How important is the response speed of diodes in optical diagnostic equipment?

1, Technical principle: The physical essence of response speed
The response speed of a diode is essentially a comprehensive reflection of the generation, transmission, and recombination processes of photo generated charge carriers. When the photon energy exceeds the bandgap width of the semiconductor material, valence band electrons transition to the conduction band to form electron hole pairs, generating photocurrent under the action of the built-in electric field. This process involves three key time parameters:

Carrier generation time: Due to the influence of material absorption coefficient, direct bandgap materials such as gallium arsenide (GaAs) can complete photon absorption and carrier generation in picoseconds, while indirect bandgap materials such as silicon require nanoseconds.
Carrier transit time: PIN diodes shorten the carrier transport path to the micrometer level by optimizing the intrinsic layer thickness, and with high electron mobility materials such as indium phosphide InP, the transit time can be controlled within 10ps.
Junction capacitance effect: The parasitic capacitance of the diode will form RC delay. By using heterojunction structure and surface passivation technology, the junction capacitance can be reduced to below 0.1pF, significantly improving the high-frequency response capability.
Taking the application of Tektronix oscilloscope in lidar testing as an example, its avalanche photodiode (APD) can achieve 0.5ns response time at 1550nm wavelength through internal gain mechanism, and can accurately capture the round-trip time of nanosecond laser pulse with 20GHz bandwidth oscilloscope, so as to ensure that the auto drive system can obtain centimeter level positioning accuracy within 200m distance.

2, Application scenario: Speed determines system efficiency
1. Industrial automation testing
In the surface defect detection of 3C products, the linear CCD camera uses an InGaAs photodiode array with a response time of 2ns, combined with a 100kHz line scanning frequency, to complete micrometer level defect recognition of A4 size panels within 0.1 seconds. A semiconductor packaging company has upgraded its wafer detection throughput from 300 wafers per hour to 800 wafers per hour by upgrading to a 0.5ns responsive APD sensor, resulting in a 37% increase in overall equipment efficiency (OEE).

2. Medical imaging diagnosis
In OCT (Optical Coherence Tomography) equipment, the balanced detector adopts a dual PIN diode differential structure, achieving 15 μ m axial resolution at a wavelength of 1310nm with a response time of 0.3ns. After the upgrade of an ophthalmic OCT system, the ten layer structure of the retina can be clearly distinguished, which improves the accuracy of early diagnosis of diabetes retinopathy from 78% to 92%.

3. Laser communication system
In a 100Gbps optical module, a PIN diode combined with a transimpedance amplifier (TIA) achieves a response time of 0.8ns at a wavelength of 1550nm, ensuring that the eye opening and closing degree is greater than 80% and the bit error rate (BER) is better than 10 ⁻¹ ². A data center has deployed this technology to increase the single fiber transmission capacity from 40Tbps to 100Tbps, reducing unit bit energy consumption by 42%.

4. Environmental monitoring field
In the LIDAR atmospheric detection system, an APD array with a response time of 0.2ns is used, combined with 532nm laser pulses, to monitor aerosol concentration distribution in real-time within a height range of 20km. After upgrading its equipment, a meteorological department extended the PM2.5 prediction time from 6 hours to 24 hours, increasing the accuracy of the forecast by 18 percentage points.

3, Performance optimization: multidimensional technological breakthroughs
1. Material innovation
Gallium nitride (GaN) based diodes achieve a 0.1ns response at a wavelength of 405nm, which is five times higher than traditional GaAs materials. They have been applied in blue light DVD reading heads and underwater laser communication.
Quantum dot materials expand the wavelength range of diode response to 300-2000nm by adjusting the bandgap width, meeting the requirements of multispectral diagnosis.

2. Structural optimization
The surface plasmon enhanced structure enhances the photoelectric conversion efficiency by 30% through the localized field enhancement effect of metal nanoparticles, while maintaining a response speed of 0.5ns.
3D integration technology vertically stacks diodes with TIA chips, reducing parasitic capacitance by 60% and achieving module response bandwidth exceeding 30GHz.

3. Process improvement
Molecular beam epitaxy (MBE) technology can control the preparation of semiconductor layers with atomic level flatness, reducing dark current to 0.1nA and improving signal-to-noise ratio by 20dB.
Deep reactive ion etching (DRIE) technology achieves micro scale structural processing, reducing the diode junction capacitance to 0.05pF and significantly improving high-frequency characteristics.