Progress in MOS-controlled bipolar devices and edge termination technologies

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Abstract

An overview of the recent developments in high-voltage power semiconductor MOS-controlled bipolar devices is presented. The Insulated Gate Bipolar Transistor (IGBT) technology is explored from its initial stage up to the latest state-of-the-art developments, in terms of cathode engineering, drift design and anode engineering to highlight the different approaches used for optimisation and the achieved trade-offs. Further, several MOS-gated thyristors, which are aimed to replace the IGBT, are analysed. Moreover, the present paper reviews the various approaches in the fabrication of edge termination used in power device MOS-controlled bipolar devices.

Introduction

Effective and Efficient power switching is a key element in a large number of electronics applications, triggering an increased demand for power semiconductor switches in recent years. Typical applications include data processing, telecommunications and consumer electronics, industrial automation for power generation, conversion and amplification, automotive and other heavy-duty industrial sectors, traditionally the field of electro-mechanical components with analogue functions. Thus, the world market for power semiconductors has grown from $4.94 billion in 1996 to $11 billion in 2001. Power semiconductor devices may be sold either as discrete devices or modules, depending on the type of applications and costs involved. The market of discrete devices is expected to increase from $6.5 billion in 1998 to $10.5 billion in 2003, while the forecast for the module market predicts a raise from $930 million in 1998 to $1.3 billion in 2003 [1].

Since its development in the mid-eighties, the Insulated Gate Bipolar Transistor (IGBT) [2], [3] has become the most commonly used device in medium power semiconductor market. It has rapidly replaced the bipolar transistor at the lower end and encroaching the Gate Turn-off Thyristor (GTO) market at the higher end, due to its attractive features, such as the ease of gate control, low on-state/switching losses, wide safe operating area, ruggedness and relative ease of manufacture. More recently, the need of higher operating frequencies for the IGBT has also become apparent, due to the necessity to optimise the performance of new designs, reduce the size of external circuitry and respond to the legislative demands of reducing noise levels. For example, Toshiba has proposed the new IGBT++ series, featuring operating frequencies of 200 kHz for 1200 V/50 A, an order more than the traditional limit of 20 kHz of this device [4]. Nowadays, IGBTs are available with voltage ratings ranging from 600 V to 6.5 kV, currents up to 3600 A and operating frequencies of up to 200 kHz.

In this article, the evolution of the IGBT from its initial concept to the latest, state-of-the-art, devices is explored. The developments in cathode and anode engineering, as well as in the design of the drift region are analysed, and the comparative advantages of various solutions proposed are given. Further, an overview of several MOS-gated thyristors, proposed as an alternative to the IGBT is presented. In addition, the evolution of the edge terminations used in power devices is explored.

Section snippets

Structure and operation of the IGBT

The IGBT combines the high input impedance of a MOSFET with the current carrying capability of a bipolar transistor [2], [3] to simplify the gate drive requirements and enhance the on-state performance. The structure of a planar IGBT and its equivalent circuit are shown in Fig. 1. Connecting the gate and source contact together and increasing the anode voltage operates the device in its forward blocking mode where the P base/N drift junction supports the positive anode voltage. The thickness

MOS-gated thyristors

The most commonly used device in high-voltage applications is the GTO. This structure employs a thyristor mode of operation to minimise the forward voltage drop, but is current controlled so the control circuitry needs to be very complex and consumes excessive power. Consequently, IGBT and the IGCT [25] are being thought of as its replacement in the range of medium to high power ratings. However, high-voltage IGBTs show excessive forward conduction losses due to the insufficient drift region

Edge terminations

A unique distinguishing feature of all semiconductor power devices is their high voltage blocking capability. Depending on the application, the breakdown voltage can range from 25 V for applications such as power supplies to over 6.5 kV for applications in power transmission and distribution. The ability to support high voltages is primarily determined by the avalanche breakdown of a reverse-biased p–n junction, which occurs when the electric field within the device structure becomes large. The

Conclusions

With the present trend toward effective device optimisation and process developments, the IGBT is set to dominate the medium power applications, and will compete with the IGCT and GTO in the high power range up to 6.5 kV. At low power end, due to increased frequency of operations, IGBT structures have significant potential to compete with MOSFETs. On the other hand, technologically, the CIGBT can benefit from all the advances of IGBT based technologies. Therefore, whether thyristor type

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