Improvement Technologies Of Strained Si/SiGe HBT On Virtual Substrate For High Breakdown Voltage And Low Temperature Sensitivity

The virtual substrate structure is proposed to reduce the strain effect of Si substrate on the epitaxial layer of SiGe in SiGe heterojunction bipolar transistor (HBT). The increase of Ge composition in SiGe base could reduce the base energy band gap, leading to an increase in current gain. The gain can subsequently be traded off against the reduced base resistance RB, which could improve the performance of SiGe HBT for high frequency and high power applications. With the development of strained-Si in the fourth-generation SiGe technology, the strained Si/SiGe HBT on virtual substrate will play a significant role in the terahertz applications, such as millimeter-wave radars, multigigabit-per-second wireless local area networks(WLAN), 100 Gb/s Ethernet, and so on. However, the breakdown voltage of strained Si/SiGe HBT on virtual substrate is lower than SiGe HBT on Si substrate due to the low breakdown field in SiGe, which could result in the decrease of the output power. Because of the poor thermal conductivity of SiGe, the self-heating effect in strained Si/SiGe HBT on virtual substrate is more seriously, which will lead to the drift of quiescent point, degenerate the device performance and aggravate the thermal instability especially when device operates at high current.In the thesis, the improvement technologies of strained Si/SiGe HBT on virtual substrate are studied for high breakdown voltage and low temperature sensitivity. The main works can be summarized as follows:First, the model of strained Si/SiGe HBT on virtual substrate is established with SILVACO/ATHENA based on the manufacturing process of the device proposed by the Newcastle University. The stress characteristic, DC characteristic and frequency characteristic of strained Si/SiGe HBT on virtual substrate are extracted, and the breakdown characteristic and temperature sensitivity of the device are analyzed in detail. It is shown that the strain effect of Si substrate on the epitaxial layer of SiGe in SiGe HBT is decreased by the use of virtual substrate. As a result, higher Ge composition in the base become viable, and the current gain (β) is improved remarkably. However, the thermal conductivity of SiGe is too low to improve the thermal dissipation capability compared with Si, which would lead to the incease of the device temperature. Considering that both β and cut-off frequency (fT) are associated with the temperature, which woud cause the drift of the bias point in circuits. At the same time, the breakdown voltage of strained Si/SiGe HBT on virtual substrate is also decereased due to the lower breakdown field of SiGe than Si, which restrict the high power application of the device.Second, two types of superjunction design in collector are proposed respectively in order to improve the breakdown voltage of strained Si/SiGe HBT on virtual substrate. It is shown that the distribution of electric field is improved by the design of vertical superjunction (VSJ) structure, which gives rise to the improvement of breakdown voltage. However, the space charge region in collector is extended due to the introduction of superjunction structure, and hence the transist time in space charge region is increased, leading to the degradation of frequency behavior. Therefore, the design of lateral superjunction (LSJ) structure in the space charge region in collector is proposed further to decrease the peak electric field in collector junction. The results show that the breakdown voltage is improved without extending the space charge region in collector, at the same time the high frequency behavior of the device is maintained.Third, the design of trapezoid Ge profile in base is proposed to improve the temperature sensitivity of strained Si/SiGe HBT on virtual substrate. Although the effective dopant concentrations can be reduced, leading to the improvement of thermal conductivity in collector material of HBT and the decrease of the device temperature by the employ of LSJ structure, the temperatre still has a great influence on β and fT-However, with the design of trapezoid Ge profile in base, an accelerating electric field is introduced in the base, the temperature dependence of/? and fi-are weakened, and fT is also improved.Finally, a novel strained Si/SiGe HBT with both the lateral superjunction structure and trapezoid Ge profile is proposed combined the superjunction design in collector with Ge profile design in base. Compared with the conventional strained Si/SiGe HBT with uniform Ge profile in base, the novel deice has a higher breakdown voltage, the whole temperature of the device is decreased, and the temperature sensitivity of β and fT are improved, which could be in thermal stability under high power operation.