Comparison of junction-isolated and soi high-voltage devices operating in the source-follower mode

Abstract

Recent developments in high-voltage devices have made it feasible to realize monolithic power integrated circuits that combine high voltage devices with control, level shifter and logic function on a single chip. One of the important uses of such circuits is to assemble bridge circuits, which are needed whenever waveforms of arbitrary shape and frequency have to be synthesized. Typical examples are electronic ballasts and motor drives. In such circuits the source of the upper of the two switches, the high-side transistor, has to float above ground potential. In this work we have studied, both theoretically and experimentally, the behavior of junction-isolated (JI) and silicon-on-insulator (SOI) devices in applications involving the source-follower configuration at voltages up to 700 V. We find that the SO1 devices exhibit significantly lower on-resistance under source-high conditions than do JI devices, and that this effect is especially pronounced at high source bias voltages. The devices used in this study were LDMOS transistors, with off-state breakdown voltages of 500-700V (Fig. 1). Both types of devices were designed according to the RESURF principle in order to maximize the breakdown voltage [l, 21. Measurements of the on-state Id - Vh characteristic of the JI transistor in common-source and source-high configurations indicate that, in the latter case, the difference between the substrate and source potentials causes the drift region of the LDMOS transistor to become depleted, resulting in an increase of the on-resistance. In the case of the the SO1 device, however, an inversion layer forms at the buried oxide-SO1 layer interface. Once strong inversion is established, the width of the depletion region does not increase with further increase in the substrate potential, so that the remainder of the SO1 layer remains undepleted. Consequently, relatively little change occurs in the on-resistance of the SO1 device after the initial onset of the inversion layer. As a consequence of the RESURF principle which imposes conditions on the drift region charge, the specific on-resistances of the JI and SO1 devices in common-source configuration are comparable. However, as the source potential increases, the SO1 on-resistance increases slightly and saturates to a constant value, while the on-resistance of the JI device increases much more drastically (Fig. 2). This effect in JI devices can be alleviated to some extent by design modifications, such as the use of a lightly-doped substrate, a thicker epitaxial layer or inclusion of additional charge-compensating regions [3]. However, the effectiveness of such modifications at very high voltages is limited. For this reason, the source-follower operation becomes more difficult to realize in very-high- breakdown-voltage JI devices. This behavior has important implications on the ability to integrate high-voltage source-follower devices in power integrated circuits. The substantial increase in the on-resistance of the JI transistor would necessitate a correspondingly large increase in the device area, making the integration impractical. Such a limitation does not exist in the SO1 technology, which is thus more suitable for integrating multiple power devices on a single chip.