22.3 Photodiodes
613
Fig. 22.32 Scheme of a traveling-wave photodetector with pin structure and coplanar contacts
In the case of a heterostructure-APD with SAM structure (Fig. 22.30b) absorption (of light with
sufficiently long wavelength with an energy smaller the the InP band gap) takes place only in the
InGaAs layer. Since no light is absorbed in the multiplication region, the device functions similarly
for front and back illumination. In [1846] a multi-stage InGaAs-based APD on InP is described that
is optimized for electron multiplication in ten subsequent gain sections; a total gain of 10
3 is achieved
with an excess noise factor F of about 40, belonging to an effective ionization ratio of k = 0.036.
22.3.7 Traveling-Wave Photodetectors
In a standard photodetector there was a tradeoff between the thickness of the absorption layer and the
speed of the detector. In a traveling-wave photodetector the light absorption occurs in a waveguide
such that for sufficient length L all incident light is absorbed. Complete absorption is achieved (‘long’
waveguide) if L ((α)
−1 , α being the absorption coefficient and ≤ 1 being the optical confinement
factor, the geometrical overlap of the optical mode with the cross section of the absorbing medium (cf.
also Sect. 23.4.4).
The electrical connections are designed along this waveguide on the sides (coplanar layout,
Fig. 22.32). The bandwidth limitation due to a RC time constant is now replaced by the velocity
match of the light wave v opt = c/n and the traveling electric wave in the contact lines v el ≈ 1/
√
LC.
While the two waves travel along the waveguide, energy is transferred from the light wave to the electric
wave. The 3 dB bandwidth due to velocity mismatch B vm (for impedance- matched, long waveguides)
is given by [1847]
B vm =
α
2π
v opt v el
v opt − v el
.
(22.54)
For a MSM structure, whose electrode separation has been designed with a self-aligned process (without extensive effort in lateral patterning) by an etch depth of a few 100 nm (Fig. 22.33), 3 dB cutoff
frequencies beyond 500 GHz have been achieved (Fig. 22.34). The quantum efficiency of this detector
was still 8.1%.
613
Fig. 22.32 Scheme of a traveling-wave photodetector with pin structure and coplanar contacts
In the case of a heterostructure-APD with SAM structure (Fig. 22.30b) absorption (of light with
sufficiently long wavelength with an energy smaller the the InP band gap) takes place only in the
InGaAs layer. Since no light is absorbed in the multiplication region, the device functions similarly
for front and back illumination. In [1846] a multi-stage InGaAs-based APD on InP is described that
is optimized for electron multiplication in ten subsequent gain sections; a total gain of 10
3 is achieved
with an excess noise factor F of about 40, belonging to an effective ionization ratio of k = 0.036.
22.3.7 Traveling-Wave Photodetectors
In a standard photodetector there was a tradeoff between the thickness of the absorption layer and the
speed of the detector. In a traveling-wave photodetector the light absorption occurs in a waveguide
such that for sufficient length L all incident light is absorbed. Complete absorption is achieved (‘long’
waveguide) if L ((α)
−1 , α being the absorption coefficient and ≤ 1 being the optical confinement
factor, the geometrical overlap of the optical mode with the cross section of the absorbing medium (cf.
also Sect. 23.4.4).
The electrical connections are designed along this waveguide on the sides (coplanar layout,
Fig. 22.32). The bandwidth limitation due to a RC time constant is now replaced by the velocity
match of the light wave v opt = c/n and the traveling electric wave in the contact lines v el ≈ 1/
√
LC.
While the two waves travel along the waveguide, energy is transferred from the light wave to the electric
wave. The 3 dB bandwidth due to velocity mismatch B vm (for impedance- matched, long waveguides)
is given by [1847]
B vm =
α
2π
v opt v el
v opt − v el
.
(22.54)
For a MSM structure, whose electrode separation has been designed with a self-aligned process (without extensive effort in lateral patterning) by an etch depth of a few 100 nm (Fig. 22.33), 3 dB cutoff
frequencies beyond 500 GHz have been achieved (Fig. 22.34). The quantum efficiency of this detector
was still 8.1%.