Design and Optimization of Nano-scaled Silicon-Germanium Heterojunction Bipolar Transistors

Abstract

In this work, we explore the design and optimization of nano-scaled SiGe HBTs. The cutoff frequency fT and the maximum oscillation frequency fmax are optimized towards Terahertz. We first start with intrinsic device designs to obtain the initial one-dimensional doping profile, which features a 7 nm base width, a high base doping of 8x10^{19}/cm^3, and a 25 nm collector width. Using this profile, the impacts of Ge designs on device performance are examined at the same film stability. After comprehensive comparisons, we conclude that graded Ge profile wins over box Ge profile in device performance metrics.

Then we focus on the 2D device scaling for the purpose of minimizing device parasitic effects and maintaining the high performance achieved by the highly scaled 1D design. The raised extrinsic base 2D structure is used, which is widely used in 200 GHz -- 350 GHz SiGe HBT technologies. To better understand 2D parasitic effects, the transit time analysis is extended to 2D, which provides a method to analyze the distributive capacitance. Two lateral scaling schemes, fixed total base width and fixed extrinsic base width, are examined. When emitter window is scaled to 60 nm, fmax can be optimized to 1090 GHz for both scaling. However, fixed 'w,exB' scaling features less extrinsic base transit time, and hence the higher fT. Therefore, fixed 'w,exB' lateral scaling is favored for those highly-scaled SiGe HBT designs.