Subsections

• 4.4.3.1 Silicon Nitride as a Wide Bandgap Semiconductor

4.4.3 Effectiveness of Silicon Nitride Passivation

The quality of Si$_3$N$_4$ thin film passivation is of importance for the reliability of III-V based HBTs. Both theoretical and experimental studies have been conducted in order to minimize base leakage currents as a major source of degradation. The effectiveness of the passivation of devices before and after electrothermal stress aging is investigated by the means of two-dimensional numerical simulations. For that purpose proper electrical and thermal models are implemented in MINIMOS-NT allowing simulation of silicon nitride as a wide bandgap semiconductor material.

4.4.3.1 Silicon Nitride as a Wide Bandgap Semiconductor

The non-ideal properties of the passivation are accounted by considering it as a semiconductor material. A bandgap energy of 5 eV, relative dielectric constant of 7, and constant carrier mobilities based on resistivity of $10^{16}$ $\Omega$-cm are used. Thus, previous results assuming an ideal insulator material are verified. However, it has to be noted that the transport properties in the insulator, being treated as a semiconductor, strongly depend on the bandgap alignment on the insulator/semiconductor interface which can be altered in the presence of surface charges.

The impact of the surface charge density is illustrated in Fig. 4.39 where simulation results for the electron current density at V$_\mathrm {BE}$ = 1.2 V without and with a surface charge density of $10^{12}$ cm$^{-2}$, respectively, are shown. Shockley-Read-Hall recombination is taken into account also in the SiN with a trap density of $10^{8}$ cm$^{-3}$. Note, the decrease in the electron current density also in the insulator in the presence of negative charges (Fig. 4.40).

Figure 4.39: Electron current density [A/cm$^2$] at V$_\mathrm {BE}$=1.2 V: Simulation without surface charges

Figure 4.40: Electron current density [A/cm$^2$] at V$_\mathrm {BE}$=1.2 V: Simulation with surface charge density of $10^{12}$ cm$^{-2}$