Samson Mil'shtein

p-HEMT with tailored field

The high electron mobility in heterostructure devices stems from fact that electrons are injected into intrinsic layer of a semiconductor material and are confined into two-dimensional space of a heterostructure potential. However, non-linear distribution of the voltage along a transistor channel results in variation of depth and width of heterostructure potential. As in case of non-uniform electrical field in conventional field effect transistor, non-uniformity of this potential causes limitation of gain gm and operational frequency ft; increased level of noise and, most important, significantly impact the non-linearity of the gain. We illustrated our study with characteristics of tri-gate pHEMT, fabricated on M/A-COM’s high volume production AlGaAs/InGaAs process. The speed at which electrons traverse the channel was modified by providing the highest bias voltage on the first gate, and gradually decreasing the bias voltage on all subsequent gates. Thus tailored electrical field along the channel was the modeling and design base of new p-HEMTs. Manufactured devices carried in agreement with modeling, better saturation of current (large Early Voltage), significant linearity of the transconductance gm and better reliability. q 2003 Published by Elsevier Science Ltd.

Shaping electrical field in heterostructure transistors

Abstract The decrease of the electron transit time in FETs is achieved mainly by design of short transistors and minimization of scattering in the intrinsic layer of the HEMT channel. A penalty of this miniaturization trend is the creation of a poorly controlled, extremely high electric field along the channel of a transistor. The non-linear distribution of the electric field along the channel of HEMTs leads to the reduction of mobility by a factor of 5–7, causing sequential worsening of the performance parameters, such as gain ( g m ) and cut-off frequency ( f t ). The appearance of hot electrons around the drain is another issue on a long list of problems resulting from extreme field profiles. Fundamental dependence of electron velocity on electrical field strength was the foundation of field tailoring in variety of multi-gate transistors. In recent years, the intensified research was focused on reshaping the field profile by using various topologies of additional electrodes, called Field Plates (FPs), such as Overlapping Gate structures, Detached FP design, Stacked 2- and 3-floor FP systems, and others. The current study presents a review of the advantages and drawbacks of various HEMT designs where additional field plates were used. Although most of the FP topologies were used to increase the breakdown voltage of the new transistors, we offer another application for the field plates. They can be used to improve the average electron velocity. It is also shown that a new pHEMT with quasi-constant g m can be designed using FPs.

Improving FET Switch Linearity

RF field-effect transistors, especially pseudomorphic high-electron mobility transistors (pHEMTs), are commonly used as switches in communication applications. These small high-speed devices are vital for routing and conveying signals in such uses. The important characteristics of pHEMTs, besides their small size, are their high-frequency capability, insertion loss, isolation, power handling, switching speed, and linearity. A topology using a pair of simple but modified series and shunt elements was designed to improve upon the linearity of an RF switch. Each element of the switch was composed of a single, unbiased, but relatively long pHEMT, which was designed for the test. By shifting the position of the gate asymmetrically toward the source terminal in these transistors, it was found that the linearity was improved without cost to other performance parameters

High speed switch using pairs of pHEMTs with shifted gates

In many radio frequency (RF) applications, small, high-power, high-speed switches are vital. Currently, pHEMTs are considered among the fastest of the semiconductor devices, capable of operating up to 100 GHz. We designed a switch topology using simple series and shunt elements. Each element was comprised of a single, unbiased pHEMT. We found shifting the position of the gate asymmetrically towards the source terminal in these transistors improved the switching time. Using very long HEMTs in our experimental chips we observed improvement of turn-on and turn-off time (both in single nanoseconds) as well as better linearity and power handling.

Low-pressure polishing of GaAs wafers

Abstract A low-pressure polishing procedure was developed where a GaAs wafer was immersed in a polishing solution of NaOCl and rotated with a speed of 6500 rpm. The strengths of the technique are illustrated by polishing non-lapped polycrystalline GaAs wafers, with final flatnesses better than 0.10 μm/cm.

Progress of quantum electronics and the future of wireless technologies

Research in quantum electronics over several decades has fueled the creation and rapid growth of todays wireless communications market. Sales of electronic components into this market exceeded

Reliability characteristics of pHEMT resulting from electron interaction with interface states under the gate

25 billion in 2006. Nearly all cellular handsets sold today include integrated circuits (ICs) based on energy gap engineered transistors-high-electron mobility transistors (HEMTs) and heterojunction base transistors (HBTs). The success of these technologies notwithstanding, future wireless communications systems will require even more demanding IC performance, especially in the areas of linearity and low noise. We propose that a new concept in transistor design, wave-function engineering, offers un-tapped opportunities to realize these needed performance improvements.

Beam testing of the electrical field in semiconductor devices

The hot electron degradation of pHEMT devices was compared in this study with recent results of stress testing of MESFETs. The off-state (two terminal) stress condition caused failures in pHEMT transistors, as was the case with two terminal stress of MESFETs. The three-terminal stress of pHEMTs led also to hot electron degradation, while similar stress did not affect the performance of MESFETs. The significant difference in the reliability of these two groups of devices is linked to interaction of electron flow in the channel with interface states in the transistor structure.

Is HEMT operating in 2D mode

Abstract Every semiconductor device has a unique distribution of a potential and a charge. This unique signature of a potential characterizes completely the successful operation or failure of the device. In the area of device design and testing it is essential to have a quantitative measurement of a field profile with high resolution on a submicron scale. Recently, the quantitative Scanning Electron Microscopy (SEM) Differential Voltage Contrast (DVC) method was developed to measure potential and field distribution across two and three terminal devices. The DVC method consists of taking two images of a tested device, one from an unbiased device and the second image taken under the same operational conditions of SEM from the same area of biased device. The subtracted image is calibrated and processed by special software to generate one and two dimensional field profiles. Current study reviews various voltage contrast techniques and presents recent DVC measurements of diodes, solar cells, MOSFETs, MESFETs and quantum well lasers.

Reliability test of MESFETs in presence of hot electrons

The complete answer to the question posed by the title of this paper requires a detailed explanation. The conceptual design of the pseudomorphic high electron mobility (p-HEMT) structure is based on 2D electron gas transport and, therefore, a HEMT is expected to manifest quantization at any point along the channel. Our main question addresses devices manufactured by a variety of semiconductor companies and research groups where a certain number of thick cap layers above the 2D channel space could work as a leakage passage for electrons out of the channel. Indeed, the measurements of output current performed by us at low temperature demonstrated limited quantization, which happens in a very small volume under the gate. In the current study, measurements of the output current of a dual-gate p-HEMT showed some control of the volume in which quantization occurs. Better control of quantization was observed in the novel HEMT we designed, where the escape of electrons from the channel was prevented. The appearance of quantization steps and the length of these steps in the output current at low temperature were dependent on the biases applied to the second gate. We tend to believe that quantization of electron energies at low temperature is responsible for the appearance of these steps in output current characteristics.