Parasitic energy barriers in SiGe HBTs

Determination of bandgap narrowing and parasitic energy barriers in SiGe HBT's integrated in a bipolar technology

“…4 and 5 show anomalous behavior on both LOCOS and mesa devices when the undoped SiGe spacer is very thin. This suggests that the explanation for the anomalous behavior may be out-diffusion of boron from the SiGe base, which leads to the formation of parasitic energy barriers at the emitter/base and collector/base junctions [11]- [13], and consequently to a reduction of the collector current. The perimeter dependence of the slope seen in Fig.…”

“…For the 900-Å thick, 2.10 cm boron-doped transistor, we have estimated a transit time in the base, of 6.3 ps, leading to a cut-off frequency, of 19.4 GHz (this simple calculation does not consider the extrinsic elements). The Slotboom model [13] gives an expression of the base transit time in the presence of barriers If we use the measured barrier characteristics with the previously calculated base transit time we can estimate a new base transit time of 32.6 ps, and a drop of the cutoff frequency to 4.6 GHz. We have measured on equivalent devices a cut-off frequency of approximately 6.3 GHz, which is much lower than to the original estimate of 19.4 GHz, and comparable to the value of 4.6 GHz obtained by taking into account parasitic barriers.…”

“…The Slotboom parasitic energy barrier model [13], [28] can be adapted to obtain an expression for the collector current density as a function of the barrier characteristics. If a barrier width and a barrier height are defined (Fig.…”

“…Boron out-diffusion from the base of a SiGe HBT can severely degrade the device performance due to the formation of parasitic energy barriers at the collector/base and emitter/base junctions [11]- [13]. Even very small amounts of out-diffusion (a few nanometers) from the SiGe base can dramatically degrade the collector current [14] and hence the current gain.…”

“…In this paper, the analysis method described earlier [10], [11], [13] are further developed to take into account outdiffusion from the heavily doped bases of SiGe HBT's. This modified analysis method is then used to study how the major processing variables influence the device electrical characteristics when a SiGe epitaxial base is inserted into an existing CMOS-derived, single polysilicon, sef-aligned bipolar process.…”