Analog/RF Performance of NPN SiGe HBT on Thin Film SOI Over -55 to +125 degC Temperature Range

The research has been carried out to integrate MOSFETs and bipolar transistors on SOI substrate for high performance mixed signal applications [1]. Jin Cai, et al [2] analyzed npn SiGe HBT on SOI substrate. The characteristics of SiGe HBT on SOI over temperature range of 27 to 330 degC are reported in [3]. The performance of SiGe HBT on bulk is reported in extreme environment (-230 to 120 degC) in [4]. In this paper, the analog/RF characteristics i.e. AC voltage gain, power gain and unity current gain frequency of npn SOI HBT [5] is studied over temperature range of -55 to +125 degC in Sentaurus device simulator [6]. Fig.1 shows the schematic of a 0.15 X 1.0 μm 2 npn SiGe HBT on thin film SOI with parameters mentioned in Table 1. In this device structure, Ge profile was varied from 10 % (baseemitter junction) to 25 % ( base-collector junction) to introduce an electric field in SiGe base. In the HBT simulation, the models that have been used are discussed in [5]. The Shockley-Read-Hall electron and hole lifetimes of SiGe are 3x10 -6 s and 1x10 -6 s respectively. The VCE was kept 1.2 V and VBE was varied from 0 V to 1.2 V during HBT simulation. The input DC characteristics and current gain were plotted over the temperature range of -55 to 125 degC as shown in Fig.3 and Fig.4 respectively. The small signal analysis of HBT has been performed with sinusoidal signal amplitude of 0.1(kT/q) at 1MHz frequency. From the analysis, the ac voltage gain (Y21/Y22) is evaluated (see Fig.5) over the specified temperature range. It has been observed that the performance to power consumption ratio of HBT is better at low temperature. The power gain, U of SiGe HBT on thin film SOI was obtained from the Y-parameters using formula from [7] (see Fig.6). The unity current gain frequency, ft of the SOI HBT is shown in Fig.7. The ft value of the HBT is better at low temperature due to high trans-conductance value. The 2D numerical simulation of NPN SiGe HBT on thin film SOI within temperature range of -55 0 C to 125 0 C (military specification) has been performed. The DC current gain, AC voltage gain, power gain of HBT is analyzed with this temperature range. At low temperature, the improvement of voltage gain is attributed to high output impedance and high trans-conductance value of the HBT. To achieve same performance, HBT at low temperature consumes lower power compared to the HBT at high temperature. The simulation result suggests that npn SiGe HBT on thin film SOI can be used for analog/RF circuit applications within the specified temperature range which is of interest for military applications.