fotodiodos

Question 1

alpha

Answer 1

Absorption coefficient α is a
material property.

Question 2

Direct bandgap semiconductor

Answer 2

• 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

Question 3

Indirect bandgap

Answer 3

• 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.

Question 4

Junction and Avalanche gain

Answer 4

-Junction or PIN do not have internal gain
Avalanche has an internal gain M

Question 5

Photoconductivity

Answer 5

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.

Question 6

Important parameters in a photoconductive process

Answer 6

*absorption coefficient (alpha)
*cut-off wavelength (λC)
*responsivity (RS)
*dark current (ID)
*quantum efficiency (ηe)

Question 7

III-V compound semiconductors

Answer 7

*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)

Question 8

Choice of photodiode materials

Answer 8

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”

Question 9

Junction photodiodes

Answer 9

-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.

Question 10

I0

Answer 10

*I0 is the “saturation current” representing thermal-generated free carriers which flow
through the junction (dark current).

Question 11

Short-circuit current and Open-circuit voltage

Answer 11

-The short-circuit current (V = 0) is the photocurrent Ip
-The open-circuit voltage (I = 0) is the photovoltage Vp

Question 12

Photocurrent and photovoltage

Answer 12

-*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.

Question 13

Open-circuit voltage

Answer 13

-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)

Question 14

Photoconductive and photovoltaic modes

Answer 14

-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.

Question 15

photoconductive mode

Answer 15

-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)

Question 16

photovoltaic mode

Answer 16

*Does not require a bias voltage but requires a large load resistance.
* RL&raquo_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)

Question 17

Reversed based p-n junction

Answer 17

-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.

Question 18

p-i-n photodiodes drawbacks:

Answer 18

-Depletion layer capacitance is not sufficiently small to allow photodetection at high modulation frequencies
-Narrow SCL

Question 19

p-i-n photodiodes

Answer 19

*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).

Question 20

Reversed based p-i-n junction

Answer 20

-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

Question 21

p-i-n photodiodes
Advantages

Answer 21

-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.

Question 22

Schottky photodiodes

Answer 22

-A thin metal layer (~ 100 Å) replaces the p-layer
in the p-n diode to avoid the surface absorption.

Question 23

Avalanche photodiodes (APD)

Answer 23

• An APD is operated under a reverse-bias voltage which is sufficient to enable avalanche
  multiplication (impact ionization) to take place.

Question 24

vantagens APD

Answer 24

– 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.

Question 25

Important parameters APD

Answer 25

– Breakdown voltage : VB
– Ionization rate (coefficient) : The number of EHPs generated by an
electron (or hole) per unit distance.
– Multiplication factor : M
The ratio of the number of electrons leaving the junction to those entering it.

Question 26

APD

Answer 26

-n internal gain obtained by having a high electric field that energizes photo-generated electrons and holes.
-These electrons and holes ionize bound electrons in the valence band upon colliding with them
*This mechanism is known as impact ionization
*The newly generated electrons and holes are also accelerated by the high electric field and they gain enough energy to cause further impact ionization
*This phenomena is called the avalanche effect

Question 27

Avalanche vs. p-i-n photodiodes

Answer 27

-APDs have greater sensitivity for a given material
-APDs have higher dark current.
-PIN diode may have greater sensitivity for long wavelength detection than an APD.
-PIN diodes are easier to fabricate.
-PIN diodes are easier to integrate (with amplifiers)
-APDs are very temperature sensitive for dark current and multiplication factor.
-APDs require a high operating voltage.