Low Noise Amplifiers

Different transistors for low noise amplifiers (LNAs) offer distinct performance characteristics, primarily due to their material properties and structures. Primary tradeoffs occur between noise figure, frequency, cost, and power handling. 

Silicon based transistors

Silicon transistors are the most common type of transistor, a fundamental component of modern electronics. Silicon is abundant (silica, found in sand), its use is the result of an extensive worldwide infrastructure and producing high purity wafers with very few defects.

• Performance: Silicon based LNAs operate at lower RF frequencies, typically up to L Band, making them suitable for consumer applications like Wi-Fi and Bluetooth.
• ​​​​​​​​​​​​​​​​​​​​​Limitations: Silicon bipolar based performance degrades at higher frequencies due to lower electron mobility compared to other compound semiconductors like gallium arsenide (GaAs). Higher parasitic capacitances of Silicon and poorer noise figures of BiPolars limit their use in high-frequency, low-noise circuits.
• Advantages: Silicon bases Transistors are cost effective, enabling the creation of complex system-on-chip (SoC)  or System-in-Package (SiP) solutions. 

Gallium arsenide (GaAs) FETs

GaAs is a compound based semiconductor known for its superior high electron mobility compared to Silicon, making it an excellent choice for higher frequency microwave circuits. 

• Performance: GaAs Metal Semiconductor Field Effect Transistors (MESFETs) offer better noise figures and higher operating frequencies than silicon based circuits. This makes them ideal for cellular and satellite communication circuits.
• Limitations: GaAs is generally more expensive than Silicon and mechanically more brittle as well. GaAs based Transistors also have a lower bandgap, which limits its power handling and makes it more susceptible to high voltage breakdowns. 

Pseudomorphic high-electron-mobility transistors (pHEMT)

A pHEMT is a type of High Electron Mobility Transistor typically grown on a GaAs substrate. The key to its performance is a strained or "pseudomorphic" layer or super lattice that improves electron mobility.   In other words, the layer has taken on the lattice structure of the substrate even though its natural structure is different.

• Performance: pHEMTs provides a significantly improved noise figure performance over standard GaAs FETs and Silicon above HF Band. For Low Noise Amplifiers, pHEMTs are now an industry standard for circuits requiring ultra-low noise performance from VHF to Ka Band.
• Substrate variations: While pHEMTs are now more common in RF Circuits, some pHEMT companies now use an InP channel on a GaAs based pHEMT substrate to further improve electron mobility. 

Indium phosphide (InP)

InP, a variation of the pHEMT, offer the lowest noise figures and highest operating frequencies of these 4 technologies, reaching into the 100+ GHz range. 

• Performance: The higher electron mobility in InP allows these transistors to achieve the highest gain and lowest noise performance at Millimeter wave frequencies.
• Specialized use: InP based LNAs, due to their higher cost and complex nature, are usually reserved for the most demanding circuits, such as radio astronomy amplifiers, military radar front end receivers, and high-frequency test and measurement equipment.
• Drawbacks: This superior performance comes at a cost. InP technology is not only significantly more expensive to manufacture than GaAs or Silicon, InP Transistors are more fragile and have lower breakdown voltages than the other three metalizations.

Metallization effects

Beyond the basic material itself, the choice of both metallization and the fab processes plays a critical role in a Transistor’s performance, particularly in minimizing noise in a circuit.

• Parasitic resistance: The resistance of the contact points, especially gate resistance, can be a significant source of thermal noise. Bu using low-resistivity metals like gold, one can minimize this extrinsic added noise source.
• Layout and design: Careful layout design in order to minimize parasitic contributions is crucial in low noise figure circuit design. Reducing trace and hybrid bond wire lengths and ensuring proper ground planes using thick gold traces all help to reduce noise figure performance. Smaller gate lengths on some modern transistor designs can also improve the noise figure performance.
• Feedback and matching networks: Improved impedance matching as well as loop and feedback circuits around the transistor based circuit also affects the overall noise figure of the low noise amp.