Semiconductor Photodetectors

Semiconductor Photodetectors

• Semiconductor photodetectors include photoconductors, p-n photodiodes, p-i-n photodiodes, avalanche photodiodes (APDs), and phototransistors.
• Common materials used in these devices are Si, Ge, GaAs, InGaAs, InP, CdS, and CdSe.
• Performance is measured by responsivity, quantum efficiency, gain, noise, and response time.

Photoconductors

• A photoconductor consists of a semiconductor with ohmic contacts on either end, connected to a voltage source and a series load resistor, with incoming light shining on the semiconductor.
• When an incoming photon has energy greater than the semiconductor's band gap ($hv > E_g$), an electron-hole pair is generated in the semiconductor material.
• The conductivity of the semiconductor increases due to the presence of photogenerated carriers affecting current flow.
• Gain (G) is defined as $G = \frac{\tau}{t_t}$, where $\tau$ is the carrier lifetime and $t_t$ is the transit time.
• Transit time is calculated by $t_t = \frac{L}{v} = \frac{L}{\mu E} = \frac{L^2}{\mu V}$, where $L$ is the length of the semiconductor, $v$ is the carrier velocity, $\mu$ is the mobility, $E$ is the electric field, and $V$ is the applied voltage.
• Photoconductors have high gain but also exhibit slow response times and high noise levels.

p-n Photodiode

• A p-n photodiode consists of a p-n junction diode exposed to incoming light.
• Electron-hole pairs are generated when photons with sufficient energy ($hv > E_g$) are absorbed in the depletion region.
• The electric field separates the photogenerated carriers, with electrons moving to the n-side and holes to the p-side, creating a current.
• p-n photodiodes have a fast response time and low noise, but low gain.

p-i-n Photodiode

• A p-i-n photodiode is similar to a p-n photodiode but includes an intrinsic region between the p and n regions.
• The wider depletion region increases the probability of photon absorption within the device.
• A wider depletion region reduces diode capacitance, improving the response time.
• High quantum efficiency, fast response time, and low noise are advantages, while low gain is a disadvantage.

Avalanche Photodiode (APD)

• An avalanche photodiode (APD) operates similarly to a p-i-n photodiode but has a high electric field in the depletion region.
• The intense electric field accelerates photogenerated carriers causing them to ionize other atoms in the lattice and create additional electron-hole pairs.
• The process is called avalanche multiplication, with gain in the range of $10 \sim 10^3$.
• APDs exhibit high gain but also high noise and require high voltage to operate.

Phototransistor

• A phototransistor consists of an NPN bipolar transistor with incoming light shining on the base-collector junction.
• When a photon with $hv > E_g$ is absorbed in the base-collector junction, an electron-hole pair is generated.
• Photogenerated electrons are injected into the base increasing base current and collector current.
• The collector current is proportional to the base current, by the transistor's current gain ($\beta$).
• Phototransistors have high gain but slow response times.

Explore semiconductor photodetectors: photoconductors, p-n photodiodes, p-i-n photodiodes, avalanche photodiodes (APDs), and phototransistors. Understand how incoming light affects conductivity and generate electron-hole pairs. Learn about responsivity, quantum efficiency, gain, and noise.