| dc.description.abstract | One of the remarkable characteristics of SiGe HBT is the ability to operate over a wide temperature range, from as low as sub 1K, to as high as over 400 K. The SiGe HBT investigated and measured in this work is a first-generation, 0.5 um SiGe HBT with fT/fmax of 50 GHz/65 GHz and BVCEO/BVCBO of 3.3 V/10.5 V at 300 K. The base doping is below but close to the Mott-transition (about 3x10^{18} cm^{-3} for boron in silicon). In this dissertation, some important SiGe HBTs physics at cryogenic temperature are analyzed. New compact models equations for SiGe HBT are developed, which can function from 43 to 393K. Device TCAD simulations are used to help understand the device physics at cryogenic temperatures.
First, the temperature dependence of semiconductors critical metrics are reviewed, including bandgap energy Eg, effective conduction band density-of-states Nc and valence band density-of-states Nv, intrinsic carrier concentration at low doping ni, bandgap narrowing Delta Eg, carrier mobility u, carrier saturation velocity vsat and carrier freezeout. The dc and ac low temperature performance of SiGe HBT are analyzed, including collector current density, current gain, Early effect, avalanche multiplication factor, transit time, cut-off frequency and maximum oscillation frequency. This illustrates why SiGe HBT demonstrates excellent analog and RF performance at cryogenic temperatures.
The current dependence of multiplication factor M-1 at low temperatures are investigated based on a substrate current based avalanche multiplication technique. The M-1 at high current is considerably lower than it at low current. Then, the temperature dependence of forced IE pinch-in maximum operation voltage limit, which is of interest for many space exploration application is investigated. In particular, we discuss how the critical base current IB* varies with temperature, and introduce the concept of critical multiplication factor (M-1)*, critical collector-base bias VCB* where M-1 reaches (M-1)*. A decrease of the voltage limit is observed with cooling, and attributed to the increase of intrinsic base resistance due to freezeout as well as increase of avalanche multiplication factor M-1. A practically high emitter current IE is shown to alleviate the decrease of VCB* with cooling, primarily due to the decrease of M-1 with increasing IE.
The existing commercial compact models are shown to fail below 110 K. In this work, new temperature scaling equations are developed. As much physics basis are implemented as possible to fit temperature dependence of SiGe HBTs dc and ac characteristics, such as ideality factor, saturation current, series resistances and thermal resistance. In particular, carrier freezeout is now modeled accounting for latest research on Mott transition, leading to successful modeling of temperature dependences for all series resistances in SiGe HBT. These new temperature equations give reasonably accurate fitting of the dc characteristics from 393 to 43 K, ac characteristics from 393 to 93 K.
Furthermore, the impact of the non-ideal temperature dependence of IC-VBE in SiGe HBTs on the output of a BGR is examined. These non-idealities actually help make the BGR output voltage vary less at cryogenic temperatures than traditional Shockley theory would predict. Successful cryogenic temperature modeling of both Delta VBE and VBE components of the BGR output is demonstrated for the first time. | en_US |
