Photonic devices Flashcards
What is a photodetector? Three steps operation?
optoelectronic device that absorb optical energy and converts it into electrical energy, which usually manifests as photocurrent
- Carrier generation by incident light;
- Carrier transfer across the device
- Interaction of the photocurrent with the external circuit to provide the output signal
Type of photodetectors
- Photoconductors: SC whose resistance depends on the absorption of light, can be intrinsic/extrinsic
- Charge coupled deviced
- Photodiodes
How does a photodiode work?
A photodiode is a pn junction optimized for absorbing light and operated under reverse bias.
A p-i-n where i = intrinsic = undoped region, sandwiched by two doped p,n regions.
The depletion region extend over the whole i-region and is larger than depletion layer of pn junction.
A photon is absorbed if ℏω >= E_gap –> e/h pairs create inside the i-region –> carriers are then swept out by E-field into an external circuit to generate photocurrent I_L
Photoconductive mode of photodiodes
Reverse biased (V_b < 0, V < 0; the power is delivered to the device by external circuit to achieve max sensitivity and fast response
–> the device acts as current source
–> works in 3rd quadrant, including the short circuit condition at V = 0
Photovoltaic mode for photodiodes
Forward biased (V_b = 0, V > 0); the power is delivered to the load by the device
–> The device acts as voltage source with V_output limited by the equilibrium built in potential
–> Works in 4th quadrant, including the open-circuit condition at I = 0
–> Used as solar cell!
What is the origin of V_open_circuit, how can V > 0 if no external bias is applied?
The photogenerated e(h) drift towards n(p) side and accumulate at the junction, reducing the built-in voltage and putting the junction in forward bias. The appearance of a forward voltage across an illuminated junction (photovoltage) is known as the photovoltaic effect.
- The open‐circuit voltage (i.e., the voltage for which I = 0) is the photovoltage Voc .
The upper limit of 𝑉oc is the built‐in barrier 𝜑b as the electric field 𝜺, hence also the drift current, vanishes when Voc= 𝜑b
What determines the photocurrent in a pin photodiode?
Not all incoming photons contributes to the photocurrent since n_air ≠ n_semiconductor –> there will be some reflection and a fraction will be absorbed.
Thus we need to account for (1-R)x(1-exp(-αl)) = fraction of light absorbed in the active region
Internal quantum efficiency
carriers generated/#absorbed photons
External quantum efficiency
carriers generated/# total incident photons
Photosensitivity R
Is the ratio of photocurrent I_L to incident light power P_in at a given wavelength
Open circuit solar cell
The current is zero, the solar cell delivers max voltage, output power is zero.
Max achievable V_oc = built-in potential
Short circuited solar cell
The voltage across the diode is zero; the solar cell delivers max current, the output power is zero
Solar cell with resistive load
The max output power is given by the area between the dark and illuminated diode equation.
Conflicting effects on a solar cell?
Large I_shortcircuit = large absorbed photons = small band gap energy
BUT small band gap energy = small built in potential = small V_oc
Solution –> multi junction solar cell
