technology GaN HEMTs
hot electron relaxation can be accompanied by other mechanisms, such as electron-phonon and interface scattering [2].
Electronic traps
Electrons passing along the channel can face other obstacles. These include electronic traps, which can have a direct or indirect impact on the life of an electron and the related current density. One type of electronic trap is associated with defect states, which capture and release electrons within a characteristic time, thereby modifying the electric field distribution near the device channel. Identifying the location of traps in degraded HEMTs is very challenging. The most common approach is to turn to simulation to estimate the location of the traps, based on a fit to electrical device characteristics. This is a valuable, but rather indirect approach.
In another class of device, a large-area capacitor, gate-capacitance-based measurements, such as conductance techniques, are widely used to evaluate trap characteristics. However, this approach cannot be transferred to short-channel devices, because inaccuracies are far larger, due to the weak capacitance signal offered by the small gate area. In contrast, dynamic transconductance does not require capacitance measurements to extract the trap conductance [3]. In this case, both the interface and the bulk traps are detected and distinguished by analysing bias dependence of the extracted conductance (see Figure 2 for an example of bulk trap conductance). In addition to bulk and AlGaN/GaN interface traps, surface and subsurface traps can impact the path of the electron as it passes along the channel. If the traps are near the gate, they affect the electric field in the channel that governs the electron transport.
UV-assisted transient trapping is capable of locating and identifying electronic traps in GaN HEMTs. This technique involves applying a trap-filling pulse at a high negative gate-source voltage and then recording the de-trapping transient in on-state operation. Exposing the transistor to UV-light prior to the filling pulse can modify the electronic trap population.
We have determined the current transient trapping characteristics for GaN HEMTs illuminated with UV light of different photon energies (see Figure 3). These measurements reveal three trap states with different time constants: Only the slow trap is strongly affected by light exposure. When illuminated with photon energies between the bandgaps of AlGaN and GaN, these devices show a large trapping amplitude, indicating that the traps are located in the AlGaN barrier close to the gate (see the inset of Figure 3).
These traps are probably caused by oxygen reactions at the device surface [4], which can reduce the
Figure
3.Transient current-trapping characteristics of an AlGaN/GaN/SiC HEMT.A trap-filling pulse was applied at VDS
= 0 V ,VGS = -10 V
with subsequent transient
measurements at VDS
= 0.5 V ,VGS = 1 V . electric field in the electron channel.
The consequences of this are not limited to reduced electron acceleration in the channel, and include partial depletion of the 2DEG and a subsequent decrease in drain current.
When surface trap density increases during device operation and electrical stressing, leakage pathways are formed through the AlGaN barrier to the device channel. Leakage current then rises during device stress, correlating with the appearance of EL spots during off-state operation [5]. We have noted that there are localised EL spots along the drain edge of the gate – where the maximum electric field is located – that multiply in number during device stress (see Figure 4). The appearance of each new spot coincides with a step increase in the gate
The current transients are labelled with the photon energy of the UV light exposed to the device prior to measurement
,
Typical experimental setup used for combined electrical and optical device testing,including Raman thermography,photoluminescence and electroluminescence analysis as well as device stressing and transient trapping analysis
January / February 2013
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