Technology and Physics of Gate Recessed GaN/AlGaN FETs

Abstract

Gallium Nitride (GaN), a wide band gap semiconductor, gained importance as Heterostructure Field Effect Transistors (HFET) in the early 90s. The fabrication of first HFET opened a door for tremendous research over GaN FETs. Currently GaAs/AlGaAs Modulation Doped FETs (MODFET) are utilized with limitations in high power applications. The reason for such limitation is poor physical and electrical properties concerning to GaAs. But now a day GaN with astonishing features compared to GaAs for high power, electrical and optoelectrical devices is a hot topic of research. The reason is based on its interesting physical properties like thermal stability, high breakdown voltage, chemical inertness and electrical properties as well as a property of wide band gap which plays an important role in blue Lasers and devices with low noise. GaN High Electron Mobility Transistors (HEMTs) and MODFETs are important electrical devices for high speed electronics.

With the technological advent to control layer thickness in crystal growth by Metal Organic Vapor Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE), HFET emerged with new horizons. Structures with different layers are grown and characterized. Group III-Nitride devices are highly promising for numerous applications. For the optical/display applications, LASERs and light emitting diodes (in the visible and UV emission range) are used. On the other hand the electrical properties of gallium nitride are being utilized in order to fabricate the electrical devices that provide high performance e.g., field effect transistors working at high temperature, high frequencies or high power. Talking about field effect transistors grown over different substrates, gate recess technology is indeed important to have better control over the channel, higher modulation speed, etc. but, off course, it is a very difficult process step which needs high precision. Gate recessed HFETs are useful to reduce pinch off voltage and the gate leakage current of the device.

In this thesis “Technology and Physics of gate recessed GaN/AlGaN HFETs” some geometrical aspects of recess and gates are investigated. Additionally some problems of the recess technology e.g. etching defects, the control of recess etching depths, misalignments of recess will be discussed. This thesis is divided into the following chapters; Chapter 2 is mainly concerned with some of the most important physical properties of III-nitrides. An overview of different types of FET based on GaN is given in Chapter 3. The theoretical Models which are used in this thesis are also illustrated. Chapter 4 introduces the technology which is used to fabricate recessed gate GaN/AlGaN HFETs. A recessed gate is formed by etching the surface down and then deposition of gate metals in this region. Theoretically it is a way to improve control over the channel [1.3]. Etching is done with Electron Cyclotron Resonance-Reactive Ion Etching (ECR-RIE). Optical and e-beam Lithography is also discussed here. Chapter 5 presents the results and discussion of realized recessed gate HFETs. Here basically characterization is done as a function of recess spacing (Lg), recess depth (trecess), and source drain spacing. Transconductance, drain currents and source resistance are important parameters in transistor characteristics. HFETs with recessed gate fabricated, show good channel control as the transconductance is as high as 220 mS/mm with 250 nm T-gate and a shift in the pinch off voltage could be seen. A detailed epitaxial layer structure and transistor layout is given in appendixes with process technology and instruments used.