It is known that the operating temperature range of microelectronic components and integrated circuits on silicon structures is very limited. For example, the operating range of industrial electronic products is from –40 to 85 °C, and the range of military electronics is from –55 to 125 °C (Fig.1). However, for some tasks the electronics with a higher maximum allowable operating temperature is demanded. Typically, these electronic components are designed for stable operation in the temperature range from –60 to 225 °С and belong to the class of high-temperature electronics (HTE) [1, 2, 3]. For creation of the high temperature circuitry silicon on insulator (SOI) structures, silicon carbide (SiC) and gallium nitride (GaN) are used . Electronic components on the SOI basis have a lower final cost and the technology is well established in the production of radiation-resistant integrated circuits [3, 4, 5]. Due to the low cost and high manufacturability the SOI circuitry is relevant for the aviation, automotive, oil and gas, aerospace and nuclear industries [1, 3, 4, 6].
The main problem of CMOS structures on a silicon basis that arises with increasing temperature are leakage currents [1, 5, 7]. In SOI technology these currents are minimized due to the complete dielectric isolation of each electronic component. This project is devoted to the comparison of the main characteristics of 0.5 µm MOS transistors, which are made on the basis of SOI structures and bulk silicon, in the temperature range from 0 to 250 °С. Based on this comparison the conclusions about the operability of SOI MOS are presented and the features of functioning of MOS transistors on a bulk silicon at temperatures above 125 °C are explained. The choice for testing at high temperatures of the MOSFETs manufactured by the technology of close to 1 µm is caused by similar studies of 0.8 µm transistors conducted by Honeywell in the framework of Energy Deep Trek project . STUDIED ELECTRONIC COMPONENTS AND DESIGN OF EXPERIMENT The n- and p-channel MOS transistors with an operating voltage of 5 V made on the SOI structures and bulk silicon are selected as objects of study. These MOS transistors have a channel length of 0.48–0.50 µm and are manufactured by Mikron (tab.1 and tab.2).
The design of experiments includes following steps: • to measure the parameters of MOS transistors on chips at temperatures from –60 to 250°С; • to obtain the dependencies of threshold voltage, saturation current and leakage current of MOS transistors on the temperature; • to compare the characteristics and dependencies for the MOS transistor on SOI structures and on bulk silicon; • based on comparison and analysis of the obtained correlations to draw conclusions about the possibility of using high-voltage MOS transistors on SOI structures and to explain the operating features of the MOS transistors on a bulk silicon at temperatures above 125 °C. OBTAINED CHARACTERISTICS OF MOS TRANSISTORS The results of measurements of parameters of MOS transistors at different temperatures are presented in the form of dependences of threshold voltage, the saturation current and leakage current on the temperature (Fig.2–4).
In particular, Fig.2 shows the dependence of threshold voltage of the MOS transistors on temperature. According to the data for the n-channel transistors (Fig.2a) the threshold voltage of SOI MOS transistor is about 1 V at the temperature of 60 °C and about 0.7 V at 250 °С. Similar values for the MOS transistor on bulk silicon are equal to 0.7 and 0.3 V, respectively. Threshold voltages of p-channel SOI transistor (Fig.2b) at minimum and maximum temperatures are –1.2 V and –0.8 V, respectively. Similar values for the transistor on bulk silicon are approximately equal to –0.9 V and –0.5 V. Given the linear nature of these relationships, we will present the values of drift of the threshold voltage per 1 °C for the investigated MOS transistors (tab.3). Tab.3 presents also the same parameters for MOS transistors developed by Honeywell in the framework of the Energy Deep Trek project . According to the table, a drift of the threshold voltage of n-channel SOI MOS transistor investigated in this work is lower than that of compared transistor on a bulk silicon and of Honeywell’s transistor. A similar parameter for the p-channel SOI MOS-transitory is less than the specific threshold voltage of the Honeywell’ device. The difference between characteristics of Mikron’s and Honeywell’s MOS transistors is caused by the inverse dependence of the drift of the threshold voltage on the concentration of dopant in the drain-source area of the transistor. Since Honeywell use 0.8 µm process (versus Mikron’s 0.5 µm process) the specific doping concentration is less than that in the Mikron’s transistor, which determines the large value of the drift of the threshold voltage per 1 °C. Dependences of saturation currents of MOS transistors at different temperatures are shown in Fig.3. Since the saturation current of the MOS transistor has a linear dependence on temperature , as in the case of the previous dependence, we present the values of the drift of the saturation currents of MOS transistors per 1 °C (tab.4). According to the comparison, the values of drift of the saturation current for the similar (by type of conductivity) MOS transistors can be considered approximately identical. At the same time, in the whole temperature range the difference between the values of saturation currents of SOI MOS transistors from saturation currents of MOS transistors on a bulk silicon is about 0.4∙10-4–0.5∙10-4 A. Dependences of leakage currents of transistors on the temperature are shown in Fig.4. For both n-channel (Fig.4a) and p-channel (Fig.4b) MOS transistors the leakage current reaches extreme values at the temperature of 250 °С. Exponential dependence of leakage current on temperature is typical for all four test transistors, and at temperatures above 125 °C a more intense change of leakage current is noticed in MOS transistors on a bulk silicon. For example, at maximum temperature, the leakage currents for n- and p-channel transistors are of 7.16∙10–9 A and –1.13∙10–9 A respectively, that in seven and three times exceed similar parameters of SOI MOS transistors. PHYSICAL INTERPRETATION OF OBTAINED RESULTS The obtained dependences allow to consider that the operation of the SOI MOS transistors is more stable at high (>125 °C) temperatures in comparison with MOS transistors on a bulk silicon. This conclusion follows from multiple excess of the level of leakage currents for MOS transistors on a bulk silicon compared with that for SOI MOS transistors at temperatures above 125 °C.
The phenomenon of increased leakage current in MOS transistors on a bulk silicon with increasing temperature is primarily caused by the drift of minority carriers in the volume of the substrate . The intensity of the drift of minority carriers increases with increasing temperature and leads, ultimately, to reduction of threshold voltage to close to zero values, when the MOS transistor is open at zero voltage on gate . As a result, the MOSFET loses its amplifying properties, that is tantamount to failure . In turn, in SOI MOS transistors due to the buried insulation layer, the area of drift of minority carriers is limited to the area between the drain and the source. As a result, the leakage currents at high temperatures are lower and threshold voltage is sufficient to provide amplifying properties. CONCLUSION Thus, this paper presented and compared the main characteristics of 0.5 µm MOS transistors on the SOI structures and bulk silicon in the temperature range from 0 to 250 °С. Based on the comparison of operating characteristics of MOS transistors on the basis of these structures at temperatures above 125 °C, conclusions on the performance of SOI MOSFET's are drawn and the distinctive features of MOS transistors on a bulk silicon are explained.
The qualitative advantage of SOI structures compared with the structures based on bulk silicon for creation of high temperature transistors is shown. It is determined that at high temperature due to the presence of a buried insulation layer the minimal (in comparison with MOS transistors on a bulk silicon) leakage current at minimal saturation current is provided. This is confirmed by the presence of the amplifying properties of SOI transistors on the whole range of the stated temperatures. Therefore, 0.5 µm SOI MOS transistors with 5 V supply voltage meet the requirements for the operation in the temperature range from 0 to 250 °С, and can be classified as high-temperature microelectronic components. ■ The project was supported by the Ministry of education and science of the Russian Federation in the framework of the Federal Targeted Programme for Research and Development in Priority Areas of Development of the Russian Scientific and Technological Complex for 2014 – 2020 according to the agreement No. 14.576.21.0063 dated October 23, 2014 (unique identifier of applied research RFMEFI57614X0063).