5.3 Performance Degradation Mechanisms of a III-V HEMT

In the following, a classification of device performance degeneration mechanisms is listed for III-V HEMTs from recent publications. The limitations can first be distinguished between reversible and non-reversible effects. For the reversible effects another distinction is made between thermal and electrical. The thermal limitations can be separated from the electrical ones by comparing the dissipated DC power.

Reversible effects are:

• Thermal reversible effects:
  • Gain reduction [169].
  • Bias shifts due to increased ohmic terminal line resistance.
  • Occurrence of self-conduction of the semiconductors for ${\it T}_\mathrm{L}$ $>$ 400 K.

  • Thermal self-confinement of the transistors in circuits, i.e., the increasing temperature leads to reduced PAE, which again increases the heat generation.

• Electrical reversible effects:
  • Clipping, i.e., the generation of non-linearities [76].

  • Enhancement of gate currents [270].

Non-reversible effects are:

• Thermal non-reversible effects:
  • Temperature enhanced ohmic contact oxidation [46].

  • Enhanced electromigration [243].

  • Intrinsic degradation of the heterostructure materials, e.g. for GaN at ${\it T}_\mathrm{L}\geq$ 873 K [72].

• Electrical non-reversible effects:
  • Changes of trap occupation at interfaces [161].

  • Schottky contact damages due to reverse gate currents caused by thermionic field emission or/and impact ionization [75].

  • Occurrence of impact ionization with increase of drain resistance (InAlAs/InGaAs)  [235] and for higher fields:

  • Avalanche breakdown with crystal damages [234].

  • Electromigration at Schottky and ohmic contacts [46].

  • Break of DC lines and interconnects [302].

  • Electrostatic discharge at Schottky interface [46].

  Other non-reversible mechanisms are:

• Irradiation damages, i.e., introduction of deep traps due to e.g. neutron radiation [210].

The thermal and electrical mechanisms are naturally combined, e.g. by the lattice temperature dependence of the impact ionization rates and the rise of ${\it k}_{\mathrm{B}}\cdot{\it T}_\mathrm{L}$.

Due to the observation of the previous section, a third group is named process enhanced failure mechanisms. This group is the most undesirable, yet, the most interesting, since it leads to a more complete understanding of the fabrication technology. Suggested mechanisms are:

• Introduction of traps due to aggressive etching techniques [88].

• Impact of interface charges due to remaining surfaces pollution before SiN deposition or gate deposition [129,190].

• Effective carrier passivation due to flourine [121] and hydrogen [43].

• Changes in alloy composition at ohmic contacts [324].

• Changes of material composition at the Schottky contact interface [44,235].

Investigations of the physical background of the changes at the metal-semiconductor Schottky interface mostly named "gate-sinking" have been recently performed. The naming arises from the similarity to an effective reduction of ${\it d}_\mathrm{gc}$ at the nanometer scale. Generally, non-reversible increased interdiffusion is assumed to be the origin [46]. An explanation of a similar reversible effect is reported by Blanchard et al. in [44].

Thus, most of the non-reversible and process enhanced failure mechanisms are related to the metal-semiconductor or semiconductor-SiN interface.