Question E6A01:
In what application is gallium arsenide used as a semiconductor material?  

Choices:  
A: In high-current rectifier circuits  
B: In high-power audio circuits  
C: In microwave circuits  
D: In very low-frequency RF circuits    

Correct Answer is  C: In microwave circuits  

Correct answer: C — In microwave circuits

Why C is correct:
- Gallium arsenide (GaAs) has much higher electron mobility than silicon (about 8500 vs. 1400 cm²/V·s), allowing devices to operate at very high frequencies with low noise.
- This enables fast transistors (MESFETs, HEMTs) and diodes (Schottky, Gunn) used in RF/microwave front-ends, mixers, LNAs, and MMICs.
- The higher carrier velocity yields higher transit/cutoff frequencies, making GaAs ideal for microwave applications.

Useful relation:
- The device transit/cutoff frequency scales inversely with gate length and directly with saturated carrier velocity:
  $$f_T \approx \frac{v_{\text{sat}}}{2\pi L}$$
  GaAs has higher $v_{\text{sat}}$, so $f_T$ is higher for a given feature size.

Why the other choices are incorrect:
- A: High-current rectifier circuits
  - Rectifiers prioritize current handling, efficiency, and cost. Silicon (and SiC for very high voltage/current) dominates. GaAs offers no advantage here and has poorer thermal properties than SiC.
- B: High-power audio circuits
  - Audio is low frequency; the high-speed advantage of GaAs is wasted. Silicon BJTs/MOSFETs provide better cost, availability, and thermal performance for power audio.
- D: Very low-frequency RF circuits
  - At LF, GaAs’s high-frequency/noise advantages are unnecessary. Silicon devices meet requirements at far lower cost.

Rules of thumb:
- GaAs/GaN: microwave to millimeter-wave RF, low-noise and/or high-frequency.
- Silicon: general-purpose, low to moderate frequency, most power/audio.
- SiC: high-voltage/high-current power electronics.