半導体工学その3



## ■ 1.1 Importance of Risk Identification in Analog IC Development

In analog IC development, it is **crucial to identify and assess potential risks** during the **early specification and planning stage**. The accuracy and depth of this risk assessment significantly influence the **development period**—especially the number of design iterations and rework.

### ◉ Key Points

* Conduct a thorough review of **potential risks** and prepare **mitigation strategies** in advance before finalizing the development plan.
* For projects with tight schedules, it may be wise to adopt **proven processes and packages** to reduce risks related to yield, performance, or quality.
* Over-engineering a product to exceed required specifications may result in extended timelines and increased costs—**a poor trade-off** if it doesn’t align with customer or market demands.
* Clear understanding of customer expectations and **balanced specification setting** are essential to efficient and successful development.

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## ■ 1.2 Analog IC Circuit Design: Methodology and Key Considerations

Analog circuit design requires the **ability to select or invent the optimal circuit topology** that meets performance targets. Unlike digital design, analog circuits offer **greater design freedom**, which means a skilled designer’s creativity and insight directly influence the outcome.

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### ◉ General Design Flow

1. **Specification Definition**

   * Define electrical, thermal, and mechanical specs based on market needs, performance targets, and production capabilities.
   * Include: power consumption, operating temperature and voltage ranges, load drive capacity, pin count, thermal resistance, and packaging.

2. **System Design**

   * Create functional block diagrams.
   * Identify required core components: voltage/current sources, current mirrors, differential pairs, operational amplifiers, comparators, etc.

3. **Circuit Design**

   * Develop detailed circuit implementations for each functional block.
   * Optimize biasing, gain-bandwidth tradeoffs, compensation networks, and ensure robust behavior under process/voltage/temperature (PVT) variations.

4. **Circuit Verification (Simulation)**

   * Use circuit simulators (SPICE-based) to verify that the design meets its specifications.
   * Common simulation types include: DC, AC, transient, noise, sensitivity, temperature sweep, and Fourier analysis.

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## ◉ SPICE Simulation: Usage and Limitations

* Widely used tools: **SPICE, HSPICE, Spectre, SmartSpice, PSpice**, etc.
* Standard device model: **BSIM3 (Berkeley Short-Channel IGFET Model 3)**

### ◉ Types of SPICE Analysis (Examples)

| Analysis Type            | Description                                              |
| ------------------------ | -------------------------------------------------------- |
| **DC Analysis**          | Calculates static bias points and current/voltage levels |
| **AC Analysis**          | Small-signal gain and frequency response                 |
| **Transient Analysis**   | Time-domain response simulation (e.g., settling time)    |
| **Noise Analysis**       | Evaluation of circuit noise levels                       |
| **Sensitivity Analysis** | Impact of parameter variations on output                 |
| **TF Analysis**          | Transfer function estimation                             |
| **Fourier Analysis**     | Frequency-domain analysis for distortion/harmonics       |
| **Temperature Sweep**    | Analyzes behavior over a range of operating temperatures |

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## ◉ TEG (Test Element Group) Usage

For characteristics that **cannot be accurately simulated**, prototype test chips known as **TEG** are fabricated:

* **Circuit TEGs**: Used to verify block-level behavior, optimize constants, or evaluate parasitic effects.
* **Device TEGs**: Used to characterize transistor models and extract accurate physical parameters.

Designers must judge where simulation is sufficient, and where **hardware validation through TEGs is necessary**.

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## ◉ Limitations of Circuit Simulators

* Circuit simulators are **not all-powerful tools**.
* They are meant to support **desk-based design**, replacing manual calculations in complex analyses.
* However, simulations have **limited accuracy** in certain regions (e.g., subthreshold leakage, near-Vth operation, OFF-state leakage currents).
* In such cases, multiple **model parameter sets** may be required for different operating regions.

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## ◉ Breadboard Testing (for Bipolar Circuits)

* In some cases (e.g., bipolar analog circuits), **discrete-component breadboards** can be built to validate key performance aspects.
* For **CMOS analog ICs**, however, breadboarding is impractical due to:

  * High-impedance nodes prone to noise
  * Sensitivity to electrostatic discharge
  * Difficulty in accurately replicating chip-level parasitics

As a result, **simulation remains the primary verification method**.

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## ◉ Final Thoughts

Analog IC design requires a **delicate balance of creativity, physical understanding, and rigorous verification**. Designers must:

* Choose architectures that meet specifications,
* Validate them through both simulation and real-world testing (if needed),
* Understand the **limitations of models and tools**, and
* Be prepared to adjust their approach based on practical results.