Impact of carbon in the buffer on power switching GaN-on-Si and RF GaN-on-SiC HEMTs

Abstract

This article addresses the impact of the buffer doping on the critical performance issues of current-collapse and dynamic R ON in GaN high electron mobility transistors. It focusses on the effect of carbon, either incorporated deliberately in GaN-on-Si power switches, or as a background impurity in iron doped RF GaN-on-SiC devices. The commonality is that carbon results in the epitaxial buffer becoming p-type and hence electrically isolated from the two-dimensional electron gas by a P–N junction. Simulations which incorporate a model for leakage along dislocations are used to show that a remarkably wide range of experimental observations can be explained including dynamic R ON and the complex time dependence of drain current transients in power switches. In RF GaN-on-SiC devices, the current-collapse, the drain current dynamics, kink effect, pulse-IV and electric field distribution in the gate-drain gap can all be explained.

Suppression of charge trapping in ON-state operation of AlGaN/GaN HEMTs by Si-rich passivation

“…In the ungated region, the derivative of the potential with respect to the distance from the gate (x) is proportional to the local resistance of the 2DEG (R(x) = V(x)/I and I is a constant throughout 2DEG). Figure 7(a) shows a schematic of negative charge distribution in wafer A during the ONstate by applying this rule (consistent with the detailed simulations in [16]). Some negative charges are accumulated under the drain and gate edges leading to the peak lateral electrical fields, and uniform negative charges are distributed within the ungated region.…”

“…After the substrate stress, there are two discharging paths as depicted in figure 7(b). The first path is vertical current flow leading to recombination between the positive and negative charges within the carbon doped GaN layer [16,17]. The second path is the same as the mechanism disccussed for ON-state stress, as the remaining negative charges spread laterally and flow to the contacts.…”

“…In contrast, the wafer D shows little negative charge storage in the GaN buffer apart from below the gate edge. The basic difference between wafers A and D can be explained in the leaky dielectric model [15,16], in terms of the relative vertical leakage through the UID GaN channel. For Figure 7.…”

Study of Drain Injected Breakdown Mechanisms in AlGaN/GaN-on-SiC HEMTs

“…The doped GaN is doped with 2 × 10 17 cm −3 C deep acceptor (CN, Ev+0.9 eV) and 1 × 10 17 cm −3 shallow donors (CGa and intrinsic donors, Ec-0.03 eV) giving a compensation ratio of 0.5. This doping profile will finally result in a p-type buffer as the Fermi level is pinned at about Ev+0.97 eV with the Fe neutral except near the surface [10]. Band-to-band leakage paths have been added under the source and drain contacts by adding heavily doped p-type shorts, which allows hole flow from the contact to the buffer [13].…”

Vertical field inhomogeneity associated with threading dislocations in GaN high electron mobility transistor epitaxial stacks

“…On the other hand, leakage along threading dislocations is also known to facilitate efficient discharging of the buffer following trapping under stressed conditions, particularly when C-doped buffer layers are involved. This hugely benefits dynamic device performance following switching from off-state to on-state [22][23][24][25] . This ultimately leads to a tradeoff between a low density of conductive dislocations to ensure low high field leakage impacting breakdown, and sufficient dislocation-originated leakage to achieve good dynamic device performance 26 .…”