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4.6 Breakdown Quantities

To describe limitations towards high terminal voltages the following quantities are used. The most simple quantities are the reverse characteristics of gate diode measurements:

    $\displaystyle {\it BV}_{\mathrm{GD}}= {\it V}_{\mathrm{GD}} \vert _{{\it I}_{\mathrm{G}} = 1 \text {mA/mm}}$ (4.21)
    $\displaystyle {\it BV}_{\mathrm{GS}}= {\it V}_{\mathrm{GS}} \vert _{{\it I}_{\mathrm{G}} = 1 \text {mA/mm}}$ (4.22)

with the third terminal floating. The definition of the voltages $ {\it BV}_{\mathrm{GD}}$, when $ {\it I}_{\mathrm{G}}$= 1 mA/mm is reached, is an arbitrary, but generally accepted measure, since 1 mA/mm is considered a significant damaging reverse current. $ {\it BV}_{\mathrm{GD}}$ and $ {\it BV}_{\mathrm{GS}}$ give first indication of the breakdown hardness of the device, especially for pseudomorphic AlGaAs/InGaAs HEMTs, where the gate diode is limiting the maximum $ {\it V}_{\mathrm{DS}}$ bias. However, as was shown by Sommerville et al. in [270], the extension path of the gate current $ {\it I}_{\mathrm{G}}$ for the three terminal device into the on-state of the transistor depends very much on the given materials system and technology. Consequently, for InAlAs/InGaAs values from (4.21) and (4.22) are of little practical importance. To evaluate a breakdown voltage for a three terminal device with the third terminal fixed, $ {\it BV}_{\mathrm{DS}}$ is described in [270]:

    $\displaystyle {\it BV}_{\mathrm{DS}}= {\it V}_{\mathrm{DS}} \vert _{{\it I}_{\mathrm{G}} = 1 \text{mA/mm}}$ (4.23)

In this case a constant current is introduced into the gate with the source at ground, while the drain current is swept up. Thus, the magnitude of the breakdown voltage $ {\it BV}_{\mathrm{DS}}$ at constant $ {\it I}_{\mathrm{G}}$ is determined. This technique is useful to understand and separate the two effects, impact ionization and thermionic field emission, as shown in Chapter 6 and 7, respectively. Starting from a large-signal perspective, a breakdown voltage $ {\it BV}_{\mathrm{DS RF}}$  [76] can be extracted from the so-called clipping behavior of a HEMT, i.e., from the generation of nonlinearities for large voltage sweeps. Performing e.g. load-pull measurements, this results in a breakdown locus which has an approximately quadratic behavior as a function of $ {\it V}_{\mathrm{DS}}$, as stated in [292].
    $\displaystyle {\it BV}_{\mathrm{DS RF}}= a+ b \cdot {\it V}_{\mathrm{DS}} + c \cdot {\it V}_{\mathrm{DS}}^2$ (4.24)

$ {\it BV}_{\mathrm{DS RF}}$ is derived from power or load-pull contours. The latter analysis is based on the generation of harmonics of the operation frequency rather than on the gate-currents, although this is related. The comparison of data obtained for $ a$, $ b$, $ c$ for the breakdown voltage is useful to determine the meaning of the DC breakdown voltage for each device technology, since only this behavior shows the limitations of the load matched device for large-signal operation.

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