Physics and Novel Device Applications of Semiconductor Homojunctions

Introduction and Background

     A junction formed by two different electrical types of the same (bandgap) material can be classified as a homojunction. Similarly, a heterojunction is formed by two chemically different materials. These types of junction structures are well known and extensively discussed in the literature. A common example for a homojunction is the heavily used, well-understood silicon p-n junction. Recently developed GaAs/Al_xGa_1-xAs structures are a good example of a *heterojunction. Here our emphasis is on crystalline semiconductor homojunctions, especially Si. Since almost all the circuit components, such as resistors, capacitors, transistors, diodes, charged coupled devices (CCDs), charge integrated devices (CIDs), shift registers, and detectors, could be fabricated using standard Si technology, putting all those components in one single chip to fabricate an integrated circuit (IC) is a major advantage of using Si.
     Semiconductor homojunction structures, especially the p⁺-n-n⁺ (p-i-n) structures on which we concentrate here, have been studied for a very long time and have been used in a variety of applications. The advent of the molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and other thin film techniques has advanced both homo- and heterojunction design and fabrication to new levels. However, our studies in recent years have demonstrated that even simple and mundane p⁺-n-n⁺ junction structures can exhibit a variety of new electrical and optical phenomena, leading to novel and intriguing device applications consistent with twenty-first century research interests. Here our focus is on homojunctions for both electronic and optoelectronic applications, which mainly involve intraband processes rather than interband processes. These devices will include different types of infrared detectors and spontaneous pulse generators, that act like biological neurons. The fact that the same semiconductor material (with different dopants or concentrations) is used in the homojunction makes the fabrication of these samples much simpler than heterostructures. The material-technology needs for the implementation of these devices in practical applications were met at least a decade ago, making the incorporation of these devices into high-performance integrated circuits just a routine exercise.
     First we will address homojunction infrared detectors based on internal photoemission mechanisms. Various IR detector approaches, using interfacial workfunctions in homojunctions and other IR detector apporaces based on homojunctions, such as a delta doped potential well approach and a room-temperature FIR detector approach, based on a p-type high-low Si junction and a charge storage approach, will be discussed. The interfacial workfunction-type structues will be subdivided into three groups based on their impurity concentrations. Recent experimental results on Si homojunctions showing spectral response wi th a long-wavelength threshold (lambda_t) varying from around 30 to 200 µm confirm the wavelength tunability of these detectors. The main significance of this detector concept is in establishing a technology base ofr the evolution of large-area, uniform detector arrays with a multispectral capability for greatly improved NEdeltaT sensitivity using the well-established Si growth and processing technology.
     Next we will discuss spontaneous pulsing in silicon p⁺-n-n⁺ homojunctions. Another mode of infrared detection will be discussed in connection with these pulses (spiketrains). This is the only mode of IR detection which does not need any preamplifiers. Compared to measuring very small (pico- or microampere) currents spread out over a long duration, counting pulses will be much easier. These spiketrains convey both analog and digital information (mixed character), which can carry more information than either situation alone. The interpulse time intervals convey analog information, whereas the presence or the absence of the almost-uniform-height pulses conveys digital information. Noise immunity is another advantage of this pulsing approach.
     Another application of these pulses is in emulating biological neurons, opening up pathways to design brainlike parallel asynchronous multispectral focal-plane image/sensor processors for various missions. These low-power, high-resolution sensor/processor innovations may have a profound impact on techniques currently being explored for defense, geological and meterological survey systems, strategic and tactical IR systems, ground, airbourne, and space-based FLIR systems, planetary probes, and medical diagnostic systems even though these are futuristic approaches. © 1995 by Academic Press, Inc.

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