Power bipolar junction transistor(BJT) and hetero-junction bipolar transistor(HBT) usually employ multiple emitter fingers in parallel to improve the current handling capability, thermal dissipation capability and RF power performance[
Journal of Infrared and Millimeter Waves, Volume. 40, Issue 3, 329(2021)
RF power performance improvement of multi-finger power bipolar transistor by non-uniform emitter finger spacing design without the use of emitter ballasting resistor
In this paper, the RF power performance and surface temperature distributions for a multi-finger power hetero-junction bipolar transistor (HBT) with non-uniform emitter finger spacing (NUEFS) without the use of emitter-ballasting-resistor (EBR) are measured, and are compared with a multi-finger power HBT with EBR. The experiment results show that for the multi-finger power HBT with NUEFS, the highest surface temperature is lowered, the uniformity of surface temperature distributions measured by US QFI Infrared TMS is improved, the RF power gain and power-added-efficiency (PAE) are increased compared with the multi-finger power HBT with EBR respectively. These results could be attributed to the improvement in positive thermoelectric feedback and thermal coupling effects among the fingers, and the riddance of adverse impact from emitter-ballasting-resistor used in traditional power HBT.
Introduction
Power bipolar junction transistor(BJT) and hetero-junction bipolar transistor(HBT) usually employ multiple emitter fingers in parallel to improve the current handling capability, thermal dissipation capability and RF power performance[
1 Mechanism and disadvantages of the introduction of emitter ballasting resistor to improve HBT current handling capability
For an HBT, emitter-base junction voltage VBE varies with temperature T.
Figure 1.The measured emitter-base junction voltage VBE as a function of T under different emitter currents of 3 mA, 1 mA, 100 µA and 10 µA respectively
In order to alleviate positive thermoelectric feedback between emitter current and temperature, and protect the HBT from thermal burn-out, it is common to add a ballasting-resistor REi at emitter as shown in
Figure 2.The schematic diagram of an N-finger HBT, where REi is the emitter ballasting resistor of the ith emitter finger[8-9]
To guarantee thermal stability of HBT within a certain limit of emitter current(IE), the engineering regulation for ballasting resistor RE is as follows: when emitter-base junction temperature varies by ±5 K, the added RE at emitter finger could able to restrict the variation of IE within ±5%, so the minimum emitter ballasting-resistor REmin can be expressed as[
where units of dVBE/dT and IE are mV/℃ and mA respectively.
We can see that REmin is determined by threshold emitter current(IE), it is only effective for an HBT to thermally stable operate under a certain of emitter current(IE).
However, on the other hand, the additional emitter ballasting-resistor RE will degrade RF power performance no matter how to optimize the RE. The parameters of power gain(Gp) and power-added-efficiency(PAE), which are employed to characterize RF power performance, are related to ballasting-resistor RE as follows:
where pout is RF output power, pin is RF input power, fT is transit frequency, rb is base resistance, CTC is collector output capacitance, PDC is DC power, ηC=pout/PDC is collector efficiency.
Therefore, Gp and PAE are degraded due to the existence of RE. Furthermore, in order to guarantee the thermal stability of HBT within a certain of emitter current(IE) and power, it is usually suggested that RE >REmin, this could further degrade Gp and PAE.
2 Structure and layouts of multi-finger power HBTs
The schematic cross section of a cell of multi-finger HBTs based on SiGe process and the material parameters of various layers of the device are shown in
Figure 3.The schematic-cross section of a cell in multi-finger HBTs
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Figure 4.Micrographs of the fabricated multi-fingers SiGe HBTs with emitter ballasting resistor.(Note: polysilicon emitter ballasting resistors locate at root terminal of each emitter finger, see the enlarged image)
Figure 5.Micrograph of the fabricated multi-fingers SiGe HBTs with non-uniform finger spacing
3 Measurement results and discussion
In this section, the measured surface temperature distributions by US QFI Infrared TMS, and the measured power gain(Gp), collector efficiency(ηC) and power-added-efficiency(PAE) for two types of multi-fingers SiGe HBTs with non-uniform finger spacing(NUEFS) and emitter ballasting resistor(EBR) respectively are shown and compared.
3.1 Surface temperature distributions measurements of multi-fingers power HBTs
The surface temperature distributions measured by temperature measurement microscope systems(TMS) from US quantum focus instruments(QFI) Corporation for two types of multi-fingers HBTs with emitter ballasting resistor(EBR) and with non-uniform emitter finger spacing(NUEFS) under IC=800 mA, VCE=5 V and case temperature of TC=80℃ are shown in
Figure 6.Measured surface temperature distribution by US QFI Infrared TMS for (a) a multi-finger power HBT with emitter ballasting resistor, and (b)for a multi-finger power HBT with non-uniform finger spacing under IC=800 mA, VCE=5 V and case temperature of TC=80 ℃
3.2 RF power performance measurements
The package and RF power measurements of two types of multi-finger power HBTs with non-uniform finger spacing(NUEFS) and emitter ballasting resistor(EBR) are performed by The 13th Research Institute of China Electronic Technology Corporation. The measurement system mainly consists of a power signal source, isolator, directional coupler, test fixture, attenuator, power meter, bias power supply, and so on, as shown in
Figure 7.The block diagram of RF power measurement system
The measured output power(pout) versus input power(pin), collector efficiency(ηC) versus input power(pin), and power-added-efficiency(PAE) versus input power(pin) for two types of multi-fingers SiGe HBTs with emitter ballasting resistor(EBR) and non-uniform emitter finger spacing(NUEFS) under class C operation and at the frequency of 1.2 GHz are shown in Figs.
Figure 8.RF Output power poutversus RF input power pin for two types of multi-fingers power HBTs with emitter ballasting resistor and with non-uniform emitter finger spacing respectively
Figure 9.Collector efficiency ηCversus RF input power pin for two types of multi-fingers power HBTs with emitter ballasting resistor and with non-uniform emitter finger spacing respectively.
Figure 10.Power-added-efficiency(PAE) versus RF input power pin for two types of multi-fingers power HBTs with emitter ballasting resistor and with non-uniform emitter finger spacing respectively
As shown, for the multi-fingers SiGe HBTs with EBR, pout increases with the increase of pin, but pout tends to saturate. At pin=0.9 W, the pout=1.9 W, power gain Gp=3.25dB, ηC=72%, and PAE=40%. For the multi-fingers SiGe HBTs with NUEFS, pout increases linearly with the increase of pin and does not saturate. At pin=0.9 W, the pout=4W, power gain Gp=6.48dB, ηC=63%, and PAE=49%. The comparison between the HBT with NUEFS and the HBT with EBR indicates that the pout is increased by 2.1W. This is because that for the same RF input voltage signal, there is not voltage drop across RE in the HBT with NUEFS. Therefore, the current gain is improved, and the collector current ic is increased [
4 Summary and conclusions
A comparative study of two types of multi-finger power HBTs with non-uniform emitter finger spacing(NUEFS) and emitter-ballasting-resistor(EBR) is performed experimentally in terms of surface temperature distributions, RF power gain and power-added-efficiency(PAE). The experiment results show that for the multi-finger power HBT with NUEFS, not only the uniformity of surface temperature distributions measured by US QFI Infrared TMS but also RF power gain and power-added-efficiency(PAE) is improved compared with a multi-finger power HBT with EBR. These results could be attributed to improvement in positive thermoelectric feedback and thermal coupling effects among the fingers, and the riddance of adverse impact from emitter-ballasting-resistor used in traditional power HBTs.
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Zheng ZHANG, Yan-Hua ZHANG, Dong-Yue JIN, Wei-Cong NA, Hong-Yun XIE. RF power performance improvement of multi-finger power bipolar transistor by non-uniform emitter finger spacing design without the use of emitter ballasting resistor[J]. Journal of Infrared and Millimeter Waves, 2021, 40(3): 329
Category: Research Articles
Received: Jun. 20, 2020
Accepted: --
Published Online: Sep. 9, 2021
The Author Email: Yan-Hua ZHANG (zhangyh@bjut.edu.cn)