Physical structure of MOSFET

Figure 3: Structure of MOSFET

The Figure 3 shows the structure of a N type MOSFET (NMOS) device. Similarly the structure of a p-type MOSFET can be drawn, just replace the p-type substrate with n - type substrate. The Source and Drain material is now p+ instead of n+, whereas the other structure remains the same. 

Also a thin silicon dioxide material separates the Gate from the Substrate, this is called in general as Oxide.

The components of the n type MOSFET ( P type MOSFET) are:

1. 2 Heavily doped n regions forming the Source and Drain 

2. A heavily doped piece of conductive Poly Silicon generally known as Poly forms the terminal called as GATE

3. The fourth terminal is the “bulk” or “Body”

Note: The device is symmetric with respect to the terminals Source and Drain

The lateral dimension of the Gate along the Source - Drain path is called the Length and the perpendicular drawn to the Length is called as Width (W)

As seen from Figure 3, the channel is the length separating the Source and Drain and is indicated by LDrawn.

But due to the side diffusion under the Gate, the effective length is Leff which is smaller than the drawn length LDrawn by 2LD. Where LD is the diffused doping under the channel.

Leff=LDrawn-2LD

The parameters Leff and tox play a very important role in the MOS technology. 

The term  Leff is termed as simply Length L from now onwards unless stated otherwise.

Note:

The MOS structure is symmetric.

If so, why do we call one n+ regions as Source and the other n+ as Drain?

Here we can define the regions in terms of the bias / voltage applied to them.

If one n+ region is the terminal that provides the charge carriers - electrons in case on N- Channel MOSFET or Holes in case of P- Channel MOSFET it is called Source and the other n+ region that collects them is called the Drain region.

As such the voltage applied to the two n+ regions decide whether it is a Source or Drain terminal and these terminals get exchanged if the applied voltage changes.

As we know MOSFET is a four terminal device and the terminals are:

1. Gate (G)

2. Source (S)

3. Drain (D) and

4. Substrate or Body (B)

For a typical MOSFET operation, the Source / Drain junction diodes must be reverse biased.

The Substrate of the NMOS device must be connected to the most negative supply in the system. If the circuit operates between 0V to 1.2 V, the substrate is connected to 0V, so also the Source terminal. In general the terminals Source and the Body are tied together.

Then how do we take connection from the Substrate or Body?

The figure 4 below shows the structure for substrate connection in N MOSFET.

It may be noted that the all dopants to be negated to obtain P MOSFET and is shown in Figure 5.

Figure 4: The N MOSFET structure showing the Body connection

Figure 5:  The P MOSFET structure showing the Body connection

It may be noted from our discussion so far that the MOSFET is a sandwich-like structure. 

The basic structure is built on wafers made up of highly pure Silicon material.  These wafers are available as thin flat circular wafers of diameter 15-30 cm. 

The MOS structure is created by superimposing several layers of conducting, insulating and transistor forming materials.

This involves several chemical process steps like - Oxidation of the Silicon, diffusion of impurities into the Silicon to give it a specific type of conduction properties, deposit and etching of Aluminum on Silicon to form interconnections.

Two types of MOSFET devices are possible with the above structure:

Case 1: 

If the substrate is of p-type, then diffusion with dopants of n type, two regions of n+ region can be formed (called as Source and Drain)

This will result in an n- type MOSFET

Case 2:

If the substrate is of n-type, then diffusion with dopants of p type, two regions of p+ region can be formed (called as Source and Drain)

This will result in an p - type MOSFET

The MOSFET device consists of 

1. Conducting Gate

2. Insulating layer of Silicon Dioxide

3. Body or Substrate

4. Two regions of heavily doped n+ or p+ regions - Source and Drain

Some critical dimensions of MOSFET:

1. Oxide thickness tOX

2. Width W

3. Gate Length L - distance between the Source and Drain

4. The ratio W/L is an important parameter

It may be noted that these dimensions have been clearly indicated in Figure 3.