fotodiodos Flashcards
alpha
Absorption coefficient α is a
material property.
Direct bandgap semiconductor
- The photon absorption does not require assistant from lattice vibrations
-The photon is absorbed and the electron is excited directly from the VB to CB without a change in its k
vector, since photon momentum is very small.
hK(cb)-hK(vb)=0
Indirect bandgap
- Indirect bandgap semiconductors, the photon absorption requires assistant from lattice vibrations (phonon)
-the probability of photon absorption is not as high as in a direct transition and the λg is not as sharp as for direct bandgap semiconductors.
Junction and Avalanche gain
-Junction or PIN do not have internal gain
Avalanche has an internal gain M
Photoconductivity
When incident light impinges on the surface of the photoconductor, EHPs are generated
either by band-to–band transition (intrinsic detector) or by transitions involving forbiddengap energy levels (extrinsic detector), resulting in an increase in conductivity.
Important parameters in a photoconductive process
*absorption coefficient (alpha)
*cut-off wavelength (λC)
*responsivity (RS)
*dark current (ID)
*quantum efficiency (ηe)
III-V compound semiconductors
*Direct-bandgap III-V compound semiconductors can be better material choices than
germanium for the longer wavelength region.
*Their bandgaps can be tailored to the desired wavelength by changing the relative
concentrations of their constituents (resulting in lower dark currents).
*They may also be fabricated in heterojunction structures (which enhances their highspeed operations)
Choice of photodiode materials
A photodiode material should be chosen with a bandgap energy slightly less than the
photon energy corresponding to the longest operating wavelength of the system.
*This gives a sufficiently high absorption coefficient to ensure a good response, and yet
limits the number of thermally generated carriers in order to attain a low “dark current”
Junction photodiodes
-The semiconductor photodiode detector is a p-n junction structure that is based on the
internal photoeffect.
-Photoresponse of a photodiode results from the photogeneration of electron-hole pairs
through band-to-band optical absorption. The threshold photon energy of a semiconductor photodiode is the bandgap energy Eg of its active region.
-The photogenerated electrons and holes in the depletion layer are subject to the local electric
field within that layer.
I0
*I0 is the “saturation current” representing thermal-generated free carriers which flow
through the junction (dark current).
Short-circuit current and Open-circuit voltage
-The short-circuit current (V = 0) is the photocurrent Ip
-The open-circuit voltage (I = 0) is the photovoltage Vp
Photocurrent and photovoltage
-*As the light intensity increases, the short-circuit current increases linearly
-The open-circuit voltage increases only logarithmically and limits by the equilibrium contact potential.
Open-circuit voltage
-The photogenerated, field-separated, majority carriers (+ charge on the p-side, - charge on
the n-side) forward-bias the junction.
-The appearance of a forward voltage across an illuminated junction (photovoltage) is known
as the photovoltaic effect.
-The limit on Vp is the equilibrium contact potential V0 as the contact potential is the maximum forward bias that can appear across a junction. (drift current vanishes with Vp =V0)
Photoconductive and photovoltaic modes
-The device functions in photoconductive mode in the third quadrant of its current-voltage
characteristics, including the short-circuit condition on the vertical axis for V = 0. (acting as a current source)
-It functions in photovoltaic mode in the fourth quadrant, including the open-circuit condition on the horizontal axis for I = 0. (acting as a voltage source with output voltage
limited by the equilibrium contact potential)
-The mode of operation is determined by the bias condition and the external circuitry.
photoconductive mode
-With a series load resistor RL< Ri
gives the load line
-Keep Vout < VB so that the photodiode is reverse biased
-vb é a fonte de tensão
-Power (+) is delivered to
the device by the external
circuit (photodetector)
photovoltaic mode
*Does not require a bias voltage but requires a large load resistance.
* RL»_space; Ri, so that the current I flowing through the diode and the internal resistance is negligibly small.
-Power (-) is delivered to
the load by the device
(solar cell/energy harvesting)
Reversed based p-n junction
-It is important that the photons are absorbed in the depletion region. Thus, it is made aslong as possible
-The depletion-layer width widens and the junction capacitance drops with reverse voltage
across the junction.
p-i-n photodiodes drawbacks:
-Depletion layer capacitance is not sufficiently small to allow photodetection at high modulation frequencies
-Narrow SCL
p-i-n photodiodes
*A p-i-n photodiode consists of an intrinsic region sandwiched between heavily doped p+ and n+ regions. The depletion layer is almost completely defined by the intrinsic region.
*In practice, the intrinsic region does not haveto be truly intrinsic but only has to be highly
resistive (lightly doped p or n region).
Reversed based p-i-n junction
-The depletion-layer width W in a p-i-n diode does not vary significantly with bias voltage but is essentially fixed by the thickness, di, of the intrinsic region so that W ≈ di
p-i-n photodiodes
Advantages
-Increasing the width of the depletion layer (where the generated carriers can be
transported by drift) increases the area available for capturing light.
-Increasing the width of the depletion layer reduces the junction capacitance and thereby
the RC time constant. Yet, the transit time increases with the width of the depletion layer.
Schottky photodiodes
-A thin metal layer (~ 100 Å) replaces the p-layer
in the p-n diode to avoid the surface absorption.
Avalanche photodiodes (APD)
- An APD is operated under a reverse-bias voltage which is sufficient to enable avalanche
multiplication (impact ionization) to take place.
vantagens APD
– vary large internal current gain
– fast response to the light modulated at microwave frequencies
– high sensitivity to low-level optical signals with about 1 ns response time, particular useful in fiber optic communication.
