[attachment=9882]
Abstract
Biological systems have inherent mechanisms which ensure their adaptation and thus survival — preservation of functionality, despite extreme and varying environments. One such environmental feature is that of temperature. Extreme temperature electronics (ETE) is a field where, similarly, these organisms (electronic solutions), have to be designed to survive in such an environment. A number of approaches that address ETE are both proposed and, in some cases, implemented in today’s technologies. Some of these approaches may be said to reduce this challenge but none may be said to solve it. However, biology has found a solution. There can, therefore, be great merit in turning to biology to identify possible solutions. However, it is important to first consider where the field is today.
This paper presents a survey of methods and techniques for tackling temperature effects in ETE — from materials to static and dynamic design techniques. Further, suggestions are provided as to where a bio-inspired approach may be applied either as an improvement to an existing approach or as a novel approach to an existing sub-challenge. Particular attention has been given to where a bio-inspired approach might provide a more dynamic solution.
1. INTRODUCTION
Electronic designs for the automotive, geothermal and petroleum deep-well drilling or space-craft industry all have one major challenge in common — they need to operate in extreme-temperature environments. This implies that Extreme Temperature Electronics (ETE) is required to operate in temperatures out with traditional commercial, industrial or military ranges i.e. out with −55/ − 65 ◦C to +125 ◦C [25]. The extremes of ETE are termed high temperature electronics (HTE) referring to the temperatures over +125 ◦C, low-temperature electronics (LTE) for the temperatures below −55/−65 ◦C, and cryogenic temperature electronics i.e. below −150 ◦C.
Living organisms are capable of survival despite dynamic and extreme environments. Finding ways to identify the fundamental mechanisms due to which such survival is achieved, may provide inspiration for ETE solutions not achievable by traditional methods. However, it is important to look at the existing traditional methods to identify where a bio-inspired approach might be advantageously combined with an existing approach or where such an approach alone might provide an improved solution or a novel solution to an unsolved problem. This paper presents a brief introduction to the challenges inherent in ETE, as well as a survey of ETE techniques and technologies under investigation. Moreover, it identifies avenues of research for bio-inspired techniques within ETE.
An overview of semiconductor properties and their variation under changing temperature conditions is provided in section 2. Existing approaches to ETE with fixed design solutions are surveyed in sections 3 and 4 — materials and design techniques respectively. Section 5 presents dynamic (adaptive) techniques and stresses the bio-inspired techniques as the promising guidelines for the novel solutions. Lastly, the conclusion remarks are given.
2. Theoretical background
Although new materials are under investigation and some materials are being applied to commercial devices, the majority of electronics today is based on semiconductor technology and, in particular, silicon. This section presents an overview of the challenges that silicon is facing with respect to extreme temperature environments and also different issues that need to be taken into account when considering semiconductors.
2.1 Operating point
The operation of an electronic device is described by its input-output characteristics. The operating point reflects the relationship between the inputs and outputswhich describes the device functionality under given conditions. When exposed to extreme temperatures, this change in conditions causes the operating point to move resulting in a deviation in the device functionality.
2.2 Carrier mobility
Semiconductor materials are, in their native state, nonconducting materials. It is the ionised dopant atoms added to these materials that provide for the majority of electricity carriers in the material. However, the semiconductor intrinsic carriers can provide a certain amount of the total electricity carriers under specific conditions— see section 2.3. Temperature variation causes an inverse exponential change in carrier mobility i.e. it is higher at low and lower at high temperatures. This kind of behaviour is a consequence of the fact that rising temperature makes atoms in the semiconductor crystal lattice vibrate causing more collisions with the dopant carriers—lattice scattering, making them less mobile [27].
2.3 Intrinsic carrier concentration
Rising temperature provides increased thermal energy to electrons in the semiconductor valence band. If this energy is higher than the semiconductor bandgap, the electron is promoted from the valence band to the conduction band thus contributing to a higher concentration of carriers and thereby increasing current. The concentration of intrinsic carriers is exponentially dependent on temperature. However, for the same temperature level and intrinsic carrier concentration, wider bandgap semiconductors exhibit lower current resulting from intrinsic electron promotion.
2.4 Threshold voltage
In CMOS technology, which is the dominating silicon technology today, threshold voltage shows linear dependence on temperature [34]. With technologies based on other materials, threshold voltage shows similar behaviour. It is only the parameters which determine its functional dependence on temperature that differ, for example the rate of the change.
2.5 Leakage current
Leakage current represents one of the challenges of silicon-based technology. In CMOS technology, there are several components which contribute to leakage current. Junction leakage, which is characteristic for reverse-biased p-n junctions, occurs at the junction between source or drain and the well due to minor carrier diffusion near the junction depletion region and also due to electron-hole pair generation within this region. Subthreshold leakage occurs when the gate voltage is below the threshold value. Further, there are also components caused by the gate electric field and other components which arise in short-channel devices [33]. Scaling towards nano-devices will increase leakage current effects.
2.6 Low-temperature behaviour
At very low temperatures MOS structures exhibit specific behaviour. Dopant carriers are frozen-out at specific low temperatures e.g. 30K for Si, where the thermal energy level is insufficient to ionise dopant atoms. This makes pwell and n-well perform similar to insulators [10]. As temperature lowers, the majority carriers are trapped at source making a bias at the well. The well bias causes threshold voltage to decrease which results in a current increase. For the particular value of the drain voltage the current saturates at a higher level than the Id-Vd characteristics. Thus the Id- Vd characteristic exhibits current kink and hysteresis at low temperatures. Other possible explanations for current kink may be seen in the literature [19, 48]. Current kink is more pronounced for n-MOS structure. The freeze-out phenomenon in bipolar transistors is pronounced at base and collector sites. It affects the transistor frequency response, dynamic switching performance and the noise properties [9].
2.7 Zero-Temperature-Coefficient (ZTC) bias points
ZTC bias points refer to the two gate bias voltage points, one in linear another in the saturation region of the MOSFET structure, for which drain current is least affected by temperature [34]. This phenomenon can be exploited for tackling the temperature effects in MOSFETs through biasing schemes, see [14].
2.8 Semiconductor Crystal Structure
All semiconductor substrate wafers have single-crystal structures. Such a structure exhibits higher periodicity, making the dopant movement through the crystal structure easier. Therefore, the quality of the crystal structure greatly influences semiconducting properties such as carrier mobility. On the other hand, all semiconductor dielectrics are amorphous. The amorphous nature of the structure preventselectricity movement yielding material dielectric properties [22].
Single-crystal structures are, therefore, sought for future electronics based on synthetic semi-conducting materials.These materials are generated through the process of epitaxial i.e. layer-by-layer growth during which impurities are added. However, for many potentialmaterials it is very hard to achieve a defect-free single-crystal structure during this process.
2.9 Packaging and Wiring
Operation of the electronic device at extreme temperatures is limited not only by the temperature-induced phenomena and temperature-dependent properties of its components, but also by the packaging and wiring. At high temperatures the interaction between materials is more pronounced due to the presence of higher thermal energy. A number of aspects need to be addressed: characterisation of materials and their interactions at high temperatures; minimisation of mechanical stresses caused by thermal expansion mismatches; provision of heat dissipation path and environmental protection; improvements in metallisation and device development tools and testing equipment