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978-3-8439-1924-1, Reihe Elektrotechnik
Threshold Voltage Engineering of GaN-based n-Channel and p-Channel Heterostructure Field Effect Transistors
182 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2014), Hardcover, A5
Alternative materials are currently considered to replace Si in diverse areas of solid-state electronics. One of these areas is power-switching. By increasing the efficiency of power-conversion systems, total energy consumption can be reduced. While on the system level, efficiency can be enhanced by improving passive components or circuits design, on the transistor level, it can be enhanced by new device designs or the use of new semiconductor materials. Among such materials, the group III nitrides (GaN, AlN, InN) are considered as suitable alternatives to Si. These materials feature inherent polarisation effects (spontaneous and piezoelectric). This polarisation can be engineered by changing the material composition, i.e. by mixing the compounds to alloys (e.g. GaN and AlN to AlGaN). By growing a nitride-based heterostructure in which a wider-bandgap material is deposited on top of a narrower-bandgap material, a polarisation difference is typically present at the material interface. This difference then leads to a fixed interface charge which is compensated by the formation of a 2-D electron gas (2DEG). Within this 2DEG, very sheet high carrier densities (up to 6x10^13 cm^-2) and large mobilities (above 2000 cm^2/Vs) are achievable. Together with the high electrical breakdown strength of the material, the 2DEG is the cornerstone of the superiority of GaN heterostructure FETs (HFETs) over Si MOSFETs in terms of power-switching performance.
When it comes to fabricate circuits for power-switching, enhancement mode (e-mode) behaviour of the power transistor is a necessity to enable fail-safe operation and to allow for high efficiency of the circuit. However, due to the intrinsic presence of the 2DEG, GaN devices are typically of depletion type. Although different approaches have been employed to render these devices e-mode, the single heterostructure limits the achievable threshold voltage (V_th) to below +1 V. This value, however, is not expected to be sufficient in a real-world application.
As solution for this fundamental problem, the single heterostructure is replaced in this thesis by a double heterostructure in which at the backside of the GaN channel another polarisation difference counteracts the one which is responsible for forming the 2DEG. Moreover, a dielectric is used such that a metal-insulator-semiconductor double hetero-structure FET (MIS-DHFET) is formed.