Abstract
Saturation current–bandwidth product (SCBP), the key of figure
of merit in high-speed and high-power photodiodes (PDs), is mainly limited
by the tradeoff between carrier drift time in depletion layer and RC-limited bandwidth of conventional
PDs. Here, we present a revolutionary photodiode structure: linear-cascade
photodiodes (LCPDs), designed to further improve the SCBP performance. Our
demonstrated LCPD structure can greatly increase the SCBP without using a
complex distributed structure of the traveling-wave PD or reducing the load
resistance (output RF power). Two flip-chip bonding packaged near-ballistic
unitraveling-carrier photodiode (NBUTC-PD) units are employed in our LCPD
structure. It exhibits a great improvement in SCBP compared to that of the
control device with a single NBUTC-PD. A two-port equivalent-circuit model
is established for the LCPDs and the modeling results clearly indicate that
the increase in SCBP can be attributed to the significant reduction in its
total capacitance due to the serial connection. Furthermore, we find that
only when each PD unit in the LCPD structure has the same amount of injected
optical power and modulated frequency of optical signal, the whole structure
exhibits a carrier transit time as short as that of a single PD. Under the
proper optical excitation, we can achieve a record high SCBP (7500 mA$\cdot$GHz and 100 GHz) for two-element LCPDs under a 50 $\Omega$ load.
© 2011 IEEE
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