A New Design Method for Emitter Finger Space of Heterojunction

Bipolar Transistors

Chuantao Ma

1, a

College of Physics and Electronic Engineering, TaiShan University, TaiAn, China

mcht1016@163com

Keywords: Heterojunction bipolar transistors, silicon–germanium, segmented emitter, thermal stability.

Abstract: A new design method for emitter finger space of multi-finger HBT was proposed to improve thermal

stability of HBT. The 3-D temperature distribution, cross section temperature distribution, cross section

temperature gradient distribution of five-finger power HBT with traditional emitter structure and non-

uniform emitter finger space structure at the same dissipation power are given. Compared with 3-D

temperature distribution and cross section temperature distribution, the difference between traditional

emitter structure and non-uniform emitter finger space structure is more distinctive, so the cross-section

temperature gradient distribution is more effective for the design technique of non-uniform finger spacing of

multiple finger power HBT.

1 INTRODUCTION

Heterojunction bipolar transistors (HBT) has

become increasingly popular in power amplifiers for

wireless communications and microwave

applications because of its high-speed performance,

low-noise, high cutoff frequency and compatibility

with BiCMOS technology (S. P. McAlister, et.al,

2004; Wang Y, et.al, 2007; A Schuppen, et.al, 1995;

C Kermarrec, et.al, 1994). HBT usually employ a

multi-finger structure to improve the current

handling capability and thermal dissipation

capability. However, self-heating effects caused by

the temperature rise due to the power dissipation and

thermal coupling effects among emitter fingers

result in a higher temperature at the center fingers.

Because of the positive temperature coefficient of

emitter current, the center fingers conduct more

current and consequently generate more heat, which

eventually makes the device to become unstable at

high power and seriously limit the power handling

capability of the device.

In order to get high thermal stability under high

power dissipation, one method of non-uniform

finger spacing is usually proposed. Analytical and

experiment are used to investigate thermal behavior

of multiple finger heterojunction bipolar transistors

(HBT’s) with the non-uniform mitter-finger spacing.

For the non-uniform finger spacing HBT, the heat

flow from adjacent to the center finger is reduced by

increasing the spacing between fingers. It is shown

that the HBT with non-uniform finger spacing have

lower peak temperature in the device center, smaller

temperature difference between fingers compared

with HBT with uniform finger spacing under the

same power dissipation. These results indicate that

the method of non-uniform finger spacing is very

useful for getting the HBT with high thermal

stability. But it is difficult to get Suitable emitter

finger space of multi-finger HBT using a

comprehensive model for the multi-finger HBT

including the effects of temperature dependence of

thermal conductivity, and non-uniform, two-

dimensional temperature distribution on the emitter

fingers.

In this paper, a 3-D thermal simulation is

performed by the finite element method (FEM) in

ANSYS (A Schuppen, et.al, 1996; P A Potyraj, et.al,

1996; W. Liu, B. Bayraktaroglu, 1993; Willian Liu,

Ali Khatibzadeh, and Jim Sweder, 1996; J. -S. Rieh,

et.al, 2002; Jae-Sung Rieh, et.al, 2005). The results

of simulation show that temperature gradient can be

used to support a new method for the design of

emitter finger space of HBT.

398

Ma, C.

A New Design Method for Emitter Finger Space of Heterojunction Bipolar Transistors.

DOI: 10.5220/0008865203980401

In Proceedings of 5th International Conference on Vehicle, Mechanical and Electrical Engineering (ICVMEE 2019), pages 398-401

ISBN: 978-989-758-412-1

2 THEORY

Figure.1 shows the bulk heat flux distribution of

multi-emitter HBT through the ANSYS finite

analysis software. When the HBT is working, the

temperature will gradually rise. The heat will send

out from the chip to the outside. When the

temperature don’t rise, the transistor achive a

thermal stable state. According to the theory of heat

conduction, the heat flux will move from the area of

the high temperature to the area of the low

temperature. Therefore, two adjacent emitter fingers

with the same power have the same heat flux

condition. The temperature of intermediate point is

the highest. The heat flux downward

conduction after meeting. The heat flux no longer

disperse to the two sides. we establish the thermal

resistance model as shown in Figure.2.

Figure 1. Bulk heat flux distribution of multi-emitter HBT.

Figure 2. The thermal resistance model of a multi-finger

HBT.

The device discussed in this study mainly

consists of two parts: active region and substrate as

shown in Fig. 3 (P A Potyraj, et.al, 1996; W. Liu, B.

Bayraktaroglu, 1993; Willian Liu, et.al, 1996). Heat

dissipation originates from the base-collector

junction, and through two opposite directions:

upward through emitter fingers and downward

through substrate. For the downward case, since the

thickness of collector layer is small enough

compared with the substrate, the thermal resistance

of substrate (Rths) was only considered. The upward

dissipation can be accounted as two pieces of

thermal resistor (Rthe and Rthb) representing the

thermal resistance from emitter and base,

respectively. The thermal resistance of the device

can be expressed as a series combination of partial

thermal resistances from very thin slabs along the

heat flux:

th th i





(1)

Where Rth is the thermal resistance, k is the

thermal conductivity of the medium, A is the

effective area of the heat conduction, t is the

thickness of the thin slab, and i is the number of the

slabs.

Figure 3. Cross-section of SiGe HBT showing heat flux,

heat source, and thermal resistance components.

The thermal resistance for the device can be

calculated from maximum temperature and

dissipation power:

max 0

diss





(2)

Where Tmax is the maximum temperature of the

device, T0 is ambient temperature.

3 RESULTS AND DISCUSSION

In order to make sure the precision of the simulation,

there are two important steps in the thermal

A New Design Method for Emitter Finger Space of Heterojunction Bipolar Transistors

399

simulation: substrate simulation and active region

simulation. The stepped simulation avoids the

negative effect of the huge disparities of dimension

among each part (the thickness of emitter is 200nm

whereas it is up to 150mm for the substrate), ensures

the view of the 3-D thermal distribution in active

region clearly. For the first step, the model contains

heat source, collector layer, and substrate. We apply

the boundary temperature to the bottom of substrate

and dissipation power to the B-C junction. It was

assumed that the operating temperature of substrate

is ambient temperature at 300K. In second step, the

model of the active region consists of three parts:

emitters, base and collector. The dissipation power is

determined by collector current (Ic) and supply

voltage (Vce). In SiGe HBT device, collector current

must be less than Icritical which is the critical

collector current before current gain collapse occurs.

The 3-D temperature distribution, cross section

temperature distribution, cross section temperature

gradient distribution of five-finger power HBT with

traditional emitter structure and non-uniform emitter

finger space structure at the same dissipation power

are shown in Fig.4- Fig.6

Figure 4. The 3-D temperature distribution of five-finger

HBT.

Figure 5. The 3-D cross section temperature distribution of

five-finger HBT.

Figure 6. The 3-D cross section temperature gradient

distribution of five-finger HBT.

ICVMEE 2019 - 5th International Conference on Vehicle, Mechanical and Electrical Engineering

400

4 CONCLUSIONS

In this paper, the 3-D temperature distribution, cross

section temperature distribution, cross section

temperature gradient distribution of five-finger

power HBT with traditional emitter structure and

non-uniform emitter finger space structure at the

same dissipation power are given. Compared with 3-

D temperature distribution and cross section

temperature distribution, the difference between

traditional emitter structure and non-uniform emitter

finger space structure is more distinctive, so the

cross section temperature gradient distribution is

more effective for the design technique of non-

uniform finger spacing of multiple finger power

HBT.

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