Mesa and planar SiGe-HBTs on MBE-wafers

SiGe-HBTs have the potential for outstanding analog and digital or mixed-signal high frequency circuits widely based on standard Si technology. Here we review on MBE grown transistors and circuits. Processes and results of a research-like SiGe HBT and two possible production relevant HBT versions are presented. The high frequency results with f_max and f_T up to 120 GHz and a minimum noise figure of 0.9 dB at 10 GHz demonstrate the advantage of using MBE samples with steep and high base doping and high germanium contents. A comparison to the concept of reported low doped, low germanium and triangular profiled SiGe base layers, realized by UHV-CVD, is given. In addition, some circuit demonstrators of SiGe-ICs will be presented.     

26-GHz etched-groove silicon permeable base transistor

A report is presented on the fabrication of etched-groove silicon permeable base transistors (PBTs) that reveal a maximum frequency of oscillation and a unity-current-gain cutoff frequency of 26 GHz, the latter being the highest value yet reported for Si PBTs. This performance was achieved with mushroom-shaped source fingers, a structure that permits both the passivation of the Si finger sidewalls and the reinforcement of the platinum gate with a gold overlayer. The doping distribution was optimized with two-dimensional numerical simulations, parasitic capacitances were reduced using a multilayer polyimide structure, and deep-UV optical lithography was used to fabricate PBTs with 0.2- to 0.4-μm finger widths     

Silicon permeable base transistors fabricated by selective epitaxial growth

A new type of silicon PBT uses lateral selective low-pressure vapour-phase epitaxy to grow ridges with mushroom-like cross-sections leading to self-aligned gate and drain electrodes. Advantages are the lack of any etching damage encountered with etched-groove PBTs and high-temperature process compatibility. First experimental devices with bar widths of 1 mu m exhibited transconductances of 7 mS/mm.<>     

The influence of emitter-base junction design on collectorsaturation current, ideality factor, Early voltage, and device switchingspeed of Si/SiGe HBT's

In advanced Si/SiGe HBT's the base is doped much higher than emitter and collector. Base outdiffusion becomes a problem because of the formation of parasitic barriers that degrade device performance. The simulations and experiments of this paper show that a strong correlation exists between (a) the drop of the collector saturation current, (b) an increase of its ideality factor and (c) a rise of the switching time due to an additional emitter delay which can no longer be neglected. Curves of these three parameters as a function of Si/SiGe heterointerface position and outdiffusion at the base-emitter interface have been calculated and indicate that only a few nm shift may cause severe device degradation. An important result is that the collector current ideality factor or the inverse Early voltage is a very sensitive indicator for the quality of the emitter-base interface. Application of these results have yielded experimental SiGe HBT's with transit frequencies above 60 GHz     

Dynamic characterisation of Si/SiGe power HBTs

Si/SiGe power heterojunction bipolar transistors (HBTs) grown by MBE were dynamically characterised in the common-base configuration. At an emitter current density of 1.1×10⁵ A/cm², a maximum frequency of oscillation of 49 GHz was observed. At 10 GHz a maximum unilateral gain of 14 dB is available, and a CW output power of 1.3 W/mm for a device with 10 parallel emitter-fingers of 1×10 μm² each was predicted, from CW measurements     

Dual-gate silicon permeable-base transistors built on LPVPE-grown material

Permeable-base transistors have been fabricated on epitaxial silicon layers grown by low-pressure vapour-phase epitaxy. Reactive ion etching is used to form the grooved-etched transistors with 0.5-2 ?m-wide channels. The emitter geometry permits self-aligned formation of two separate base electrodes which can be useful in mixer applications. Measured gm values of 62 mS/mm are the highest yet reported for Si PBTs.     

Silicon etched-groove permeable base transistors with 90-nm fingerwidth

Silicon etched-groove permeable base transistors (PBTs) which utilize a new structure to eliminate surface depletion effects are discussed. The 90-nm-wide fingers are n⁺-doped, and the channel region is buried below the bottom of the grooves. Doping and thickness of the active layer were optimized using two-dimensional computer simulations. The maximum measured transconductance of 155 mS/mm is the highest reported for Si PBTs and demonstrates the potential of silicon as substrate material. The measured transit frequency was 12 GHz; f_max reached 13 GHz. It has been recognized that for improved high-frequency performance a reduction of the gate capacitance is necessary, demanding a more precise control of groove depth and geometry     

MBE-grown Si/SiGe HBTs with high β, fT,and fmax

Si/SiGe heterojunction bipolar transistors (HBTs) were fabricated by growing the complete layer structure with molecular beam epitaxy (MBE). The typical base doping of 2×10¹⁹ cm^-3 largely exceeded the emitter impurity level and led to sheet resistances of about 1 kΩ/□. The devices exhibited a 500-V Early voltage and a maximum room-temperature current gain of 550, rising to 13000 at 77 K. Devices built on buried-layer substrates had an f_max of 40 GHz. The transit frequency reached 42 GHz     

1/f noise in self-aligned Si/SiGe heterojunction bipolar transistor

The first characterization of the low-frequency noise in a self-aligned Si/SiGe heterojunction bipolar transistor (HBT) is reported. The observed low-frequency noise exhibits a pure 1/f shape, probably related to carrier number fluctuations at the pseudomorphic emitter-base heterointerface