衬底二极管对硅锗光电晶体管性能的影响

This paper provides a study on the substrate effect  on the opto-microwave behavior of Silicon-Germanium  Heterojunction bipolar Photo-Transistors (HPT). An OptoMicrowave Scanning Near Field Optical Microscopy (OMSNOM) is performed to observe the distribution of photocurrent  and dynamic behavior over the structure of the phototransistor.  The photocurrent generated in the photodiode created by a n++  sub-collector and p+ substrate is extracted and analyzed. A  maximum substrate diode current of 700ȝ A is observed at 850nm  with a related cutoff frequency of 0.42GHz. We have extracted  low frequency responsivity (at 50MHz) bandwidth product of  109.2 MHzA/W. Finally, this study will provide a design guide  line for Si base phototransistors.

Nowaday’s wireless technologies have been developed to  replace wire lines installed in the Home Area Network (HAN).  The increase of new services and wireless devices leads us  towards high data rate reaching Giga bits per second. For this  purpose, new wireless network standards arise such as the  IEEE802.11.ad which is the extension of the Wi-Fi toward  the millimeter wave ranges (60GHz). However, the  propagation is limiting to a single room, due to both the high  propagation attenuation at 60GHz and to the wall absorption  and reflections . Therefore, an infrastructure is needed to  cover the whole home area so as to distribute the signal from  one room to another one. As a result an interest has been  recently put to Radio-over-Fiber (RoF) home area network  application , for which low cost silicon based  optoelectronics are highly desirable. SiGe hetrojunction  bipolar phototransistor (SiGe HPT) are potential candidates  for light detection that were proposed first in 2003 to be  integrated in standard SiGe HBT technology. Since then,  several laboratories are working on SiGe HPTs using different  industrial process technologies like TSMC, AMS SiGe  BiCMOS and IBM SiGe BiCMOS. Hybrid photoreceiver based on SiGe HPT for 60 GHz intermediatefrequency RoF applications was implemented in.

This paper investigates the effect of the substrate  photodiode created by the p type substrate and n++ sub  collector on the dynamic response the HPT. It also provides  relavat information interms of bandwidth responsivity product  to be used this HPT into microwave photonics applications.  Finally a conclusion will be made on the design aspects of  SiGe/Si HPT structure.

The SiGe phototransistor (HPT) was fabricated using the  existing SiGe2RF Telefunken GmbH SiGe Bipolar process  technology. Indeed, the phototransistor fabrication does not  modify the vertical stacks of layers that are used to define a  standard SiGe2RF HBT technology. This ensures the  compatibility with the process technology and potential  integration of complete OE-RF circuits.

The basic HPT structure is designed by extending the  emitter, base and collector layers of the reference HBT.  The optical opening is made through the emitter. To improve  the optical illumination, the superficial Silicon oxide and  nitride layers on top of the defined optical window are  removed by using Reactive-Ion Etching (RIE) process. A  cross-section representation of the phototransistor structure is  given in Figure 1. The light paths goes through the polysilicon  of the emitter before entering the Si emitter, SiGe base and Si  collector region. This HPT is essentially one large HBT whose  emitter metallization was removed on the side.

Fig1

By using the bench setup described in [13], we perform the  experimental mapping of the HPT at two different bias  conditions (photodiode (PD) and phototransistor mode) and,  also, at different optical probe position. For this study, we use  a multimode optical source at 850nm and optical probe in  order to make sure that our study is more realistic for HAN  application where multimode source and fiber is used. The  phototransistor mode is studied at a fixed collector-emitter  voltage of 3V and fixed base emitter voltage of 0.857V. The  photodiode mode is studied by setting collector emitter  voltage of 3V and base emitter voltage of 0V. These biasing  conditions are the optimum biasing conditions in terms of  opto-microwave responsivity.

The base current mapping is symmetric along both x and y axis as shown in Figure 3 in both HPT (a) and PD (b) mode. In  HPT mode, the sign of the base current is changed when the  optical probe is moving in to the center of the phototransistor. The negative sign, when the active area is illuminated,  indicates that parts of holes generated in the active area are  flowing out through the base contact. However, parts of the  holes are moving to the emitter for transistor amplification. In  the PD mode, Ib has negative sign at all position of the optical  probe as there is no any transistor action, all the holes  generated are flowing out through the base contact. The base  current (Ib) measured in PD mode operation is the sum of the  dark current and the photogenerated current. Thus, the  photocurrent generated in the structure, called the primary  photocurrent, can be computed from the difference between the  base current measured with and without light illumination.  

The peaks of the substrate photocurrent can be explained  from the vertical and lateral structure of the HPT shown in  Figure 1. When the optical probe moves over the structure, the  optical beam passes through different stacks depending on the  position of the probe. The two peaks of the substrate  photocurrent, one near the collector contact and the other near  the base contact, are due to the illumination of the photodiode  created by n++ sub-collector and p type Si substrate.

Figure 7 shows the frequency behavior of HPT at the center  of the optical window. We recognize a slope close to -20dB /  decade in phototransistor mode, characteristic of the behavior  of HPT, while the photodiode mode has a slope of about -10  dB/decade. The latter characterizes the behavior of the  substrate, due to the difference in transit time of the photocarriers based on the detection depth into the substrate.