technology GaN HEMTs electric fields or its high operating temperatures.
Those involved in these efforts must ask themselves one essential question, which will shed light on all the limitations in AlGaN/GaN HEMTs arising from device degradation mechanisms: What happens to the electrons in the device? Or, to put it another way, which obstacles does an electron face on its way from source to drain?
To gain insight into the life of an electron in a GaN HEMT, our team at the University of Bristol, UK, is trying to follow its path through the device and track its behaviour. To do this, we employ a combination of electrical and optical techniques.
We know that once an electron leaves the source, it
Figure 1. Electroluminescence spectrum produced by an AlGaN/GaN HEMT formed on SiC during on-state
operation.This device features an InGaN back-barrier,which enhances the optical output
joins its fellows in the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface, before it is accelerated towards the finish line, the drain, by the high electric field. As the electron heads towards this chequered flag, the chance that it collides with lattice atoms on the way increases. And where collisions occur, it is possible that they spawn a process known as impact ionization. If this does happen, holes formed in the channel layer will create severe device damage once they are accelerated towards the source and gate contact.
This device-degrading event is a common affliction for GaAs HEMTs. Whether it also occurs in GaN HEMTs, however, is far from clear. It’s a controversial, hotly debated issue, with some arguing that the bandgap of GaN is too large for impact ionization to occur. In GaAs HEMTs, impact ionization is easy to detect – you simply directly measure the generated hole current. But in GaN HEMTs, the higher leakage currents swamp the expected hole current from impact ionization, diminishing the chances of observation of any potential impact ionization electrically.
Optical methods
Hunting for holes with optical techniques offers a more promising route to determining whether impact ionization occurs in GaN HEMTs. Modifying the device structure helps in this endeavour, with the addition of an InGaN back-barrier several nanometres beneath the AlGaN/GaN interface introducing a hole collector and provoking photon emission with very high quantum efficiency. That’s not surprising, given that InGaN is the material of choice for forming wells in the active region of blue LEDs.
Figure 2.Bulk trap conductance,Gp/ω,can be determined from transconductance measurements as a function of frequency for different
base-plate
temperatures.The inability of channel electrons to follow the pulsed gate signal leads the conductance to peak at a certain frequency related to the trap-specific time
constant.This technique enables the detection of traps at a density of 1011
cm-2 eV-1
particular those located at or near the AlGaN/GaN interface in GaN
HEMTs.In this case,the traps were identified as bulk traps with a temperature-dependent time constant ranging from 10-2 activation energy of 0.7 eV
to 10-4
Electroluminescence (EL) produced by this modified HEMT features a distinct interband recombination peak at the InGaN bandgap, in addition to broad luminescence from hot electron relaxation (see Figure 1). The bandedge luminescence provides evidence of holes in the device, generated at high electric fields and high electron densities. Our measurements show that this strong bandedge luminescence also occurs at cryogenic temperatures, allowing us to rule out hole emission from traps as the hole generation process [1]. Based on these measurements, we can draw the important conclusion that impact ionization is the dominant cause of hole generation and does occur in GaN HEMTs.
and below,in s and an ,located in the GaN buffer
layer.This device
has a 0.25 µm gate length,and the data was fitted to a model (solid lines) representing a continuum of trap energy levels
70
www.compoundsemiconductor.net January / February 2013
More can be learnt about the behaviour of electrons as they pass through the transistor from the low energy tail in the EL spectrum. This reveals that, in the channel of the device, electrons undergo relaxation processes that don’t involve recombination with holes. The spectral distribution offers an insight into the temperature of the electron and its fellows, showing that they can easily reach several thousand degrees – ten or more times higher than that of the lattice. Impact ionization and
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