R.J. Trew

The potential of diamond and SiC electronic devices for microwave and millimeter-wave power applications

The potential of SiC and diamond for producing microwave and millimeter-wave electronic devices is reviewed. It is shown that both of these materials possess characteristics that may permit RF electronic devices with performance similar to or greater than what is available from devices fabricated from the commonly used semiconductors, Si, GaAs, and InP. Theoretical calculations of the RF performance potential of several candidate high-frequency device structures are presented: the metal semiconductor field-effect transistor (MESFET), the impact avalanche transit-time (IMPATT) diode, and the bipolar junction transistor (BJT). Diamond MESFETs are capable of producing over 200 W of X-band power as compared to about 8 W for GaAs MESFETs. Devices fabricated from SiC should perform between these limits. Diamond and SiC IMPATT diodes also are capable of producing improved RF power compared to Si, GaAs, and InP devices at microwave frequencies. RF performance degrades with frequency and only marginal improvements are indicated at millimeter-wave frequencies. Bipolar transistors fabricated from wide bandgap material probably offer improved RF performance only at UHF and low microwave frequencies. >

Improved breakdown voltage in GaAs MESFETs utilizing surface layers of GaAs grown at a low temperature by MBE

A metal-semiconductor field-effect transistor (MESFET) utilizing surface layers of GaAs grown at a low temperature by MBE (LT GaAs) under the gate electrode has been fabricated. The high trap density of LT GaAs reduces the surface fields of the FET, suppresses gate leakage, and increases the gate-drain breakdown voltage without sacrificing current drive capability. An undoped AlAs layer is incorporated between the LT GaAs layer and the channel as a barrier to the diffusion of excess As from the LT GaAs layer to the channel. A 74- mu m-gate-width device demonstrated an improved breakdown voltage of 34.85 V with a g/sub m/ of 144 mS/mm and an I/sub dss/ of 248 mA/mm.<<ETX>>

A large-signal, analytic model for the GaAs MESFET

An analytic, large-signal model for the GaAs MESFET is presented. The device model is physics-based and describes the conduction and displacement currents of the FET as a function of instantaneous terminal voltages and their time derivatives. The model allows arbitrary doping profiles in the channel and is thus suitable for the optimization of ion-implanted and buried-channel FETs. It also accounts for charge accumulation in the conducting channel at high electric fields and the associated capacitance in a self-consistent manner. Theoretical predictions of the model are correlated with experimental data on X-band power FETs and excellent agreement is obtained. >

Silicon Carbide Electronic Materials and Devices

The development of SiC for electronic applications has been a subject of intense research for nearly 40 years. Much of this research is motivated by the extraordinary combination of physical properties possessed by SiC, especially in the development of SiC-based devices for specific high-temperature, high-power, or high-frequency applications that are not suitable for Si- or GaAs-based devices. During the early years of SiC research and development, a significant amount of good, fundamental research was performed, but the development of commercially available SiC-based devices was retarded by low-quality bulk materials and inadequate epitaxial processes. In the late 1980s, research at academic institutions, such as North Carolina State University, and industrial laboratories, such as Westinghouse (now Northrup-Grumman), Advanced Technology Materials, Inc. (ATMI), and Cree Research, Inc., coupled with the commercial offering of highquality SiC wafers from Cree, created an opportunity for further advancement. Improvements in epitaxial processes and device processing strategies were also realized during this time. Together these factors have enabled the fabrication of high-quality device structures and have generated increased research and funding activities in SiC electronic devices.

AlGaN/GaN HFET reliability

High-voltage AlGaN/GaN HFETs can produce high RF output power with nearly ideal power-added efficiency. But widespread adoption of these HFETs has been limited by a lack of acceptable reliability data for practical communications and radar applications. Device problems that have been observed include dc current and RF output power degradation as a function of time when the device is operating. Sudden and permanent degradation shifts in device performance have also been observed under certain operating conditions. Identified causes of the reliability problems include the quantum mechanical tunneling of electrons on the gate electrode to the surface of the semiconductor adjacent to the gate on the drain side, and a defect generation mechanism that occurs at a high, critical electric field. The gate leakage phenomenon described in this article produces electrons on the surface of the AlGaN layer adjacent to the gate electrode, and this creates a negative charge layer that partially depletes the conducting channel, thereby producing a degradation in dc current and RF output power. The gate leakage current is present when the device is biased and driven with an RF signal, and therefore the charge accumulation increases as a function of operation time. The gate tunnel current is a very sensitive function of surface state density, particularly near the gate edge, and of the magnitude of the electric field at this location. In addition, at a critical magnitude of the electric field defects in the AlGaN layer are created due to mechanical stress on the crystal structure, and these defects act as charge trapping centers. This mechanism is not well understood at this time and is currently the subject of research and investigation. Parameters that affect reliability are a function of device design and surface processing. Improvements in device reliability have been achieved through design modifications to produce improved surface passivation layers that reduce the gate and surface leakage currents and further modifications to reduce the magnitude of the electric field internal to the device. Continuing reliability study is required to fully elucidate the link between observed degradation behavior and physical failure mechanisms in a statistically significant manner.

Gate breakdown in MESFETs and HEMTs

A new model for gate breakdown in MESFETs and HEMTs is presented. The model is based upon a combination of thermally assisted tunneling and avalanche breakdown. When thermal effects are considered it is demonstrated that the model predicts increasing drain-source breakdown as the gate electrode is biased towards pinch-off, in agreement with experimental data. The model also predicts the gate current versus bias behavior observed in experimental data. The model is consistent with various reports of breakdown and light emission phenomena reported in the literature.<<ETX>>

Nonlinear source resistance in high-voltage microwave AlGaN/GaN HFETs

Wide bandgap semiconductors are used to fabricate field-effect transistors with significantly improved RF output power compared to GaAs- and InP-based devices. Nitride-based heterostructure field-effect transistors can be biased at high drain voltages, up to and exceeding 100 V, which results in high RF output power. However, the operation of these devices at high drain bias introduces physical phenomena within the device that affect both dc and RF performance. In this study, the existence of a nonlinear source resistance due to space-charge limited current conditions is demonstrated and verified. Inclusion of the nonlinear source resistance in a physics-based device simulator produces excellent agreement between simulated and measured data. The nonlinear source resistance degrades RF performance and limits amplifier linearity.

Microwave AlGaN/GaN HFETs

This article presents the operating physics, performance potential, and status of device development of microwave AlGaN/GaN heterostructure field-effect transistors. AlGaN/GaN HFETs show potential for use in improved RF performance microwave amplifier applications. Development progress has been rapid, and prototype devices have demonstrated RF output power density as high as 30 W/mm. Microwave amplifier output power is rapidly approaching 100 W for single-chip operation, and these devices may soon find application for cellular base station transmitter applications. Devices are being developed for use in X-band radars, and RF performance is rapidly improving. The HFET devices experience several physical effects that can limit performance. These effects consist of nonlinearities introduced during the high-current and high-voltage portions of the RF cycle. High-current phenomena involve the operation of the conducting channel above the critical current density for initiation of space-charge effects. The source resistance is modulated in magnitude by the channel current, and high source resistance results. High voltage effects include reverse leakage of the gate electrode and subsequent charge trapping effects on the semiconductor surface, and RF breakdown in the conducting channel. These effects can produce premature saturation effects. Also, under certain conditions, high voltage operation of the device can initiate an IMPATT mode of operation. When this occurs, the channel current increases and RF gain is increased. This phenomenon enhances the RF output power of the device. The physical limiting effects can be controlled with proper design, and the outlook for use of these devices in practical applications is excellent.

High-frequency solid-state electronic devices

Starting with exploratory work in the 1930s and development work in the 1940s a variety of two-terminal and three-terminal solid-state device structures have been proposed, fabricated, and developed. This work parallels the development effort on vacuum electronic devices, and the two technologies share many applications. The solid-state and vacuum electronic devices work in tandem to enable numerous commercial and military systems. Solid-state device development is closely linked to semiconductor materials growth and processing technology, and advances such as the introduction of heterojunction growth technology, permit complex multiple layer device structures to be fabricated and optimized for maximized device performance. This work has been very successful and a variety of high-performance diodes and transistors are now available for use from UHF into the millimeter-wave spectrum, approaching terahertz frequencies. The development, operating principles, and state-of-the-art of various diode and transistor structures are reviewed.

Principles of large-signal MESFET operation

The large-signal RF operating principles of MESFET amplifiers are investigated using a circuit simulator that incorporates a physics based MESFET model which has been augmented with a new gate breakdown model. It is demonstrated that the main saturating mechanisms of the MESFET under large-signal RF operation are forward and reverse conduction of the gate electrode. Maximized RF performance of MESFET amplifiers is obtained by optimally positioning the dynamic load line relative to the RF-IV plane. The position of the dynamic v i characteristic is determined by device breakdown, bias, and circuit tuning conditions.