technology  photodiodes


Figure 3.(a) The low mobility of holes limits the speed of conventional p-i-n photodetectors.(b) The UTC photodiode overcomes the limitation of low hole mobility by getting these carriers to relax in the p-contact metal without drift,diffusion or accumulation in the photo-absorption layer.However,if the electron is subjected to a high electric field,it can easily be swept to another electron potential valley with a heavier effective mass and a slower drift-velocity (c) With the NBUTC photodiode,a p-type charge layer is inserted inside the collector to control the distribution of the electrical field and minimize intervalley scattering


eliminating holes, which are far slower at moving through the device than electrons.


Thanks to their superior transport properties, vertical- illuminated type, waveguide type, and distributed type UTC-PDs can all deliver a high power-bandwidth product. However, there are still some weaknesses when the operating frequency of the UTC-PD is in the MMW (sub-THz) regime. In this frequency regime, the internal electron transient time, rather than the RC- limited bandwidth, tends to limit device bandwidth. In these devices the electron drift-velocity can saturate under a high external applied electric field due to intervalley scattering (see Figure 3). To prevent this, a very small reverse bias voltage, such as -0.75 V, can be applied to the thin collector layer (~200 nm thick). This is necessary to maintain the overshoot drift-velocity of the electron and maximise the speed performance of the UTC-PD.


The downside of using a very small reverse bias voltage to realise a high drift-velocity in a sub-MMW UTC-PD is that it tends to be screened by the output AC voltage produced by this device (see Figure 2a). A small load resistance, such as less than 25 Ω can minimize the amplitude of the output AC voltage, but this addition slashes the output power of the photodiode, which normally operates under a standard 50 Ω load.


To overcome such problems, our group at National Central University in Jhungli, Taiwan, has developed a near-ballistic uni-traveling carrier photodiode (NBUTC- PD). This outperforms a UTC-PD in the sub-MMW regime, in terms of the product of saturation current and bandwidth. One key feature of our photodiode is its p- type charge layer inside the collector layer, which can properly control the distribution of the electrical field and minimize intervalley scattering. Near-ballistic transport of electrons in the moderately high optimum reverse bias regime (-2 to -3 V) minimises external saturation. We note that a similar concept has also been reported in the near-ballistic collection HBT by researchers at the Electrical Communication Labs of NTT, Kanagawa.


Photonic transmitters


The backbone of future wireless communication networks operating at Gbit/s speeds will be the optical fibre based network, which connects different base stations, each distributing large volumes of data. Every base station in the MMW wireless network will need many pico-cells (or femto-cells) to radiate the photo- generated MMW power from the photodiode to the last- mile for the end-user.


Each photodiode is integrated with an antenna to form a photonic transmitter that radiates the photo-generated


March 2012 www.compoundsemiconductor.net 29


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