Pin and Schottky photodetectors I. Pin – photodiodes Electric field profiles in a regular p+ - n diode: V2 > V1
V3 > V2
E
Slope ~ q×ND1/εε0 V1
p+
n
x Metal
The disadvantages: The field and the electron velocity is not uniform. The space charge region width (the responsivity and the capacitance ) depends on the bias High bias voltage needed for high speed operation.
As the doping in the active layer decreases the field becomes more uniform Slope ~ q×ND2/εε0
Slope ~ q×ND1/εε0
ND2 V1
V3 > V2
V2 > V1
E
V3 > V2
E x
V1
V1 x
p+
n
Metal
p+
n
Metal
If the n- layer thickness, d < W, where W is the space-charge thickness at zero bias: The electric field, the electron velocity are nearly uniform; The capacitance is minimal and voltage independent
p-i-n photodiode The concept of p-i-n diode design is to: 1) enlarge the drift region for photocarriers and 2) decrease the junction capacitance. The p-i-n diode consists of p+ - n+ junction with low-doped n- or p- region in between. It can be considered as p+ - n- or n+ - p- junction.
p-i-n photodiode
The device features: • Dark current is small (highly doped n- and p- sides), large potential barrier between nand p-sides; • Photocurrent is due to strong electric field in the i-region - no carrier loss, high efficiency • High electric field - small drift time - fast drift response • Large n- and p- side separation - low capacitance - fast RC response
Si pin photodiode
AlGaN/GaN pin photodiode
Si is not a direct bandgap material.
• Fast response
The absorption length is big
• Visible-blind operation
(> 10 µm for visible/IR light) • Low loss in p+ layer • Thick n- -layer needed for full absorption. • Low speed of response
Quantum efficiency and frequency response of the pin photodiode In general, photogenerated carriers move by drift and diffusion and therefore the total current density through the reverse-biased depletion layer is
The drift component is due to carrier generation in the depletion region. The diffusion component is due to carrier generation OUTSIDE the depletion region. The device can be optimized to have the diffusion component as small as possible (for the fastest response).
1) The drift current density
Incident photon flux (the number of photons per unit area, per second):
Pinc φ0 = (1 − Θ R A hν
ΘR is the reflection coefficient of the top surface, A is the device area, Pinc is the incident optical power. The drift current density, Jdr assuming that all the carriers are swept out by the electric field in the depletion region:
(
J dr = q φ0 1 − e−αW
)
In case the depletion region is thick enough, i.e. αW >> 1,
Jdr max = q ϕ0
)
2) Diffusion current density, Jdiff:
This component contributes to the total current due to the carriers generated OUTSIDE the depletion region (which are LOST for the drift current)
The diffusion current (the absolute value) for the pin-diode is given by:
The total photocurrent density, J = Jdr + Jdif:
Note that pN0 is very small in the n-type region, therefore,
The external quantum efficiency of pin-photodetector:
is given by the same expression as that for p-n junction detector
Frequency response of p-i-n diode depends on three major factors:
1) the drift time through the depletion region, 2) the diffusion time for the carriers generated outside the depletion region, 3) the RC constant of the device.
Frequency response of p-i-n diode:
1) the carrier drift time:
ttr e,h = W2/(µn,p V) for the electrons and holes correspondingly. If the electric field in the depletion region is strong enough, both electrons and holes move with the saturation velocity, vS ≈ 107 cm/s. In this case, ttr = W/vS pin-photodiode is a very fast device.
2) the effect of diffusion current on the frequency response:
3) RC component of the frequency response
The equivalent circuit of the photodiode:
Cj is the junction capacitance. For the pin diode, Cj
= εε0×A/W;
The RC time constant, τRC is (ignoring the package capacitance):
τRC = (RS+RL)×Cj = εε0×A× (RS+RL) / W; When W increases, τRC decreases, however, ttr increases. Optimal design corresponds to τRC ≈ ttr
