6.2 Breakdown Voltages

For device comparison the most commonly used criterion is a simple gate-diode measurement, see Chapter 4. However, more elaborate methods have been suggested. The method used is described by [270], which measures the ${\it V}_{\mathrm{DS}}$ path for a constant gate current ${\it I}_{\mathrm{G}}$ as a function of ${\it V}_{\mathrm{GS}}$. This path changes with the technology under investigation. The considerations are extended to high-power HEMTs and higher voltages in order to understand the limitations also for these devices.

Figure 6.8: Current dependence of the breakdown voltage $BV_{DS}$ of Technology B, Variation A.

Figure 6.9: Current dependence of the breakdown voltage $BV_{DS}$ of Technology B, Variation B.

Contributions to the gate currents ${\it I}_{\mathrm{G}}$ arise technology dependent from several sources. Fig. 6.8 and Fig. 6.9 show the comparison of two different variations of the same pseudomorphic technology. The most important difference between the two variations is a change of the $\delta$-doping. Comparing Fig. 6.8 and Fig. 6.9 for a given gate current ${\it I}_{\mathrm{G}}$, the HEMT with the relatively higher $\delta$-doping (Variation B) has a lower breakdown voltage, which is not only justified by the diode measurement, but also as a function of ${\it I}_{\mathrm{D}}$.

Fig. 6.10-Fig. 6.16 show various data sets in order to analyze different contributions to the gate current.

Figure 6.10: Current dependence of the breakdown voltage $BV_{DS}$ of Technology A.

Figure 6.11: Current dependence of the breakdown voltage $BV_{DS}$ of Technology C.

The two HEMTs in Fig. 6.10 and Fig. 6.11 have the same gate length ${\it l}_{\mathrm{g}}$, but originate from completely different fabrication technologies. Fig. 6.11 shows the extension to very high currents ${\it I}_{\mathrm{D}}$, which has been performed only selectively for some ${\it I}_{\mathrm{G}}$ to obtain reproducible measurements as function of ${\it I}_{\mathrm{G}}$. The reproducibility was always confirmed. It can be seen in both Fig. 6.10 and Fig. 6.11, that both technologies exhibit breakdown voltages for low currents $\geq$ 5 V, however, that for class A operation defined as ${\it I}_{\mathrm{D}}$ $\approx$ 0.5 $\cdot {\it I}_{\mathrm{Dmax}}$ a DC bias ${\it V}_{\mathrm{DS}}$$\geq$ 3.5 V cannot be recommended. The large signal voltage sweep would cause significant gate currents which reduce long term reliability.

Figure 6.12: Temperature dependence of the breakdown voltages $BV_{GS}$ and $BV_{GD}$ of Technology B.

Figure 6.13: Temperature dependence of the breakdown voltage $BV_{DS}$ of the InP-based composite channel HEMT of Technology E.

In Fig. 6.12 the temperature dependence of the off-state breakdown voltage of a high-power pseudomorphic AlGaAs/InGaAs HEMT is shown. First, it can be seen that the change can be neglected relative to the asymmetry of the two voltages ${\it BV}_{\mathrm{GS}}$ and ${\it BV}_{\mathrm{GD}}$ due to non-symmetric processing. The breakdown voltage rises slightly with temperature ${\it T}_\mathrm{L}$ for a given bias for this device with an In content of $x$= 0.25. Fig. 6.13 shows the dependence of the breakdown voltage for a composite channel In$_{0.52}$Al$_{0.48}$As/In$_{0.66}$Ga$_{0.34}$As/ In$_{0.53}$Ga$_{0.47}$As HEMT on InP. The general decrease of ${\it BV}_{\mathrm{DS}}$ with rising ${\it T}_\mathrm{L}$ is a severe problem in terms of reliability for this materials system, as discussed in Chapter 7.

Figure 6.14: Drain ledge dependence of the breakdown voltage $BV_{DS}$ for $I_G$= 1 mA/mm of Technology D.

Figure 6.15: Breakdown voltage $BV_{DS}$ versus gate length $l_g$ of Technology D.

Fig. 6.14 shows the drain ledge dependence of the on-state breakdown voltage ${\it BV}_{\mathrm{DS}}$ for constant ${\it I}_{\mathrm{G}}$= 1 mA/mm in a pseudomorphic high-power AlGaAs/InGaAs/GaAs HEMT. The investigations are performed for devices with ${\it l}_{\mathrm{g}}$= 150 nm and the breakdown voltages were measured in a row of transistors in the same cell on the same wafer. With increasing length of the drain ledge the breakdown voltage ${\it BV}_{\mathrm{DS}}$ shows maximum values up to ${\it V}_{\mathrm{DS}}$= 28 V up to ${\it I}_{\mathrm{D}}$= 100 mA/mm measured for ${\it W}_{\mathrm{g}}=$6$\times$100 $\mu$m transistors. These results are best values obtained, but at the same time confirm the potential of pseudomorphic HEMTs for high-power applications.

Figure 6.16: Temperature dependence of the breakdown voltage $BV_{DS}$ of Technology D.

Fig. 6.15 shows the comparison of the measured maximum ${\it BV}_{\mathrm{DS}}$ for ${\it I}_{\mathrm{G}}$= 1 mA/mm as a function of gate length ${\it l}_{\mathrm{g}}$ for the same lateral recess configuration. For the off-state and low ${\it I}_{\mathrm{D}}$ levels the breakdown voltage ${\it BV}_{\mathrm{DS}}$ decreases with decreasing gate length ${\it l}_{\mathrm{g}}$ for the two transistors with gate lengths of ${\it l}_{\mathrm{g}}$= 150 nm and ${\it l}_{\mathrm{g}}$= 200 nm. Fig. 6.16 shows the temperature dependence of the on-state breakdown voltage of the pseudomorhic Technology D. A rising breakdown voltage with increasing temperature ${\it T}_\mathrm{L}$ is found which demonstrates the dominance of the thermionic field emission processes at the gate in the breakdown process for these HEMTs.