Introduction: Multistate Devices and Logic
Chapter 1 discusses the multivalued logic and different negative tunneling devices to implement this multivalued logic. Different problems of different negative-resistance device and possible solution using quantum dot gate FET (QDGFET). There are advanta
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Introduction: Multistate Devices and Logic
This chapter discusses the multi-valued logic and different negative tunneling devices to implement this multi-valued logic. Problems of different negative resistance devices and possible solution using quantum dot gate FET (QDGFET) are studied. Advantages of QDGFET based circuits over the conventional FET based circuits are also explored. A metal-oxide-semiconductor field-effect transistor (MOSFET) produces two states based on its applied gate voltage. When the gate voltage is below the threshold voltage, the transistor is off, and when the gate voltage is more than the threshold voltage, an inversion channel forms below the gate and current conduction occurs between the source and the drain terminals of the device. This allows MOSFET to be suitable only for binary logic. The trend predicted by Moore’s law has called for these MOS devices to shrink in half every 18–24 months [1, 2]. As feature sizes have started to approach sub-22-nm regime, several issues have begun to make further miniaturization difficult [3–5]. As transistors have decreased in size, the thickness of the gate dielectric needs to decrease to increase the gate capacitance and thereby the drive current and device performance [6–9]. As the gate dielectric thickness scales down below 2 nm, leakage currents due to the tunneling of charge carriers increase drastically, leading to unwieldy power consumption and reduced device reliability [10–12]. Another approach to increase integration is to increase the bit handling capacity of a fabricated device. Traditional MOSFETs can only process one bit at a time. Higher level of bit density can be achieved using multivalued logic (MVL) [13]. Multivalued logic is defined as a nonbinary logic and involves the switching between more than two states. Research is ongoing towards the development of devices that are suitable for multivalued operation. Resonant tunneling transistors (RTTs) [14–16] and resonant tunneling diodes (RTDs) [17, 18] are the major promising semiconductor devices for MVL applications. All these devices produce negative differential resistance (NDR) in the specific voltage range of operations. S. Karmakar, Novel Three-state Quantum Dot Gate Field Effect Transistor: Fabrication, Modeling and Applications, DOI 10.1007/978-81-322-1635-3_1, © Springer India 2014
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1 Introduction: Multistate Devices and Logic
Resonant Tunneling Diode (RTD)
A resonant tunneling device is similar in construction to a p-n diode with two terminals [19]. The device has a quantum well layer separated from two conducting ˚ thick). The barriers can be regions by quantum barriers (on the order of 50–100 A AlAs, for example, while the well and conduction regions can be InGaAs. Electrons tunnel through two barriers separated by a well when flowing from the source to the drain in a resonant tunneling diode, which is also known as quantum mechanical tunneling [17]. The energy level in the well is quantized because the well is in the order of de Broglie wavelength. The flow
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