Use of an InP/InGaAs double heterostructure phototransistor in a 40GHz OEMMIC photoreceiver

This paper by Hideki Kamitsuna, Yatuka Matsuoka, Shoji Yamahata and Naoteru Shigekawa was awarded the Microwave Prize at the 30th European Microwave Conference. It presents an InP/InGaAs double-heterostructure phototransistor (DHPT) with a record optical gain cutoff frequency of 82GHz, and an OEMMIC photoreceiver based this structure. The photoreceiver operates at 40GHz, the highest operating frequency currently reported for such a device.

InP-based phototransistors have been widely studied as one of the key devices for optoelectronic integrated circuits [1]. Among phototransistor structures, layer and process-compatible heterojunction phototransistors (HPTs) based on heterojunction bipolar transistor (HBT) technology are the most promising for high performance monolithic integrated photoreceivers [2-4]. Unfortunately, the measured and designed operation frequency of monolithic integrated HPT/HBT photoreceivers remains below 28GHz [3], [4] and millimetre-wave (over 30GHz) operation has not been reported to date. This is because high speed HPTs for millimetre-wave application require both extremely high speed base/collector junction photodiode operation and high maximum oscillation frequency (f max ) of a transistor.

This paper presents an InP/InGaAs double-heterostructure phototransistor (DHPT) with a record optical gain cutoff frequency of 82GHz. This performance is suitable for millimetre-wave photodetector and optically injection-locked oscillator [5] applications. Such excellent performance comes from (1) the elimination of low speed holes by the double-heterostructure, (2) high unity current gain cutoff frequency (fT ), and (3) high f max through device size reduction by a self-aligned process.

To demonstrate the excellent performance of the DHPT, this paper also presents a 40GHz band optoelectronic millimetre- wave monolithic integrated circuit (OEMMIC) photoreceiver utilising ultra high speed DHPTs and double heterostructure bipolar transistors (DHBTs).

Device structure

Figure 1: Cross sectional view of DHPT and DHBT.

Figure 1 shows a schematic cross section of a DHPT with a simultaneously fabricated DHBT. The DHPT has the same epitaxial layer structure as the DHBT, but with a different emitter electrode pattern for optical access from the top. A 1.3 or 1.55ým optical signal can easily penetrate base/collector layers since most of the emitter layers consist of "transparent" InP. An HPT can be thought as a base/ collector junction photodiode (PD mode operation) plus internal amplifier. Therefore, such layer and process compatibility with the high performance DHBT is indispensable in realising high-performance OEMMICs based on a HPT/HBT structure. For DHPT operation, this double-heterostructure, i.e. the valence band discontinuity at the interface between the InP collector and In-GaAs subcollector (Figure 2), can eliminate the effect of low-speed holes generated in the subcollector layer, and thus enables the PD mode HPT to achieve a high-speed photoresponse [6]. In addition, a self-aligned structure [7] fabricated by using the emitter electrode as a mask for emitter mesa etching is adopted to increase the high-frequency photoresponse of the DHPT through a device size reduction. This size reduction directly leads to a reduction of internal collector capacitance CBC , thus reducing the CR time constant of the PD mode DHPT. Furthermore, f max defined as follows,

can be increased. Therefore, an ultra high speed DHPT with a high optical gain cut-off frequency can be obtained.

Figure 2: Novel double heterostructure for high-speed DHPT

A new self-aligned DHPT, which has an optical access window (area: 20ým²) cut into the emitter electrode (emitter area: 24 ým²), was fabricated and tested. Due to the self-aligned process, the emitter area is successfully reduced by 29.4%, while the optical access window area is exactly the same as that in our previously reported non-self-aligned DHPT [3]. For comparison, a non-self-aligned DHPT (emitter area: 34m² ; optical access window area: 20ým²) was also fabricated and tested.

Device performance

A. Microwave Characteristics of the DHPT

Figure 3: Measured f T /f max of DHPTs and DHBT

Figure 3 shows measured f T and f max of the fabricated self-aligned DHPT, non-self-aligned DHPT, and a DHBT (emitter area: 2 x 10ým²) at VCE =1.3V. Extremely high f T and f max of 126GHz and 99GHz were obtained for the self-aligned DHPT. Those of the DHBT are 139GHz and 152GHz, respectively. Layer compatible DHPTs receive benefit from such excellent microwave performance of the DHBT. Furthermore, a large increase in f max of the self-aligned DHPT against that of the non-self-aligned DHPT (f max = 82GHz) is observed. This originates from the device size reduction since the non-self-aligned DHPT's f T (122GHz) is almost the same as the self-aligned DHPT's. The measured f max increase ratio (20.7%) well matches the estimated one from the reduction of emitter area, which is proportional to the internal collector capacitance CBC . Therefore, the higher f max of the self-aligned DHPT comes from the device size reduction in addition to the layer-compatibility of the DHBT. Such excellent microwave performance is expected to make millimetre-wave photodetector application possible.

B. DHPT Photoresponses
Photoresponse was measured by using a Cascade Microtech lightwave probe and an HP83467C lightwave component analyser ( = 1.55ým). Figure 4 shows the measured photoresponses of DHPT's under both Tr-mode (V CE = 1.3 V, I C = 20 mA) and PD-mode (V CB = 1.3 V, V BE = 0 V) operation [3]. In the measurement, the base was 50 terminated. The vertical axis expresses 20*log[R], where R is the responsivity in units of A/W. The DC responsivity of the PD-mode for both DHPTs, including the illuminated photo-spot size mismatch, is exactly the same value (0.25 A/W) at 1.55ým optical wavelength. The higher speed performance of the self-aligned PD-mode DHPT comes from the reduction of capacitance. In addition, a high optical gain cutoff frequency of 82GHz, the point where the DC responsivity of the PD-mode and the extrapolation of 6 dB/oct cross, was achieved by the new high f max self-aligned DHPT structure, while that of the non-self-aligned DHPT remains 60GHz. To our knowledge, this is the highest optical gain cutoff frequency ever reported for HPT's [4].

C. Photoresponse Simulation
To evaluate the measured photoresponse, an equivalent circuit analysis was conducted. The simulation used the equivalent circuit of the DHPT as derived from measured S-parameters under Tr- and PD-mode operation and a current source for photoexcited carriers [3].

Figure 4: Measured photoresponses of self-aligned and non-self-aligned DHPTs

Figure 5: Simulated relative photoresponses of self-aligned and non-self-aligned DHPTs. That of the DHBT with emitter area of 2 x 10ým 2 is also plotted

Figure 5 shows the simulated relative photoresponse of self-aligned/non-self-aligned DHPTs under both Tr- and PD-mode operation when the DC responsivity of the PD-mode is set to 0dB. 3-dB bandwidths are 54GHz for the self-aligned PD-mode DHPT and 23GHz for the non-self- aligned PD-mode DHPT. Since these values are derived on condition that the base is 50 terminated, the measured results in Figure 4 correspond to these values. Due to the double-heterosturucture shown in Figure 2 the photoresponses of PD-mode DHPTs, unlike conventional single-heterostructure HPTs, is limited by the CR time constant. Simulated intrinsic 3-dB bandwidths with the base terminal shorted are 76GHz for the self-aligned DHPT and 43GHz for the non-self-aligned DHPT. Optical gain cutoff frequencies of 90GHz and 56GHz were determined for the self-aligned DHPT and non-self-aligned DHPT, respectively. These values are consistent with the measured ones within 10% accuracy. Therefore, we can say that the self-aligned DHPT's excellent performance originates from both increase in the band-width of PD-mode operation and f max .

DHPT/DHBT OEMMIC photoreceiver

Figure: 6. Circuit diagram of the 40GHz band DHPT/DHBT OEMMIC photoreceiver.

Figure 7: Microphotograph of a DHPT/DHBT photoreceiver. (Chip size: 1.054 mm x 0.6 mm).

To demonstrate the excellent performance of the new DHPT, which can be simultaneously integrated with a high-performance DHBT, a 40GHz band DHPT/DHBT OEMMIC photoreceiver was designed and fabricated. Figures 6 and 7 show a circuit diagram and microphotograph of the fabricated DHPT/DHBT OEMMIC photoreceiver. The photoreceiver consists of a DHPT as a photodetector- plus-amplifier followed by a reactively matched 2-stage DHBT amplifier. The inductor L at the DHPT's base terminal increases the photoresponse at the 40GHz band [3]. The design process uses the equivalent circuit models of the DHBT/DHPT and a current source model for the O/E conversion part as was used in the discrete DHPT analysis. An extremely small chip size of 1.054 x 0.6mm² was achieved resulting from the lumped uniplanar MMIC structure. Figure 8 shows the measured photoresponse and output return loss of the DHPT/DHBTOEMMIC photoreceiver at the bias condition of V CC =1.3V, I DHPT =20mA, and I DHPT x 2=20mA. The measured photoresponse of the discrete DHPT under PD-mode is also plotted. Due to the combination of the ultra high speed DHPT with the base inductor and the 2-stage DHBT amplifier, the DHPT/DHBT OEMMIC photoreceiver yields a very large photodetection gain of 22dB compared with the discrete PD-mode DHPT at 40GHz. The designed photoresponse and return loss of the OEMMIC photoreceiver are also plotted in Figure 8. Fairly good agreement is observed between the designed and measured values.

Figure 8: Measured and designed photoresponse and return loss of the DHPT/DHBT OEMMIC photoreceiver

Discussion

When an optical signal is illuminated through the substrate [8], a DHBT can be utilised as a photodetector. Almost the same PD-mode DC responsivity for the DHBT with an emitter area of 2 x 10ým² against the DHPT's can be expected since the whole base/collector area under the emitter contributes to the photoabsorption. In what follows, the possibility of much higher frequency operation is discussed. Figure 5 also shows the simulated photoresponse of the "DHBT" with an emitter area of 2 x10ým². Extremely high optical gain cutoff frequency of 137GHz is expected. Furthermore, photoresponse of the DHBT at 60GHz is al-most the same as that of the self-aligned DHPT at 40GHz. Therefore, by using the DHBT as a photodetector, a 60GHz band OEMMIC photoreceiver, whose photoresponse corresponds to that of the 40GHz band OEMMIC photoreceiver, would be expected. An InP sub-collector [9] will enable us to achieve such high-performance photodetector/photoreceiver.

Conclusion

Owing to the double-heterostructure compatible with high performance DHBT and the self-aligned process, we have achieved an ultra high-speed DHPT with a record optical gain cutoff frequency of 82GHz. Such excellent performance originates from the ultra high-speed PD mode DHPT with a 3dB bandwidth of 76GHz (simulated) and extremely high f max of 99GHz. A 40GHz-band OEMMIC photoreceiver based on the DHPT/DHBT structure has also been presented. Much higher frequency operation, e.g. 60GHz band, is possible by using a high-f max "DHBT" with optical access through the substrate. The developed DHPTs are also promising for other OEMMICs, such as millimetre-wave direct optical injec-tion- locked oscillators and optoelectronic mixers.

Acknowledgments

The authors thank H. Niiyama and K. Takahata for helpful advice in measurements.
They also thank Y. Ishii and T. Enoki for their continuous support and encouragement.

References
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