10.1.4. Considerations for Circuit and Device Designers

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 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 this section, we learn about how voltage affects the flow of current in a circuit. When voltage (V) is applied to a circuit, it must also account for the configuration of other terminals like body and source. Even though there may be insulation preventing current from flowing through certain paths, current can still flow due to the applied voltage. Specifically, the flow of current is predominantly driven by the movement of electrons which are influenced by an electric field created by the applied voltage.

Imagine a water pipe where water (representing electrons) flows through the pipe due to pressure (which is like voltage). Even if some sections of the pipe are sealed off (like insulators), water can still find its path through open sections, driven by the pressure difference, akin to how voltage encourages electron flow.

So, we can say this vertical field it is getting created by V , which is changing the concentration of the electron on the other hand, the horizontal field getting created by V , which is helping for the movement of the electron from left to right and as a result we do have the I .

Two types of electric fields are mentioned here: a vertical field created by the gate-source voltage (V_GS) which affects the concentration of electrons, and a horizontal field generated by the drain-source voltage (V_DS) which facilitates the movement of electrons across the channel. The interaction of these fields is crucial for current flow (I) within the device, as they influence how many electrons can move and in which direction.

Think of it like navigating through a crowd. The vertical field represents barriers or attractions that can change how many people are in a specific area (electron concentration), while the horizontal field is like paths through the crowd that help people (electrons) move from one place to another efficiently, leading to a steady flow of movement (current).

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.

In electronic circuits, the 'source' is the terminal where electrons originate while the 'drain' is where they exit the device. This nomenclature helps in understanding the direction of current flow: electrons flow from source to drain. Knowing this relationship is critical for circuit analysis and design.

Consider a battery as the source of power. The positive terminal (source) provides electrons that flow through a circuit until they reach the negative terminal (drain) where they complete the circuit, similar to how water flows from a reservoir (source) through pipes to the taps (drain) where it is used.

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. Maybe it is referred as t , then dielectric constant here of this portion it is referred as ε and of course, the mobility of the electrons in the channel region.

Current flow (I) is affected by several geometric and physical parameters: the voltages (V_GS, V_DS), the dimensions of the device (length and width), as well as material properties such as the thickness of the insulating oxide layer (t_ox), the dielectric constant (ε), and the electron mobility within the semiconductor material. These parameters together dictate how effectively the device operates.

Imagine a basketball court where the players (electrons) are influenced by various factors: the size of the court (dimensions), the type of flooring (material properties), and how crowded it is (mobility). If the court is too small or too crowded, players won’t be able to move efficiently, just like how poor parameters can impede current flow.

So as a device engineer of course, so, there are different tricks to improve the device performance, but as a circuit designer we may assume that these parameters are given to us and we may consider they are fixed. 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.

Device engineers have the flexibility to modify certain parameters to enhance device performance, such as changing layer thicknesses or modifying material properties. However, once a device is fabricated, circuit designers often treat these parameters as constants as they have limited ability to modify them. In VLSI design, while some device specifications are predetermined based on technology, designers retain the ability to adjust specific parameters like the width (W) and length (L) of transistors.

Consider a chef (device engineer) who can experiment with each ingredient to create a better dish, changing things like spice levels or cooking time. In contrast, a home cook (circuit designer) working with a pre-packaged meal must follow the instructions (fixed parameters), adjusting only certain aspects like portion sizes (dimensions of transistors) to still achieve a satisfying meal.

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