diode concepts. Different diode structures are fabricated and evaluated. The new concept is demonstrated on a large gate width 600 V-device with on-state currents up to 30 A and an on-state resistance of 215 mΩ. Furthermore, the device achieves a very low gate-charge of below 3 nC and a reverse recovery charge of 8 nC.

High-voltage stress time-dependent dispersion effects in AlGaN/GaN HEMTs

Abstract: In this work we investigate the dispersion effects of GaN based HEMTs as a function of the off-state stress voltage and the stress time. We characterize the reduction of the drain current in on-state after off-state stress time from 2 μs up to 10 s. In addition, we compare different source- and gate-terminated field plate configurations. High-voltage (HV) pulsed stress tests with several stress rates are carried out to measure the increase of dynamic on-resistance over time. A new analytic model is developed to estimate the relevant trapping and detrapping time constants of the devices. By HV pulsed stress tests at different temperatures it is possible to estimate the activation energies for both processes.

Switching frequency modulation for GaN-based power converters

Abstract: The effects on the EMI spectrum for various switching frequency modulation (SFM) scenarios in a high frequency boost converter are investigated in this paper. A GaN-device and a Si-device are compared with respect to their EMI behavior, which results from different gate charges and therefore different voltage gradients dv/dt on the power lines. First, the dynamic characteristics of the GaN-HEMT are demonstrated in detail. Then the behavior in the time domain and the frequency domain for switching operations at 300 kHz with various frequency modulation settings and an output power of 250 W are presented.

High-Gain AlGaN/GaN HEMT Single Chip E-Band Power Amplifier MMIC with 30 dBm Output Power

Abstract: This paper reports on the design, fabrication and measured performance of an E-band power amplifier MMIC delivering >1 W of output power. AlGaN/GaN based high-electron-mobility transistors (HEMTs) with 100 nm of gate length have been processed in a grounded coplanar transmission line technology in order to realize the four-stage high power amplifier chip. For the designed prototype, measurement results yielded small-signal gain of 16-18 dB in the frequency range from 71 to 76 GHz (low E-band). A saturated output power of 30.8 dBm (1.2 W) has been reported under continuous wave (CW) operation with more than 11 dB of power gain, corresponding to a power density of around 1 W/mm at the output of the final stage. This demonstrates superior performance of GaN technology in the millimeter-wave design over other competing technologies.

With electroluminescence microcopy towards more reliable AlGaN/GaN transistors

Abstract: Long-term stability and reliability of AlGaN/GaN high electron mobility transistors (HEMT) can be validated by various stress tests which allow studying the physical mechanisms responsible for degradation. As the electroluminescence (EL) intensity is related to the kinetic energy and density of the channel electrons accelerated in the electric field, both local current and electric field changes can result in an increase or decrease of the EL intensity. The electric field distribution in source drain direction peaks at the drain side edge of the gate foot and the field plates. It is strongly dependent on the gate design and the passivation /semiconductor interface trap density. Although the spatial resolution of the electroluminescence microscopy (ELM) set-up is limited to a few microns, the technique allows to monitor submicron changes in the device geometry and hence to identify elevated electric field peaks being responsible for the acceleration of the degradation process. ELM also benefits from the fact that submicron imperfections at the Schottky interface of the gate electrode result in strong local current variations. It has been used to selectively identify suitable positions for further failure analysis of focused ion beam prepared cross sections by scanning and transmission electron microscopy. Process induced imperfections as voids at the Schottky interface after stress have been localized.