PIN diode structure identification — doping profile and intrinsic region Which option correctly describes the doping of a PIN diode's terminals relative to its intrinsic (i) region?

Introduction / Context:
PIN diodes are essential RF/microwave control elements acting as current-controlled resistors at high frequency. Their behavior arises from a specific layer structure that enables charge storage in the intrinsic region, giving a nearly resistive RF impedance under forward bias and high impedance under reverse bias.

Given Data / Assumptions:

• Device under test: PIN diode.
• Asked: which doping configuration is correct around the intrinsic layer.

Concept / Approach:
A PIN diode consists of a heavily doped p-type region (p+), an intrinsic (undoped or very lightly doped) region (i), and a heavily doped n-type region (n+). The heavy doping lowers series resistance and facilitates charge injection; the i-region stores charge, yielding a controllable RF resistance with bias current.

Step-by-Step Solution:

Verification / Alternative check:
Equivalent-circuit models and datasheets emphasize the low series resistance due to heavy terminal doping and the bias-dependent resistance from the stored charge in the i-region.

Why Other Options Are Wrong:

• p+ and n (lightly doped n): not the standard PIN structure.
• p- and n-: incorrectly suggests lightly doped terminals; would raise series resistance.
• None of the above: incorrect because p+ – i – n+ is standard.

Common Pitfalls:
Confusing PIN with ordinary PN junction; overlooking the role of the intrinsic region in RF behavior.

Final Answer:
p+ and n+ layers separated by i layer