Threshold Voltage Engineering of GaN-based n-Channel and p-Channel Heterostructure Field Effect Transistors

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 6$\times$10$^\mathrm{13}$\,cm$^\mathrm{-2}$) and large mobilities (above 2000\,cm$^\mathrm{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. These two changes of the device structure allow for diverse approaches to be applied for achieving a high positive $V_{th}$. These different approaches are obtained on basis of a newly derived electrostatic equation and the most effective ones in terms of shifting $V_{th}$ are analysed experimentally. The single steps to render GaN-based devices e-mode are discussed separately. Hence, it is possible to optimise the approaches individually. Impact on improved device performance and in particular on the $V_{th}$ shift, as well as possible drawbacks are investigated. With the optimised parameters for each approach, a device which combines all of them is demonstrated, yielding a high positive $V_{th}$ of +2.3\,V. Despite the large effort to fabricate GaN-based devices for RF power amplification and power-switching applications, the technology based on this material system is limited to n-channel devices. p-Channel devices, on the contrary, have not been targeted. Prior to the work discussed in this thesis, only two reports on p-channel devices with very limited data have been published. Yet, by understanding the working principle and by developing well-performing p-channel devices, numerous fields of application not accessible to date would be materialised, thereunder digital logic and voltage amplification under harsh environment. Also, for power-switching applications, the gate driver could then be monolithically integrated on a single chip with the power transistors.In analogy with the n-channel part, for the first time, an electrostatic relation is derived for a complex GaN-based heterostructure involving the presence of a 2-D hole gas (2DHG). Based on the derived relation, means to form p-channel e-mode devices are obtained. Prior to applying these approaches, the p-channel HFET is subjected to optimisations of basic device properties. On basis of the optimised devices, concepts which have already been employed for n-channel transistors are analysed. The p-channel HFETs discussed demonstrate unprecedented performance. These devices also enable to gain a deeper insight into the proposed device structure and physics by employing various characterisation methods not used before for GaN-based p-channel HFETs.In a final chapter, n- and p-channel devices are monolithically integrated on a single wafer. By rendering both devices e-mode, the first demonstration of a GaN-based digital inverter is presented