Bandgap Engineering in SiGe:C HBTs For Power Amplifier Applications
| Autor: | Sebastien Haendler, Sebastien Jouan, A. Monroy, Pierre-Marie Mans, Alexandre Talbot, A. Perrotin |
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| Rok vydání: | 2006 |
| Předmět: | |
| Zdroj: | ECS Transactions. 3:913-918 |
| ISSN: | 1938-6737 1938-5862 |
| DOI: | 10.1149/1.2355885 |
| Popis: | Since the introduction of Germanium in Silicon Bipolar transistors, and more recently the introduction of Carbon, the base band gap of SiGe:C heterojunction Bipolar transistors have been engineered to enhance device performance, thereby making them suitable for a wide range of high speed analog and RF applications. In the most recent development of Power Amplifiers (PAs) for wireless communications applications, current gain roll-off at elevated temperatures in SiGe:C HBTs has become an issue. Circuit designers have to accommodate the thermal runaway instability by designing ballast resistors which helps getting a current handling capability. Moreover, designing a large ballast resistor leads to increase ruggedness, to improve second breakdown but leads to a degradation of the efficiency. An HBT whose current gain is insensitive to temperature can help alleviate these opposing constraints and has the benefit of reducing the requirement for emitter ballasting [1]. Considering that a silicon BJT’s gain has an opposite temperature dependence (its gain increases as temperature increases) compared to a typical Si:Ge HBT, Band Gap engineering studies have been performed on an SiGe:C HBT in order to get its temperature behavior between a HBT and a BJT (i.e. insensible). Figure 1 illustrates current gain roll off with temperature for a SiGe:C HBT based on a mature 0.25μm BiCMOS technology. Current gain dramatically decreases from 220@25°C to 140@125°C (measured at VBE = 0.75V). The observed current gain roll off is caused by the band gap grading in the base and by the doping profile in the E/B junction. Using an existing method for characterizing the bang gap narrowing in the base of the bipolar (which will be detailed in the final paper), band gap energies are extracted and summarized in Figure 2 [2]. From these energies we can estimate the amount of Ge at the quasineutral base edge of the emitter-base space charge region. Then, using the measured emitter and base doping concentration, we can calculate the required Ge concentration at the emitter base junction to synthesize a device with gain insensitive to temperature. For our emitter and base doping concentration, it has been found that a target value of 3% Ge at the E/B junction is required. An optimized SiGe:C base profile has been fabricated, and current gain versus temperature has been plotted in Figure 3. As expected the current gain is invariant over a wide range of temperatures [-25°C, 125°C]. Since we have lowered the Ge concentration at the E/B junction, the current gain is reduced and reaches a maximum value of 95@VBE = 0.75V. It should be pointed out that, in these experiments, no specific effort has been made to recover the high current gain values of the reference HBTs. Higher BVCEO is obtained (7.3V compared to 6.3V) at the expense to a slightly decrease of Ft (Figure 4). Product FtxBVCEO remains constant. In conclusion, we have shown that the band gap energy can be modified in a predictable way to achieve a current gain which is invariant with temperature, an important consideration for SiGe:C HBTs used in PA applications. The optimized device electrical characteristics, DC and HF are presented. |
| Databáze: | OpenAIRE |
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