Chin. Phys. B Vol. 26, No. 9 (2017) 098502
Improved high-frequency equivalent circuit model based on
distributed effects for SiGe HBTs with CBE layout
∗
Ya-Bin Sun(孙亚宾), Xiao-Jin Li(李小进)
†
, Jin-Zhong Zhang(张金中)
‡
, and Yan-Ling Shi(石艳玲)
Shanghai Key Laboratory of Multidimensional Information Processing, Department of Electrical Engineering,
East China Normal University, Shanghai 200241, China
(Received 23 March 2017; revised manuscript received 2 June 2017; published online 31 July 2017)
In this paper, we present an improved high-frequency equivalent circuit for SiGe heterojunction bipolar transistors
(HBTs) with a CBE layout, where we consider the distributed effects along the base region. The actual device structure is
divided into three parts: a link base region under a spacer oxide, an intrinsic transistor region under the emitter window,
and an extrinsic base region. Each region is considered as a two-port network, and is composed of a distributed resistance
and capacitance. We solve the admittance parameters by solving the transmission-line equation. Then, we obtain the small-
signal equivalent circuit depending on the reasonable approximations. Unlike previous compact models, in our proposed
model, we introduce an additional internal base node, and the intrinsic base resistance is shifted into this internal base node,
which can theoretically explain the anomalous change in the intrinsic bias-dependent collector resistance in the conventional
compact model.
Keywords: SiGe heterojunction bipolar transistors (HBT), small-signal equivalent circuit, distributed effects,
CBE layout
PACS: 85.30.De, 85.30.Pq, 85.40.Bh DOI: 10.1088/1674-1056/26/9/098502
1. Introduction
In recent years, SiGe heterojunction bipolar transistors
(HBTs) have undergone rapid growth because of their high
speed, high driving power, and low noise, and they have been
applied to fields such as wireless communication, analog cir-
cuit, fast data acquisition, and conversion.
[1–3]
Small-signal
equivalent circuit models, such as the lumped SGP, VBIC,
HICUM, and Mextram, are often used to characterize tran-
sistor performance, optimize the device structure, and guide
circuit design.
[4–6]
However, bipolar transistors are actually
large distributed networks that are composed of a basic re-
sistance and capacitance, and their design is not always as
easy as described in the above lumped models. Although most
transistor electrical performances can be effectively character-
ized by lumped models, there remain fundamental limitations
with respect to actual device structures.
[7]
When extracting the
small-signal model parameters in conventional Mextram, we
find that the bias-dependent collector epilayer resistance R
C1C2
monotonously decreases as the base voltage V
BC
increases,
which clearly deviates from basic device physics theory, and
the underlying physical mechanism should therefore be inves-
tigated.
Owing to its special base structure, SiGe HBTs show
a built-in multi-Mrad total dose hardness with no inten-
tional hardening.
[8–10]
However, single-event effects (SEE) re-
main a serious problem, with recent results demonstrating a
low linear energy-transfer threshold and high saturated cross-
sections.
[11–14]
A reduction in the sensitive area enclosed by
deep trench isolation is considered an effective method of im-
proving the net upset cross section. Therefore, a transistor with
minimum feature size, i.e., using only a single collector, base,
and emitter (CBE) contacts, possesses a high SEE immunity,
compared with standard devices with double collector and
base contacts (CBEBC).
[11,12]
Furthermore, when the transis-
tors are exposed to the space-energetic particle environment,
the distributed effect is more significant because the irradia-
tion damages are generally not uniformly distributed through-
out the whole transistor structure.
[8]
Therefore, it is necessary
to investigate the small-signal equivalent circuit based on the
distributed effects for SiGe HBTs with a CBE layout.
In the present work, in order to determine the phys-
ical mechanisms by which the extracted R
CC
decreases as
V
BC
increases, we propose an improved small-signal equiv-
alent circuit that is based on the distributed effects for SiGe
HBTs with a CBE layout. The whole transistor is divided
into three parts along the base region. Then, we obtain the
high-frequency equivalent circuit under the cut-off mode by
solving transmission line equations, and taking into account
the distributed effects. The intrinsic base resistance R
BI
is
pushed into the internal base node, and the added component
of R
BI
(C
TE
+C
TC
)/3C
TC
is found to contribute to the declined
∗
Project supported by the National Natural Science Funds of China (Grant Nos. 61574056 and 61504156), the Natural Science Foundation of Shanghai,
China (Grant No. 14ZR1412000), Shanghai Sailing Program, China (Grant No. 17YF1404700), and the Science and Technology Commission of Shanghai
Municipality, China (Grant No. 14DZ2260800).
†
Corresponding author. E-mail: xjli@ee.ecnu.edu.cn
‡
Corresponding author. E-mail: jzzhang@ee.ecnu.edu.cn
© 2017 Chinese Physical Society and IOP Publishing Ltd http://iopscience.iop.org/cpb http://cpb.iphy.ac.cn
098502-1