Semiconductor Material & Fabrication Process for RF Transistors

Most of new RF & Microwave Engineer still confused with RF Semiconductor Materials & Process Technology and can't really differentiates them well.

Semiconductor Materials for example:

1. Silicon (Si)
   
2. Silicon Germanium (SiGe)
3. Gallium Arsenide (GaAs)
4. Gallium Nitride (GaN)
5. Silicon Carbide (SiC)

Transistor Fabrication Process for example:

1. Metal-Semiconductor FET (MESFET)
2. Bipolar Junction Transistor (BJT)
   
3. Heterojunction Bipolar Transistor (HBT)
4. Laterally Diffused Metal-Oxide Semiconductor (LDMOS)
5. High Electron Mobility Transistor (HEMT)
6. Pseudomorphic HEMT (pHEMT)
7. Enhanced-Mode pHEMT (E-pHEMT)
   

Five Common Methods for LNA Stabilization

1. Input resistive loading: One major and obvious drawback - degrades NF of the LNA and is almost never used.
2. Output resistive loading: Should be carefully used because the effects are lower Gain and lower OP1dB.
3. RLC feedback at the Collector to Base: This is to lower the gain at the lower frequencies and therefore improve the stability.
4. Filter matching network at the transistor's output: The objective is to decrease the gain at a specific frequency or narrowband. This method is frequently used for eliminating gain at high frequencies, far above the operating band. Short circuit quarter wave lines designed for problematic frequencies, or simple capacitors with the same resonant frequency as the frequency of oscillation (or excessive gain) can be used to stabilize the circuit.
5. Emitter feedback inductor: A small inductor/inductance can stabilize the circuit at higher frequencies. However, excessive source inductance will cause the K-factor at higher frequencies falls bellow unity. This effect limits the amount of source inductance that can safely be used.

Precautions When Measuring IMD Products

Background:

1. Whenever measuring distortion products, remember that the instrumentation contains analog components.
2. This analog elements are subject to produce the exact same symptoms that we are trying to measure (harmonics and IMD products) when they are driven into their nonlinear regions of operation.
   
3. The IMD generated by the instruments will add to the measured response from our device and give us an incorrect reading.

A simple step to check whether the instrument is being overdriven are:

1. Manually increase the input attenuation and observe the levels of the intermodulation products.
   
2. If the intermodulation products do not change, then the instrument was not being overdriven and we can restore the attenuation to the previous setting.
   
3. If the instrument was generating intermodulation products before we raised the attenuation, then we would observe that the intermodulation products are reduced after we change the attenuation.
   
4. Measurements should be taken with the attenuation set so that the intermodulation products do not change when we increase it.
   

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