Beatrix Weiss

Integrated reverse-diodes for GaN-HEMT structures

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.

Quasi-normally-off GaN gate driver for high slew-rate d-mode GaN-on-Si HEMTs

This work presents a quasi-normally-off gallium nitride (GaN) transistor with positive gate threshold voltage based on depletion-mode technology, suitable for gate drivers or logic circuits. Quasi-normally-off behaviour is achieved by the series connection of multiple Schottky diodes in the source path of an initially normally-on transistor. As opposed to conventional approaches, a novel quasi-normally-off gate driver circuit avoids the static shoot-through current path in the driver final stage and ensures a safe blocking state of a d-mode power switch in case of driver failure with only one negative driver supply voltage. For evaluation a hybrid integrated GaN power module is built, comprising a 2.4 A gate driver and 600 V/ 24 A boost converter switching cell. Measurements of pulsed inductive switching up to 274 V/ 12 A show gate voltage rise and fall times of 5.4 ns and 3.8 ns, boost converter switch node transition times as low as 1.6 ns and 1.2 ns, and maximum slew-rates up to 91 V/ns during turn-on transitions, and up to 177 V/ns during turn-off transitions, respectively.

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

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.

Linear temperature sensors in high-voltage GaN-HEMT power devices

This work presents a high-voltage GaN-based power HEMT with a highly-linear, monolithically-integrated temperature sensor. The principle is shown and compared to other concepts. The sensor is fabricated by using a interconnect metallization without additional process steps. The performance of the sensor as well as of the power device is characterized. The 600 V power device achieves an on-state resistance of RON = 55 mO at a corresponding drain current ID = 30 A and an advanced dynamic performance with a low gate charge of 20 nC.

Characterization of AlGaN/GaN-on-Si HFETs in high-power converter applications

AlGaN/GaN-on-Si Heterostructure Field-Effect Transistors (HFETs) for power switching are investigated in this paper. A design study for a fast switching environment for power devices is provided including an analysis of the technology, the fabrication, and the performances of large-area AlGaN/GaN-on-Si HFETs. Different power packages and high-current drivers are compared and the results are being discussed. Furthermore the power conversion efficiency is being evaluated for different power- and switching-frequency ranges by means of a converter test board. At 100 kHz efficiencies up to 98.7 % at power levels of up to 1.6 kW could be achieved. At 1 MHz and 1 kW input power an efficiency of 97.1 % was measured.

Analysis and modeling of GaN-based multi field plate Schottky power diodes

Lateral GaN-based Schottky diodes are promising for high-voltage, high frequency, low-loss power applications due to their low forward- and reverse charges resulting from the physical properties of the AlGaN/GaN-heterojunction. For GaN-Schottky diodes in the 600V/10A power range, reverse charges as low as 6nC can be achieved. The reverse current IRRM does not exceed 150mA at a switch-off slope of 25A/μs and reverse voltage of 400V, for higher switching-speeds of 170A/μs an IRRM of 510rmA is measured. This work focuses on compact modeling of non-linear capacitances of lateral, high-voltage Schottky diodes realized in a multi field plate design. All parameters for the model in forward and reverse direction are extracted from capacitance-voltage (C-V)- and IV-measurements presented in this work.

Switching frequency modulation for GaN-based power converters

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.