Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

Abstract

A one-dimensional self-consistent bipolar Monte Carlo simulation code has been used to model carrier mobilities in strained doped SiGe and the base-collector region of Si/SiGe/Si and SiC/Si heterojunction bipolar transistors (HBTs) with wide collectors, to study the variation of the cutoff frequency f T with collector current density J C . Our results show that while the presence of strain enhances the electron mobility, the scattering from alloy disorder and from ionized impurities reduces the electron mobility so much that it is less than that of Si at the same doping level, leading to larger base transit times τ B and hence poorer f T performance for large J C for an Si/SiGe/Si HBT than for an SiC/Si HBT. At high values of J C , we demonstrate the formation of a parasitic electron barrier at the base-collector interface which causes a sharp increase in τ B and hence a dramatic reduction in f T . Based on a comparison of the height of this parasitic barrier with estimates from an analytical model, we suggest a physical mechanism for base pushout after barrier formation that differs somewhat from that given for the analytical model.

Linewidth for interconduction subband transition in Si/Si1−xGex quantum wells

“…(1), e is the elementary charge, is the reduced Planck's constant, m* is the electron effective mass, f 10 is the oscillator strength, f(E) is the Fermi distribution function, which is calculated for a chosen sheet carrier concentration (N s ), and E 10 is the intersubband energy difference. This energy difference is calculated by solving the Schrödinger equations in the well and barrier regions considering different masses for well and barrier [25].…”

High current effects in double heterojunction bipolar transistors

“…The above analytical equations were used to demonstrate the formation of the parasitic electron barrier in Si/Si 0.8 Ge 0.2 /Si HBTs with base doping of 5 × 10 18 cm −3 and a 7500 Å-thick collector with a doping of 3 × 10 16 cm −3 . These results are compared with those obtained by using Mazhari and Morkoc's analytical model [6] and Briggs et al's Monte Carlo simulation [8] in figure 2. φ C obtained using equation (7) shows clear signs of saturation at high collector current density and its peak value is comparable to the simulated results obtained by Briggs et al (φ C ≈ 56 mV) and Mushini and Roenker [10] (φ C ≈ 60 mV).…”

“…However, the formation of φ C causes an accumulation of excess electrons in the base edge and an increase in base recombination with a consequent reduction in gain. The simulated results of Briggs et al [8] and Mushini and Roenker [10] showed that, at high current density, this excess electron density at the edge of the base depletion region edge, n B , is comparable with the background doping density. Briggs et al suggested that the extra holes injected into the base to offset this excess electron charge will influence the hole density in the CIB region.…”

“…Recently, several studies have been made to investigate the formation of the electron barrier in Si/SiGe DHBTs at the onset of the Kirk effect using simple analytical models [5][6][7]. The development of the electron barrier at high current density in SiGe DHBTs has also been investigated using Monte Carlo [8] and various numerical simulators [9,10]. However, the analytical models do not agree with the simulated results (the former showing larger parasitic electron barriers) and do not have a clear saturation tendency in the barrier height as indicated by the numerical simulations.…”