What is the use of LDD (Lightly Doped Drain) in transistor design?

The Lightly Doped Drain (LDD) structure is used to reduces parasitic capacitances between the gate and the source/drain regions. Also its enhance the voltage capability of transistors.

It increases the breakdown voltage at the transition between the drain region and the channel. By lightly doping the regions at the channel ends, it reduces the electric field intensity near the drain, minimizing hot-carrier effects. This configuration helps improve the reliability of the transistor, especially in high-voltage applications, and also , which improves overall performance.

When we talk about Lightly Doped Drain (LDD) technology, the focus is on managing electric fields at the drain junction of a MOSFET. You know that high electric fields can cause hot-carrier effects, which lead to reliability issues. To address this, we introduce a lightly doped n- region between the heavily doped source/drain (n+) and the channel.

Here’s how it works: the LDD spreads out the electric field near the drain. Instead of having a sharp field peak at the n+/channel junction, the n- region softens this transition. This reduced field intensity decreases the chances of hot carriers being injected into the gate oxide. As a result, we not only improve device reliability but also enable the MOSFET to operate at higher voltages or with shorter channel lengths without compromising performance.

By carefully optimizing the doping concentration and length of the n- region, we achieve a balance where the electric field is controlled while ensuring minimal resistance. That’s the key to how LDD helps in modern semiconductor design!

While we use LDD (Lightly Doped Drain) to reduce the electric field and improve device reliability, it does come with some trade-offs. One of the main disadvantages is the increased series resistance introduced by the lightly doped n- region. You might notice this resistance impacting the overall current drive of the MOSFET, leading to slightly reduced performance.

We also face challenges in manufacturing when integrating the LDD structure. The precise doping profile and alignment required can make the fabrication process more complex and costly. Additionally, for high-speed circuits, the added resistance and capacitance in the LDD region could slightly slow down the switching speed.

So, while LDD is great for addressing hot-carrier effects and enhancing reliability, you’ll want to carefully balance these trade-offs based on the application and performance requirements of your design.