High upconversion gain optoelectronic mixer using uni-travelling carrier phototransistors

Abstract

The optoelectronic mixing performance of a uni-travelling carrier heterojunction phototransistor (UTC-HPT) was compared with its performance as a photodiode (PD). The upconverted output power in HPT mode was 24 dBm higher than in PD mode.Introduction: As the use of mobile devices has rapidly spread and data traffic for multimedia has surged, the demand for bandwidth and mobile accessibility for high data rate communication has greatly increased in the past few years. Radio-over-fibre (ROF) systems have been considered as a possible solution for future high data rate wireless broadband communication. An optoelectronic mixer (OEM) is one of the key components in ROF communication systems because of the requirement for frequency up-and downconversion. Several optoelectronic mixers using uni-travelling carrier (UTC) photodiodes (PDs) [1] or heterojunction phototransistors (HPTs) have been demonstrated [2] in ROF applications. Although the UTC-PD OEM showed great potential, high optical incident power is required to get high output current because the UTC-PD lacks internal gain. When used in ROF link applications, the UTC-PD OEM results in a large optical link loss budget. On the contrary, ROF links using HPTs can be designed with a smaller optical link loss budget due to the HPT's internal gain. Lower link loss decreases the total link noise figure [3]. Therefore, more energy-efficient ROF links can be built if HPTs were employed.Another advantage of using a HPT in a ROF link is the possibility of making an on-chip base station because a HPT can be used in several different components such as an oscillator [4] or a mixer. Monolithic integration of these devices enables the accommodation of most of the optoelectronic and electrical components in a base station, such as PDs and amplifiers into a single chip. Monolithic integration of a HPT with a heterojunction bipolar transistor (HBT) and its implementation as an oscillator [5] have already been demonstrated. The simplified base station circuitry will result in reducing the cost of base stations, thus lowering the cost of ROF links [3].In this reported work, the OE mixing performance of a UTC-HPT was compared with its PD mode operation to explore the possibility of building ROF links with low power consumption. In our previous report [6], we showed that our UTC-HPT could work as a HBT by showing that it had a common emitter current gain which varied from 140 to 240. Our UTC-HPT is a double heterojunction phototransistor (DHPT) that puts in an InP emitter layer on top of a UTC PD structure. An OEM based on a UTC PD can be easily switched to an OEM based on a UTC-HPT to take advantage of an internal gain.

High-Frequency InP-InGaAsP Heterojunction Phototransistor Employing a UTC Structure

“…The optic generated holes dismiss/drift slowly from collector to emitter, which limits device high frequency performance and thus the photoelectric response speed [7,8] . In an attempt to overcome these problems, we utilize an uni-traveling-carrier (UTC) structure [9,11] , which employs two hetero-junctions at emitter/base junction and base/collector junction separately. In this case, both electrons and holes are generated only in base region to hope to get faster response speed, better dc characteristics and higher optical transit frequency.…”