Semiconductor Device Physics for RF Design

Course 183

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Summary

This course provides microwave circuit designers with an in-depth look at their “toolkit” of semiconductor devices. Starting with a brief look at quantum mechanics, the course develops a picture of how electrons behave in semiconductor materials. This is applied to functional descriptions of the basic semiconductor devices: the P-N junction, the bipolar transistor and the FET. Further material describes how properties of different semiconductor materials and the ability to create certain material structures leads to the large variety of modern devices, each with its own characteristics, advantages and disadvantages. A final section describes principals of semiconductor fabrication and how limitations in materials and fabrication lead to limitations in performance and repeatability of microwave devices.

Learning objectives

Upon completing the course you will be able to:

• Describe the significance of energy bands for the conductivity of semiconductors
• Visualize the operation of diode rectifiers, bipolar transistors and FETs in terms of drift and diffusion of charge carriers.
• Identify the structures and microwave applications of GaAs FETs, HEMTs, and HBTs.
• Understand the MMIC fabrication process in overview.
• List major factors and failure mechanisms that limit device performance.

Target Audience

Circuit engineers and engineering managers who can benefit from a deeper understanding of the devices they use in design and/or manufacture of microwave products. Familiarity with undergraduate Physics, Electromagnetics and Calculus is strongly recommended.

Outline

 • Quantum Mechanics - Schroedinger’s Wave Equation and energy levels • Band Theory of Solids - Valence and conduction energy bands, Fermi-Dirac distribution, generation and recombination, intrinsic carriers, direct and indirect materials • Doping in Semiconductors - Donor and acceptor levels, Fermi Level, Law of Mass Action • Advanced Concepts (as class interest dictates) - k-space, density of states, effective mass, Brilloun zones • Carrier Transport - Drift and mobility, excess carriers and diffusion, Carrier Continuity Equation, concept of a plasma • PN junction at equilibrium - depletion region, built-in potential, band curvature • PN junction at reverse bias - thermal saturation current, avalanche breakdown (as class interest dictates) • PN junction at forward bias - minority injection, diffusion current, Law of the Junction, Carrier Continuity and diode I-V relation • Circuit properties - junction capacitance and conductance • Schottky diodes - M-S junction, switching speed, reverse leakage

Transistor Structure

 • BJT in Saturation - bias state, Carrier Continuity in base, injection efficiency, Beta • Other effects in BJTs - base width modulation and the Early Effect, doping gradients • Circuit properties of BJTs - small signal model with major and secondary effects, intrinsic and extrinsic elements • FET basics - Gradual Channel Approximation and Threshold Voltage • Microwave FET Structures - Short channel effects and Schottky gate

Device Materials

 • Characteristics of Microwave Materials • Silicon, GaAs, InP • Hybrid Materials and Special Transistor Structures - SiGe, HEMT, HBT • Device Fabrication - Diffusion, Ion Implantation, Epitaxial Growth • Fabrication Issues - Parasitic Transistors and latchup, traps, surface states, hydrogen poisoning

Subject Areas:

• RF Circuit Design (Linear): S-Parameters, Smith Chart, Passive Components, LNA's
• RF/MW Integrated Circuit (RFIC/MMIC) Design