Understanding Noise in Electronic Devices: Diode, Transistor, APD, FET

Electronic components generate unwanted random fluctuations in voltage or current, which we call noise. This noise can limit the performance of circuits, especially in sensitive applications like communications and sensing. Different types of semiconductor devices generate different amounts and types of noise depending on their operating principles and structure.

Analyzing Noise Levels of Specific Devices

Let's examine the noise characteristics of the devices mentioned: Diode, Transistor, Avalanche photo diode, and FET.

• Diode (A): A simple PN junction diode primarily generates shot noise due to the discrete nature of charge carriers flowing across the junction. Under forward bias, shot noise is significant. Under reverse bias, the noise is mainly due to leakage current. Compared to active amplifying devices, a simple diode typically generates relatively low noise, especially when not in breakdown or avalanche mode.
• Transistor (B): This usually refers to a Bipolar Junction Transistor (BJT). BJTs generate several types of noise, including shot noise (due to carrier flow across junctions), thermal noise (in the base resistance), and flicker noise (low-frequency noise related to surface effects and manufacturing imperfections). BJTs generally have higher noise figures than FETs at certain frequencies, but their overall noise performance depends heavily on biasing and frequency. They are active devices capable of amplification, which also amplifies the internal noise.
• Avalanche Photo Diode (APD) (C): APDs are designed to operate with high reverse bias, utilizing the avalanche multiplication effect to achieve gain. While this provides high sensitivity to light, the avalanche multiplication process itself introduces significant excess noise. The random nature of the carrier collisions and ionization events during multiplication adds substantial noise, making APDs one of the noisiest types of diodes, and often noisier than standard transistors or FETs, especially when significant gain is achieved.
• FET (Field-Effect Transistor) (D): FETs (like JFETs or MOSFETs) primarily generate thermal noise in the conducting channel (e.g., the drain-source channel) due to random thermal motion of carriers. They also exhibit flicker noise at low frequencies. FETs generally have lower input referred voltage noise than BJTs and are often preferred for low-noise amplification at high frequencies or high input impedance applications. However, they still generate more noise than a simple non-amplifying diode.

Ranking Devices by Noise Generation

Based on the typical noise levels and primary noise mechanisms:

• The Avalanche Photo Diode (C) introduces significant excess noise due to its internal gain mechanism (avalanche multiplication), making it generally the noisiest among the given options, especially at high gain.
• A Transistor (B) (like a BJT) is an active device that amplifies, and its internal noise sources (shot, thermal, flicker) result in a measurable noise contribution, typically higher than simple passive or non-multiplying devices.
• A FET (D) also generates noise (thermal, flicker) but is often designed for lower noise than BJTs, particularly in certain configurations or frequency ranges. However, it is still an active device generating more noise than a simple diode.
• A simple Diode (A) operating without avalanche breakdown generates shot noise, which is usually the lowest level of noise compared to the active amplifying or multiplying devices listed.

Therefore, arranging them in decreasing order of noise generated is:

Avalanche photo diode > Transistor > FET > Diode

Which corresponds to the order: (C), (B), (D), (A).

Revision Table: Electronic Device Noise

Additional Information: Types of Electronic Noise

Understanding the different types of noise helps in analyzing device noise performance:

• Thermal Noise (Johnson-Nyquist Noise): This noise is generated by the random thermal motion of charge carriers within a conductor. It is present in all components with resistance and is proportional to temperature and bandwidth. It has a flat power spectral density (white noise). The mean square voltage noise $\bar{v_n^2} = 4kTR\Delta f$, where $k$ is Boltzmann's constant, $T$ is the absolute temperature, $R$ is the resistance, and $\Delta f$ is the bandwidth.
• Shot Noise: This noise arises from the random arrival or flow of discrete charge carriers (electrons or holes). It is prominent in devices where carriers cross a potential barrier randomly, such as PN junctions (diodes, transistors). Like thermal noise, it is generally white noise. The mean square current noise $\bar{i_n^2} = 2qI\Delta f$, where $q$ is the electron charge, $I$ is the average current, and $\Delta f$ is the bandwidth.
• Flicker Noise (1/f Noise or Pink Noise): The power spectral density of this noise is inversely proportional to frequency, hence the name 1/f noise. Its origin is often related to surface effects, traps, and crystal imperfections in semiconductor devices. It is most significant at low frequencies.
• Avalanche Noise: This is excess noise generated in devices operating in avalanche breakdown (like APDs). The random nature of the impact ionization process during carrier multiplication adds significant noise above the basic shot noise.