10.1.5. VLSI Circuit Designer Flexibility

So, here what you see it is that, suppose if we apply the voltage here V and also we apply V keeping body and source they are connected. So, we call this is V and this is DS DS V the current it will be of course, we do have insulator. So, through this terminal there will not be any current, but then there will be a current flow. So, this I it is flowing here. And, of course, this current it is carried by electrons. So, these electrons are really moving from left to right by this field or by this voltage, you may say this is lateral field.

In VLSI circuit design, we start by applying voltages to certain terminals of a device. When a voltage is applied, such as 'V' and 'V_DS', a flow of current (denoted as 'I') occurs due to the movement of electrons. The specific setup ensures that the body and source are connected, enabling effective operation. Electrons move under the influence of the electric field created by the applied voltages. The lateral electric field is crucial as it drives the flow of these electrons across the device.

Imagine a water slide in a water park. When you pour water (representing the voltage) at the top of the slide, it flows down the slide (representing the current), while the children (representing the electrons) move from the top to the bottom. The height of the slide (the electric field created) determines how fast the children can slide down.

So, note that these electrons are coming from this side. So, that is why you call this is source and it is getting drained to this terminal that is why you call drain. Now, it is very clear that why you call this is drain and source. So, this current flow I it is a strong function of this V , V and also it is strong function of the spacing from here to here namely the length of the device. So, it is a strong function of the length it is strong function of the other geometry namely width of the device and also it is strong function of the device parameter, which includes the thickness of this oxide.

The flow of current is not just reliant on the applied voltages; it also depends significantly on the geometry of the device. The source (where the current starts) and the drain (where the current exits) play crucial roles in determining how effectively current can flow. Variables such as the length and width of the device, along with the thickness of the oxide layer, influence the characteristics of the current. Thus, these parameters must be carefully considered during the design process for optimal performance.

Think of a narrow garden hose. The length of the hose represents the length of the device, and if it's long, the water (current) will flow more slowly. If you were to widen the hose, that would be like increasing the width of the device; more water can flow through at once. Similarly, if there are any obstructions (like an oxide layer being too thick), this would restrict the flow further.

So, as a circuit designer what will be looking for if the device it is already fabricated. So, W’s and L’s they are already defined then we will be looking for the dependency of I as function of V and V . And, as a device engineer you may try to change this t you may try to change probably in the surface so that the mobility it will be better and so and so.

Circuit designers often work with existing devices, where dimensions such as width (W) and length (L) have already been established during manufacturing. Their focus is on understanding how changes in the applied voltages (V and V_DS) affect current flow (I). In contrast, device engineers actively modify device parameters, such as thickness and surface properties, to enhance performance characteristics, like electron mobility, which directly influences current flow.

Consider a chef (circuit designer) who has a pre-made dish (device) in front of them. They can adjust the spices (voltages) to affect the flavor (current). On the other hand, the food scientist (device engineer) can change the ingredients (device parameters) to create new recipes that improve nutritional content (device performance).

On the other hand if it is you are a VLSI circuit designer where the device yet to be implemented. However, technology is fixed; that means, these parameters are fixed. So, you may say that whenever we say technology is fixed device parameters are fixed, but then you also have the flexibility to change the W’s and L’s of the devices.

In VLSI design, even though some technology dictates fixed parameters, circuit designers still enjoy a certain degree of flexibility. They can adjust dimensions like width and length to optimize device performance before the physical implementation of the circuit. This ability to fine-tune dimensions allows designers to tailor circuit behavior to specific application needs or performance criteria.

Imagine building a custom pair of shoes (VLSI circuit). You have a fixed type of material (technology) to use, but you can choose the length and width of your shoes (W’s and L’s) based on the fit and style you want. This choice allows you to create something comfortable and functional for your unique needs.

So, for VLSI circuit designer not only you will be playing with the applied voltages, but also the geometry of the transistor.

VLSI circuit designers are not limited to adjusting applied voltages; they also manipulate the physical geometry of the transistors. This aspect of design is crucial because the shape and size of the transistors affect how efficiently current flows through the circuit. By altering these parameters, designers can optimize the performance characteristics of the devices they are creating.

Consider an artist sculpting a statue from clay. While the initial clay block (base technology) is fixed, the artist can shape and carve (change the geometry of the transistor) to create a masterpiece. Each cut and shape determines the final appearance and quality of the statue, just as geometry impacts circuit performance.

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