1 Dual-Heterojunction High Electron Mobility Transistors on GaAs Substrate Maysam Ghovanloo EECS-521 Final Report University of Michigan, Ann Arbor MI 48109-2122 maysamgh@engin.umich.edu , Tel: (734) 763-6650, Fax: (734) 763-9324 Abstract – The high electron mobility transistors (HEMT’s) in general and AlGaAs/GaAs/AlGaAs dual- heterojunction high electron mobility transistors (DH- HEMT) specifically have been studied in this paper. These devices show superior characteristics over other types of FET’s for high frequency microwave power applications. The structure of a heterojunction have been discussed followed by two and multiple-channel HEMT fabricated device examples. Several models have been reviewed for calculating the density of charge in the dual 2-dimensional electron gas (2DEG) channels present in DH-HEMT undoped GaAs layer, which leads to the device current-voltage characteristics. In the last part, An AlGaAs/GaAs/AlGaAs DH-HEMT has been designed based on the theories provided in earlier sections with describing several steps taken to optimize its performance and simulations showing the effects of various design parametrers. Keywords – Heterojuction, HEMT, 2DEG, GaAs, AlGaAs, Dual-channel I. INTRODUCTION The high electron mobility transistor (HEMT) also called heterostructure field-effect transistor (HFET) has an important role in the field of ultra-high-speed microwave and digital electronics. HEMT’s demonstrate extremely high performance with low noise figures of 0.4 dB at 15 GHz and a gain of 10dB for discrete microwave HEMT’s [1]. The initial concept of the accumulation of charge at a heterojunction interface and its potential for devices was introduced by L. Esaki and R. Tsu at IBM in the late 1960s. However the realization of these devices was not possible without the extremely high precision epitaxial-growth techniques developed in the 1970s to fabricate high quality heterostructures in the III-V compound semiconductors. In 1980 Mimura et al. demonstrated the first HEMT [3] and in the same year the a similar device with a reasonable microwave performance was fabricated by the University of Illinois and Rockwell, which was called modulation-doped FET (MODFET). During 1980s, the application of these techniques to HFETs led to the design of more complicated devices with exceptional performance such as dual-heterojunction (DH-HEMT) and quantum-well HEMT’s, which were first proposed in 1984 [4] and will be discussed in this paper. II. HEMT DEVICE PRINCIPLES In MESFET’s the electrons are obtained by incorporating donor impurities (n-type material), which share the same space with electrons and interact with them. In small geometry devices, the thickness of the conducting Fig. 1. Multiple interface AlGaAs/GaAs modulation doped heterostructures where only the center region (shaded0 of AlGaAs is doped with Si donors. Positive signs indicate the ionized donors and negative ones represent the transferred electrons. If the parameters are chosen correctly, all of the free electrons will be located in GaAs layers where they show enhanced transport parallel to the heterointerfaces [2].