A systematic study of impact ionization in pseudomorphic high electron mobility transistors (PHEMTs) has been carried out using temperature-dependent electrical measurements as well as modeling for optimizing the power performance of the devices through the best layout parameters. A measurement procedure makes it possible to define a safe transistor operation region is proposed. Impact ionization in the channel is parameterized by specific gate current and voltage values. Temperature-dependent measurements are shown to provide distinction between the impact ionization current and the thermionic field emission current. A methodology for defining an optimum vertical structure and a lateral layout for a given application and operational conditions is developed. Empirical models for optimum lateral layout for a power application were developed based on a statistical "Device Zoo" approach. The results point to an optimal gate-to-drain distance for minimum impact ionization current.Index Terms-Breakdown, empirical models, impact ionization, pseudomorphic high electron mobility transistors (PHEMT).

AlGaN/GaN high electron mobility transistor, AlGaAs/InGAs/GaAs pseudomorphic HEMT, and InAlAs/InGaAs metamorphic HEMT ͑MHEMT͒ epitaxial structures have been characterized using surface photovoltage spectroscopy. The effects of the transistor top and bottom delta-doping levels ␦ top , ␦ bot , and surface charge Q sur on the spectrum features have been studied using numerical simulations. Based on the latter, an empirical model has been developed, which allows extraction and comparison of ␦ top , ␦ bot , and Q sur and is applicable for both double-sided and single-sided delta-doped structures. Prediction of the final device performance by the model is shown for two MHEMT structures. Devices produced on these structures show maximum drain currents, which correlate well with ␦ top values calculated using the model.

Effect of fluoride‐based plasma treatment on performance stability of AlGaN/GaN HFET was systematically investigated. The plasma‐based processes can introduce different type of damage that can be permanent or temporary, depending on electrical field in the device, plasma density, and energy of radicals and total process thermal budget. In order to understand more clearly the mechanism of fluorine based plasma treatment on device performance, we compare the effects of Ar, O2 and CF4 based plasma treatments applied before gate definition. Ar plasma was chosen for comparison with CF4 because of high ionization rate of Ar and chemically neutral behavior of Ar ions. Under the same medium range of 40 V DC bias in RIE system for these two gases, the shift of drain current and threshold voltage was similar that indicate the common mechanism of plasma damage. We supposed that the major effect for this phenomenon is a negative electron charge inserted in semiconductor during plasma treatment, because at the same accelerated field the penetration depth for electrons is higher than the penetration depth for fluoride ions. Devices stressed under impact ionization conditions show an improvement of breakdown voltage (for O2 plasma treatment) and some recovery of threshold voltage similar to “walkout effect”. This performance non‐stability raise the serious concern about application of fluorine based technology for power applications, where large voltage/current swing may push device to the impact ionization region. The experimental results show that the implementation of fluoride based plasma treatment for power device manufacturing can affect the performance stability of AlGaN/GaN power HFET. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Suppression of surface segregation of silicon dopants during molecular beam epitaxy of ( 411 ) A In 0.75 Ga 0.25 As ∕ In 0.52 Al 0.48 As pseudomorphic high electron mobility transistor structures

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