Elsevier

Microelectronics Reliability

Volume 47, Issues 9–11, September–November 2007, Pages 1394-1399
Microelectronics Reliability

Study of hot-carrier effects on power RF LDMOS device reliability

https://doi.org/10.1016/j.microrel.2007.07.064Get rights and content

Abstract

This paper reports comparative reliability of the hot carrier induced electrical performance degradation in power RF LDMOS transistors after RF life-tests and novel methods for accelerated ageing tests under various conditions (electrical and/or thermal stress): thermal shock tests (TST, air–air test) and thermal cycling tests (TCT, air–air test) under various conditions (with and without DC bias, TST cold and hot, different channel current IDS and different extremes temperatures ΔT values). It is important to understand the effects of the reliability degradation mechanisms on the S-parameters and in turn on static and dynamic parameters. The analysis of the experimental results is presented and the physical processes responsible for the observed degradation at different stress conditions are studied by means of 2D ATLAS-SILVACO simulations. The RF performance degradation of hot-carrier effects power RF LDMOS transistors can be explained by the transconductance and miller capacitance shifts, which are resulted from the interface state generation and trapped electrons, thereafter results in a build up of negative charge at Si/SiO2 interface.

Introduction

LDMOS is widely used in RF power applications. In the radar field, a crucial issue to tackle with is the reliability of RF LDMOS submitted to RF pulses with high drain-source DC bias for maximum output power under wide temperature range [1]. For this purpose, we designed and implemented an innovative reliability bench able to keep track of all RF powers, voltages and device base-plate temperatures whose values correspond to stress operating conditions [2].

Temperature is a critical parameter, particularly in RF power electronic devices. This element has a considerable influence on reliability and performances [3], [4], can limit the lifetime of semiconductors and consequently plays an essential part in failure mechanisms [3]. For these reasons thermal shocks and cycling conditions are becoming important for RF LDMOS in many applications. Hot carrier induced electrical device parameter degradation is one of the major concerns in device reliability. The electric parameters of MOS transistors are more and more sensitive to boundary defects, to the presence of charges in the gate oxide and at the Si/SiO2 interface [5]. The miniaturisation of MOS transistor (reduction of the channel length and oxide thickness) leads to the presence of higher electric fields, which are the major source of degradation and lifetime reduction of the MOS transistor. Our work methodology consists in characterizing the device before and after ageing on two different benches (reliability bench and thermal bench). Thereafter, we compare the degraded parameters (static, dynamic and RF) according to the applied stress.

Section snippets

Life-test conditions (reliability bench)

Life-tests are run in the working conditions (pulsed RF) using various device base plate temperatures (10 °C, 25 °C and 150 °C) and a high drain-source voltage (44 V) in order to get more power from the device for radar applications. The discrete RF device used for reliability testing is a commercial 10 W telecom dedicated transistor (LG = 0.8 μm) that operates in class-B at saturation. The parameters set for the tests are the following:

  • Frequency = 2.9 GHz and pulse width/duty

  • cycle = 500 μs/50%.

The RF

Thermal bench and accelerated ageing conditions

In our experiments, the devices are stressed with an applied drain-source voltage VDS of 40 V and a gate-source voltage VGS necessary to obtain a permanent drain-source current IDS less than 20 mA (without self-heating effect), which corresponds to the quiescent current at ambient temperature. The accelerated ageing tests were performed with a THERMONICS T-2820 precision temperature forcing system (PTFS); the system is designed for trouble-free temperature testing of electronic components (for

Comparison and results discussion

Two critical parameters are monitored during ageing tests (RF life-tests). For high power devices working at saturation point, the significant performance parameters concern output power and drain-source current (measured during RF pulse). Measurements were plotted under these two figures of merit and for three different temperature conditions (see Fig. 2, Fig. 3).

The means for each parameter from the 24, 48, 168, 500, 1000 and 1500 h test down-points have been empirically fitted to log curves,

Summary and conclusion

The reliability was reviewed under microwave operating conditions. Then the critical parameters were put forward by linking them to the RF degradations ones (PSAT and IDSS in RF amplification). This study brings new elements to clarify the problems related to hot carriers and impact ionization under operating conditions met by the RF LDMOS.

The TCT and TST seem to be equivalent because they apparently produce the same degradation. The TST cold test seems to induce faster degradation than the hot

References (14)

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