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GaN Transistor Technology: Problems of Self-Heating, Traps and Flicker Noise

GaN and GaN-based III-V alloy semiconductors are likely the most promising new materials for the next generation of high-power electronic, microwave and optoelectronic devices. Apart from the continuing search for the optimum substrate and p-type doping method, other crucial issues for the GaN transistor technology (see Figure 1) are the self-heating effects, traps and the high-levels of flicker noise. Professor Balandin's Nano-Device Laboratory (NDL) research group is involved in investigation of the thermal transport in AlGaN materials and study of the traps and trap-relarted flicker noise in GaN-based transistors. The self-heating effects in GaN transistors are related to the extremely high power densities involved and a large thermal resistance of the device structure. Self-heating in GaN-based field-effect transistors (FETs) results in abrupt increase of local temperature and can lead to the drain-source current reduction and thermal breakdowns at bias voltages much lower than the theoretically predicted (see V.O. Tourin and A.A. Balandin, Electron Letters, 2004). To solve the problem of thermal management of the high-power GaN transistors, NDL researchers carry out experimental and theoretical studies of thermal conduction in GaN thin films and AlGaN alloys used in fabrication of AlGaN/GaN heterostructure field-effect transistors (HFETs).

Figure 1. GaN/AlGaN heterostructure field-effect transistors (HFETs)
used for experimental study of self-heating effects.

NDL researchers have carried out a systematic study of the thermal conductivity in the AlGaN alloy system as a function of the Al mole fraction in the temperature range from 4K to 600K (see Figure 2). The experimental data have been explained on the basis of the the virtual crystal model. The thermal conductivity variation with Al mole fraction, as demonstrated in both the theoretical and experimental data, shows a characteristic abrupt reduction when the Al content increases from 0.0 to about 0.1, followed by a graduate approaches to minimum. Details of the study have been reported in W.L. Liu and A.A. Balandin, J. Applied Physics, 2005. This study clarified the role of the alloy and boundary phonon scattering in AlGaN thin films, and allowed the NDL researchers to extract the parameters required for accurate modeling of GaN FET performance.

Figure 2. Measured thermal conductivity in AlGaN thin films with different Al mole fraction.
The results are after W.L. Liu and and A.A. Balandin, Appl. Phys. Lett., 2004.

In collaboration with the group of Professor Kang L. Wang (UCLA), the NDL researchers investigated the effects of the ambient temperature on the performance of GaN/AlGaN HFETs in the temperature range from 25 C to 250 C (see Figure 3). Understanding the ambient effects on the transistor performance is important for designing a reliable communication system. The measured data indicates about 30% degradation in the saturation current and transconductance with temperature increase to 250 C. Experimental results have been used as input parameters for the physics-based modeling using ISE DESSIS software (see also NDL Device Modeling). The obtained experimental dependencies and simulation data can be used for predicting GaN-based device performance and reliability in changing, high temperature environment. Details of the work have been published in W.L. Liu, V.O. Turin, A.A. Balandin, Y.L. Chen and K.L. Wang, J. Nitride Research, 04.

Figure 3. Ambient temperature effects on the GaN transistor performance.

Professor A.A. Balandin in collaboration with Professor K.L. Wang (UCLA) were first to demonstrate experimentally that GaN HFETs can operate with the low flicker noise levels (Hooge parameter ~0.0001), required for the high-tech microwave applications (see A.A. Balandin et al, IEEE Electron Device Letters, 1998). The flicker noise, which manifests itself at low-frequencies (0.01 kHz 100 kHz) with the ~1/f spectral density, is an important figure-of-merit for the semiconductor device technology since this type of noise is the limiting phase-noise factor for all kinds of transistors. In addition, the low-frequency noise spectral density is a characteristic of the material quality. Professors Balandin and Wang have also proposed an original method for the noise suppresion in GaN/AlGaN HFETs using the high-Al content ("piezo-dopped") barriers without the channel doping, which led to the two-orders-of-magnitude noise spectral density suppression (see Figure 4).

Figure 4. Noise spectra density in the conventional and "piezo-doped"
AlGaN/GaN HFET. After A.A. Balandin and K.L. Wang, Appl. Phys. Lett., 2000.

In order to minimize the device noise one should understand where the noise sources are located: in the device channel, contacts, etc. Analysis of the gate and drain-source bias dependences may clarify the noise type and sources. Figure 5 shows that in the subsaturation region, the noise spectral density measured for GaN/AlGaN HFET has two different regions characterized by the dependence ~1/V^m, where m is close to 1 (for small Vg<3 V), or is equal or larger than 3. This corresponds to the case when the dominant noise source is in the device channel. Details of the noise-source analysis have been reported in A.A. Balandin, Electron Letters, 2000.

Figure 5. Effective gate-bias dependence of flicker noise in AlGaN/GaN HFET.

Capacitance-voltage (CV) profiling is a well-developed characterization technique for investigation of the carrier and trap distribution in semiconductor heterostructures. In CV measurements, the frequency dispersion provides useful information on the trap states . Dr. W.L. Liu and other NDL group members apply this technique to study surface and channel layer trap characteristics in in GaN/AlGaN HFETs (see Figure 6). The results of the CV measurements performed for surface passivated GaN-based transistors have been recently reported at the SPIE Noise Conference.

Figure 6. Capacitance-voltage spectroscopy of interface traps in GaN/AlGaN HFETs.
Results are after W.L. Liu, A.A. in collaboration with K.L. Wang, 2005.

More information on the projects currently under way in the Nano-Device Laboratory (NDL) can be found HERE. To join NDL as a graduate student or postdoctoral research visit the web-page HERE. To learn more about course offering in the field of Materials, Devices and Circuits visit the web-page HERE.

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