V. Semiconductor Photodetectors (PD)

1. V. Semiconductor Photodetectors (PD) • Introduction • Photoconductors • P-i-N photodiodes • Schottky Barrier (M-S) Photodetectors • Avalanche photodiodes (APD)

2. Intrisic Extrisic by a deep level Extrisic by inter-subband in a QW Introduction to the Photodetectors • Photodetectors are semiconductor devices that can convert optical signals into electrical signals. • Three steps for the operation of a photodetector: • (i) carrier generation by incident light absorption and generation • (ii) carrier transport (and/or multiplication) by current gain mechanism photocurrent • (iii) interaction of current with the external circuits to provide the output signal current collection • Basic requirement for the photodetectors: • high sensitivity at the operating wavelengths • high response speed • low noise • compact (small size) • low biasing voltages or current • high reliability • Development for advanced photodetectors: • tunability • speed • integration

3. Photoconductivity • 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 forbidden-gap energy levels (extrinsic detector), resulting in an increase in conductivity. • Important parameters in a photoconductive process: • absorption coefficient () • cut-off wavelength (λC) • responsivity (RS) • dark current (ID) • quantum efficiency (ηe) • Strong absorption condition:  L (absorption thickness of the sample) > 1/ • the absorption coefficient for the extrinsic absorption is quite small (~10 cm-1) , so a thick sample is needed for extrinsic detectors. • Extrinsic detectors are important for detection of long wavelength radiation • Intrinsic detectors need a very narrow bandgap for long wavelength detection and it is difficult to fabricate high-quality devices from such materials. Typical spectrum of a photodetector

4. Important semiconductor materials for light detection

5. Photoconductors • The photoconductor is the simplest type of the detectors and consists of a simple slab of semiconductor across which a bias is applied. • An important benefit of the photoconductor is the gain in the device, i.e., one can collect more than one electron (or hole) for each photon impinging. • The response time is determined by the transit time. • To achieve short transit time, small electrode spacing and a high electric field must be used. • The photoconductor suffers from the presence of a large dark current noise in the detector. • A reverse biased p-n diode has a very low dark current. (p-n or p-i-n photodiode) • The photoconductors are extensively used for infra-red detection especially for wavelength greater than a few microns.

6. Photodiodes — the basic concepts • A p-n junction diode operated under reverse bias. • The electric field in the depletion region of the diode serves to separate photo-generated EHPs, and an electric current will flow in the external circuit. • The thickness of the depletion layer WD, the quantum efficiency η, but transit time (response speed ) • Hence, there is a trade-off between the response speed and quantum efficiency.

7. Photodiodes — the quantum efficiency • The quantum efficiency η is defined as the number of e-h pairs generated for each incident photon. • One of the key factors that determines the quantum efficiency η is the absorption coefficient α. • Since α is a strong function of the wavelength, the wavelength range in which appreciable photocurrent can be generated is limited. • For light wavelength λ > λc, the values of  are too small to give appreciable band-to-band absorption. • For short wavelengths, the values of  are very large (~105 cm-1), and hence the radiation is mostly absorbed by surface recombination. • The UV and visible region ⇒ The M-S photodiodesThe near-infrared region ⇒ The Si photodiodesThe 1.0 ~ 1.6-μm region ⇒ The Ge photodiodes and GaInAs photodiodes For longer wavelengths ⇒ cooled photodiodes

8. Photodiodes — the response speed • The response speed is limited by three factors: (1) diffusion time for carriers generated outside the depletion region • minimized by making the junction very close to the surface (2) the drift time of carriers through the depletion region (charge collection time) • the depletion width (WD) , carrier mobility (μ) , and applied bias (V)  ⇒ drift time  (3) RC time constant of the device • the depletion width (WD) ⇒ capacitance of the depletion region (C)  ⇒ RC  • The optimal compromise is the width at which the depletion layer transit time is approximately one half the modulation period. • For a modulation frequency of 2 GHz, the optimal WD in a Si photodiode (with a saturation velocity of 107 cm/s) is 0.5  (2 GHz)-1 107 cm/s =0.0025 cm = 25 m

9. The p-i-n Photodiodes • For a p-i-n photodiode, the depletion region thickness can be tailored by controlling the i- layer thickness to optimize the quantum efficiency and frequency response. • The i-region need not be truly intrinsic, as long as the resistivity is high. • When the device is reverse biased, the applied voltage appears almost entirely across the i region (thus depleted entirely), width of the depletion layer is approximately equal to that of the i-layer. • The light absorption region is extended as compared to a p-n photodiode, so higher quantum efficiency can be obtained. • The device response is fast since the photocurrent is primarily due to the prompt photocurrent (in contrast to the diffusion photocurrent) • Anti-reflection coating is used to increase the quantum efficiency by minimizing the surface reflection.

10. Heterojunction p-i-n Photodiodes • Advantages: (1) Using the large-bandgap material as a window for the transmission of optical power, the quantum efficiency does not depend critically on the distance of the junction from the surface. • Broad absorption range since the efficiency is flat in the wavelength as a result of lack of near surface absorption. • Ga0.47In0.53As (Eg = 0.73eV), InP (Eg = 1.35 eV) (2) The heterojunction can provide unique material combinations so that the quantum efficiency and response speed can be optimized for a given optical-signal wavelength. • Because absorption is in the depletion region, no light generated diffusion current occurs. It is a very fast device.

11. Schottky Barrier Photodetectors • A thin metal layer (~ 100 Å) replaces the p-layer in the p-n diode to avoid the surface absorption. Metal-Semiconductor photodiode • The absorption region is the depletion layer formed by the Schottky barrier between the M/S junction  Schottky-Barrier photodiode • A key advantage is it is a majority carrier device, it does not suffer from speed delay arising from minority carrier lifetime issues.  very fast response speed (up to 150 GHz) • The diode can response in two important regimes: • (i) hν > qφbn : electrons are excited in the metal barrier to overcome the Schottky barrier. (a source of dark current) • (ii) hν > Eg : e-h pairs are created in the depletion and swept out to produce photocurrent.

12. Metal-Semiconductor-Metal (MSM) Photodetectors • A planar version of the Schottky barrier photodiode. (The two metal contacts are on the top of the device) • The spacing between the interdigitated finger contacts is small (~ 1 to 5 μm)so that, the region between the fingers can be completely depleted when biased. • When biased, the device represents two diodes in series, one forward biased and the other reverse biased. • Advantages of the MSM photodiode over the standard Schottky one: • planar structure is easy to fabricate. • The parasitic capacitance between contacts is much small than for a vertical diode smaller RC time constant • smaller contact spacing due to the narrow the finger spacing  shorter carrier transit time • easier integration with amplifiers or waveguide very attractive for OEIC applications

13. An APD is operated under a reverse-bias voltage which is sufficient to enable avalanche multiplication (impact ionization) to take place. 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. Important parameters Breakdown voltage : VB Ionization rate (coefficient) : e and h 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. The Avalanche Multiplication Process Avalanche Photodiodes (APD)

14. Comparisons between PIN and APD detectors • APDs have greater sensitivity for a given material • detection of weaker signals over longer distances of fiber at a higher bit rate. • APDs have higher dark current. • PIN diode may have greater sensitivity for long wavelength detection than an APD. • PIN diodes are easier to fabricate. • Hence produce higher yield devices and array at lower cost. • PIN diodes are easier to integrate (with amplifiers) • APDs are very temperature sensitive for dark current and multiplication factor. • Temperature stabilization circuitry is usually needed to increase their reliability. • APDs require a high (tens or hundreds of volts) operating voltage. • The use of APDs is thus presently limited to applications where high gain is of paramount important. • In an APD one has to maintain very stable voltage and temperature values which males the system costly and somewhat unreliable especially if it is placed in a region that is difficult to access (like in undersea regenerators for long distant optical communications)

15. Phototransistors • The phototransistor provides high gain due to the transistor action and is a low noise device device as compared to an APD. • The device usually operated with the base open-circuited. • Holes generated in the reverse biased base-collector junction region provide a base current which causes the electrons to be injected from the emitter. • Due to the transistor action, a small photocurrent induced base current produces a large collector current. • The device does not have a good high frequency response due to very large capacitance associated with the base-collector junction. • The phototransistor finds important uses due to its low noise and high gain.

16. Comparison of various photodetectors