Investigation of buffer traps in an AlGaN/GaN/Si high electron mobility transistor by backgating current deep level transient spectroscopy

Abstract

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ͑HEMT͒ on silicon ͑111͒ substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 30 to 100°C. With this method, known as backgating current deep level transient spectroscopy, majority carrier traps with activation energy of 200 meV as well as minority carrier traps at 370 meV are identified. The experiments are performed on completed HEMTs, allowing the investigation of the influence of device fabrication technology.Sapphire and SiC are commonly used as substrate materials for GaN based electronic and optoelectronic devices. Recently Si has been found as a useful alternative because of its low cost and good thermal conductivity. AlGaN/GaN/Si high electron mobility transistors ͑HEMT͒ with unity current gain frequencies comparable to those known for devices using sapphire or SiC substrates have been reported. 1 In contrast to sapphire the electrical conductivity of the Si substrate allows the application of a voltage at the backside of the devices through a substrate contact. This provides the control of backgating to study the physical properties of the device and of the material ͑defect states, trapping effects͒. The backgating effect is well known from GaAs based metalsemiconductor field effect transistors ͑MESFET͒ and HEMTs. 2 The junction between substrate and buffer creates a depletion layer that controls the transistor channel in the form of a gate from the backside. This effect can be used to control the drain current by a substrate voltage. The backgating effect is also one origin of the very high optoelectronic responsivity of MESFETs and HEMTs. 3 For HEMTs the backgating depletion region and therefore the influence of the substrate voltage is limited to the region below the twodimensional electron gas ͑2DEG͒. Therefore, all related effects have their origin in the bulk without contribution of surface effects. Uren et al. 4 have profited from the influence of a backgating voltage on the pinch-off voltage of an AlGaN/GaN HEMT on a conducting SiC substrate to investigate the electrically active centers in the GaN buffer. They have shown that the backgating depletion layer of the AlGaN/GaN HEMTs extends from the 2DEG into the buffer and is not located between substrate and buffer. Therefore, the observed effects do not originate from traps in the lower, more defective part of the buffer, but they are due to traps near the heterointerface. In this work we use backgating current deep level transient spectroscopy ͑DLTS͒ 5 to investigate the activation energy of traps in the buffer of an AlGaN/GaN HEMT on a Si substrate. Figure 1 shows the schematic band diagram of the substrate biased device.The device was fabricated using an AlGaN/GaN heterostructure grown by metalorganic vapor phase epitaxy on a 2 in. ͑111͒-o...

Trapping Analysis of AlGaN/GaN Schottky Diodes via Current Transient Spectroscopy

“…The most popular one is deep-level transient spectroscopy (DLTS), which can be performed in capacitance-mode (C-DLTS) [7][8][9] or current-mode (I-DLTS) [10][11][12]. However, capacitance or current transient measurement on real HFET devices can be limited to a degree because of their small dimensions.…”

Buffer transport mechanisms in intentionally carbon doped GaN heterojunction field effect transistors

“…Changes in the conductivity can then be used to quantify bulk charge storage and trapping. The key advantage of this approach, 9,6,10,11 is that surface effects and trapping are excluded, allowing unambiguous measurement of buffer trapping alone. Only negative substrate bias, V SUB , is considered here since this corresponds to the polarity experienced by a transistor under OFF state conditions.…”

Investigation of Buffer Traps in AlGaN/GaN Heterostructure Field-Effect Transistors Using a Simple Test Structure

“…However, recent reports show that the bulk traps in the semi-insulating GaN buffer also can cause the serious current collapse in AlGaN/GaN HFETs [3]. Previously, the bulk traps in the semi-insulating GaN buffer have been mainly investigated using the capacitance-mode deep level transient spectroscopy (C-DLTS) [4], the backgating current transient spectroscopy [5,6], and the drain current transient method [7][8][9]. C-DLTS is a powerful and well-established technique for the extraction of bulk traps in semiconductor devices.…”