A Physics-Based Trap-Assisted Tunneling Current Model for Cryogenic Temperature Compact Modeling of SiGe HBTs

Abstract

The work presents a new physics-based compact model of trapassisted tunneling (TAT) current in forward biased base-emitter junction. Such TAT current is negligible at room temperature, but becomes significant at cryogenic temperatures. Inclusion of TAT current and its separation from the total base current is not only important for lower V BE base current, but also affects extraction of the ideal base current. The model is successfully applied to a firstgeneration SiGe HBT technology from 43 to 110 K. IntroductionSiGe HBTs have been proven to operate over wide temperature ranges and under radiation exposure, and have attracted significant attention for extreme environment applications [1]. For instance, SiGe HBTs can function normally in the extreme ambient lunar surface environment, where temperature cycles from 43 to 393 K. Accurate device modeling is required to enable robust circuit design. We previously developed a temperature-scalable model of the terminal currents covering the temperature range of 43 to 393 K [2] [3].The models in [2][3] only address the "ideal" collector and base currents. Fig. 1 (a) and (b) show the measured base currents as a function of electrical base-emitter junction voltage (V BE for forward mode, and V BC for reverse mode) obtained from typical forward and reverse Gummel measurements. Clearly, a non-ideal base current component in the forward mode I B is observed below 100 K. Such non-ideal base current, however, is not observed in reverse mode, which is determined by characteristics of the base-collector junction. This suggests that the non-ideal base current is related to the heavy doping nature of the base-emitter junction.To model this large non-ideal I B , an understanding of its physical mechanism is highly desired. One may first think of the space charge region Shockley-Read-Hall (SRH) recombination current, which is used to model non-ideal base current in SPICE VBIC and Mextram [4]. That, however, does not work here for a number of reasons. First, the space charge region recombination current has an exponential-slope of q/2kT , while the measured non-ideal base current has an exponential-slope that is nearly temperature independent. Second, the magnitude of space charge region recombination current is proportional to the intrinsic carrier concentration, and shows a very strong temperature dependence, while the measured non-ideal base current shows a weak temperature. 301) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.-85.28 Downloaded on 2015-04-05 to IP 0.6 0.8 1 1.2 10 −10 10 −5 V BE (V) I B (A) 300K 43K TAT (a) 0.6 0.8 1 1.2 10 −10 10 −5 I B (A) V BC (V) 300K 43K (b) V CB = 0 V V EB = 0 V Figure 1: (a)I B − V BE from forward Gummel measurement; (b)I B − V BC from reverse Gummel measurement.The weak temperature dependences are signatures of quantum mechanical trap-assisted tunneling (TAT) currents in heavily-doped base-emitter junctions [5][6], which we believe is the case here. The f...

Operation of silicon-germanium heterojunction bipolar transistors with different structures at deep cryogenic temperature

“…4. This non-ideal base current is related to the heavy doping nature of the baseemitter, which is called trap-assisted tunneling (TAT) current [41,42]. Due to the decreasing defect density in the state-of-art device fabrication, the TAT current is typically negligible at room temperature.…”

Compact Modeling of Forward Operation Band-to-Band andTrap-Assisted Tunneling Currents in SiGe HBTs

“…At present, to our knowledge, compact models for the forward operation BTBT base current do not exist. A compact model of the forward TAT base current was developed by our group for wide temperature range operation of first-generation SiGe HBTs [4]. In this paper, we present a new compact model of forward bias base-emitter junction BTBT current and extend the TAT model of [4] for commercial temperature range applications.…”

“…A compact model of the forward TAT base current was developed by our group for wide temperature range operation of first-generation SiGe HBTs [4]. In this paper, we present a new compact model of forward bias base-emitter junction BTBT current and extend the TAT model of [4] for commercial temperature range applications. The models are implemented in an experimental version of Mextram 505, and demonstrated on SiGe HBTs featuring 250 GHz peak f T and peak f max .…”

“…With further increase of V BE , the measured I B slowly increases, with a large nonideality factor of 3.78 at -25 • C, which we believe is due to TAT [2] [3] [4]. TAT can occur through multiple paths, as illustrated in Fig.…”

“…2 (b) [2]. In this work, we follow [4] in assuming that the dominant path is the A → B → D path and denote the TAT current as I T AT . This I T AT should not be confused with the space-charge-region (SCR) recombination current denoted as I B2 in Mextram [7], which has an ideality factor close to 2.…”