Research On High Frequency Feature Of Inp-based Uni-traveling-carrier Double Heterojunction Phototransistor

Compared with PIN detector and APD detector, Heterojunction phototransistor detector have many advantages, such as large optical gain, high frequency response, low noise, high responsivity and easy integration with heterojunction bipolar transistor(HBT). Because of its internal gain and nonlinear processing capability, HPTs have a wide application in high-speed optical fiber communication network and optical microwave communication system when they are used as optical detectors. But since HPTs always choose HBT’s base and collector as the absorption region, several factors like the low mobility of photo generated holes make that there is a contradiction between the optimization of its responsibility and its response speed. In order to ease tensions between both high responsivity and high response speed of the device, the idea of uni-traveling-carrier(UTC) is used in InP-based HPT to form InP-based uni-traveling-carrier double heterojunction phototransistor(UTC-DHPT) with only base as optical region. For the proposed UTC-DHPT detectors, the main work is as follows:Firstly, a two-dimensional(2D) physical model of UTC-DHPT is established. The behavior of photon-generated carriers in base region and other internal carriers of UTC-DHPT are demonstrated. Optical responsivty and optical gain of the UTC-DHPT are analyzed based on the 2D physical model. The UTC-DHPT structure, including the epitaxy structure and the lateral dimension, is designed and optimized.Secondly, a 2D simulation model of UTC-DHPT based on Silvaco TCAD software is established. The basic performance of UTC-DHPT devices and traditional single hetero-junction phototransistor(SHPT) are compared and analyzed. Result shows that UTC-DHPT can obtain high optical response and optical gain. Its maximum optical responsivity reaches to 37.5 A/W, and UTC-DHPT is more suitable to deal with large power optical signals. At the same time, UTC-DHPT also provides good electronic current gain, which makes easy to integrate the phototransistor detector and the forward amplifier in a optical receiver.Thirdly, we analyze in detail the optical high-frequency performance of UTC-DHPT and the expression of optical transition frequency is obtained. The influences of photo-generated carriers on the capacitances in its small-signal equivalent model, such as the diffusion capacitance and the barrier capacitance are discussed under different working modes(two-terminal working mode 2T, three-terminal working mode 3T and DHBT working mode). Based on established Silvaco TCAD simulation model, optical transition frequency and optical short-circuit current gain of UTC-DHPT, SHPT and an ordinary HBT at different working mode are analyzed. The results show that UTC-DHPT have better frequency characteristics than traditional SHPT, and UTC-DHPT can provide high responsivity and high response speed simultaneously at three-terminal 3T working mode.Fourthly, the photocurrent is added reasonably into the small-signal equivalent circuit of a bipolar heterojunction transistor. The influences of photo-generated carriers on equivalent diffusion capacitance and barrier capacitance are fully considered too. The small signal equivalent circuit of the UTC-DHPT device at two-terminal working mode 2T, three-terminal working mode 3T and DHBT working mode are presented and the optical high-frequency performance of UTC-DHPT at three different working modes are analyzed further.Finally, an InP-based high speed and high responsivity UTC-DHPT is fabricated and testing. Test results show that compared with SHPT and PIN detector, UTC-DHPT provides higher responsivity of 10.46 A/W. The optical transition frequency of UTC-DHPT reaches to 50.5 GHz.