Analog IC Review Part I

Index
1. Introduction
2. Active Devices in IC
3. Transistor Technology

1. Introduction

This is a comprehensive review of analog IC design mostly based on the book Analysis and Design of Analog Integrated Circuits, Fourth Edition by Gray, Hurst, Lewis, and Meyer. There might be considerable overlap between this post and CMOS Analog IC Review; however, this post covers much more materials including device physics and BJT. On the other hand, it is assumed that most of the constants, symbols, formulae, circuits, blocks, and analysis are known to the sole reader, myself, hence explanations are appropriately omitted. 

2. Active Devices in IC

Depletion Region

Semiconductor electronics start from pn junction. A pn junction is formed by joining a p-type material and an n-type material. Due to carrier diffusion and recombination, the section around the junction is depleted of majority charge carrier, and this section is called depletion region. Depletion region at the p-type material contains negatively charged, immobile ions and vice versa; this creates an electric field that opposes further diffusion and reaches equilibrium, which is described by a built-in potential ψ₀ across the junction (from n-type material to p-type material). Suppose the p-type and n-type material have constant doping densities of N_A and N_D atoms/cm³, respectively, then

ψ₀ = V_T ln(N_AN_D/n_i²).

If a reverse voltage V_R (+ on n side and – on p side) is applied to the pn junction, the depletion region width and maximum electric field can be calculated based on N_A, N_D, and V_R. The formula is omitted here.

Junction Capacitance

The charge in the depletion region is dependent on V_D = -V_R, hence a small-signal junction capacitance C_j can be modeled as follows:

Junction Breakdown

Any reversed-biased pn junction has a small reverse current flow due to the presence of minority-carriers in the vicinity of the depletion region. At the breakdown voltage BV, the carriers traversing the depletion region acquires sufficient energy to trigger avalanche process which leads to a sudden increase in the reverse-bias leakage current.

BJT

A BJT is formed by joining 3 materials of alternating types (npn or pnp).