On Common–Base Avalanche Instabilities in SiGe HBTs

Theoretical analysis and modeling of bipolar transistor operation under reversal base current conditions

“…However, experimental results for forced-I E operation do not show the existence of such a forbidden region. Instead, some discontinuities appear in the CB output characteristics [1][2][3][4]. To understand the experimental CB behavior we need to include high current effects in our analysis.…”

“…This, however, is not a trivial task due to the existence of several concurring (and interacting) mechanisms (impact ionization, self-heating, hot carrier degradation), and the fact that the SOA is also dependent on the driving conditions at the input port [1][2][3][4][5]. It is well known that the widely cited open-base breakdown voltage BV CEO , which limits the operation under forced-I B conditions, does not fully describe the operating limits in practical cases, since the base is not commonly driven by a large impedance [1,2].…”

“…However, the analysis of the operating limits under forced-V BE /I E conditions is made complex by the base current reversal which, combined to the ohmic drop across the base region, yields a current focusing in the center of the base [2]. This current crowding effect is thought to be responsible of anomalous current discontinuities observed in common base (CB) output characteristics [1][2][3][4].…”

Avalanche multiplication and pinch-in models for simulating electrical instability effects in SiGe HBTs

“…A commonly quoted parameter used to define the operating limits of the bipolar transistor is the openbase breakdown voltage BV CEO . However, it has been noted that the maximum operating voltage in practical circuit configurations is actually larger than BV CEO [4]. In fact, common circuit configurations are not characterized by a large impedance on the base, so that SiGe HBTs can be safely biased above open-base breakdown voltage; practical circuit topologies using a common-base configuration include cascode stages, output buffers and differential pairs.…”

“…Unfortunately, when the device is biased above BV CEO the operation is more complex, since avalanche multiplication effects cause a base current reversal. This in turn results in a distributed ohmic drop across the base region yielding a current focusing in the centre of the base, known as the pinch-in effect [4][5][6]. Therefore, in order to predict the SOA boundaries encountered in realistic circuit applications, an accurate model for the pinch-in effect is required.…”