Introduction to Diode: What is Diode ? V-I characteristics of the Diode Explained

What is a Diode and Its V-I Characteristics?

Introduction to Diodes

• The video series focuses on understanding diodes, starting with their definition and V-I characteristics.

• A diode is defined as a two-terminal semiconductor device that allows current flow in one direction only, unlike resistors which allow bidirectional flow.

Structure of a Diode

• The diode consists of an anode (positive terminal) and a cathode (negative terminal), determining the direction of current flow based on voltage polarity.

• Current flows when positive voltage is applied to the anode relative to the cathode; negative voltage prevents current flow.

V-I Characteristics of Diodes

• Unlike resistors, diodes exhibit non-linear characteristics; their V-I relationship cannot be described by Ohm's law.

• The graph representing diode characteristics appears symmetrical but has different scales for positive and negative axes, leading to potential confusion.

Analyzing Circuits with Diodes

• In reverse bias, the current through the diode is negligible. Understanding this characteristic helps analyze circuits containing diodes effectively.

• Due to non-linear behavior, approximating diode characteristics can simplify circuit analysis.

Ideal Diode Model

• An ideal diode acts as a closed switch when forward biased (positive voltage across it), and as an open switch when reverse biased (negative voltage).

• The V-I characteristic for an ideal diode shows vertical lines for forward bias and horizontal lines for reverse bias.

Practical Example with Ideal Diode

• When connected in series with a resistor and voltage source, if 10V is applied across the ideal diode, it behaves like a closed switch allowing 0.1A through a 100-ohm resistor.

Understanding Diode Behavior and Thevenin's Equivalent

Thevenin's Equivalent Voltage

• To analyze the circuit, remove the diode and find the Thevenin's equivalent voltage between the terminals. In this case, it is determined to be -10V.

Diode Conductivity

• A diode will not conduct if the voltage across it is negative (e.g., -10V). Conversely, if 10V appears across its terminals, it behaves like a closed switch.

Ideal vs Actual Diodes

• An ideal diode starts conducting with any positive voltage (e.g., 0.1V), while an actual diode requires a threshold or cut-in voltage to begin conduction. This threshold for silicon diodes is typically between 0.6V to 0.7V, and for germanium diodes around 0.3V.

Current Flow in Circuits

• When using a silicon diode with a threshold of 0.7V and applying 10V, current flows through a connected resistor (100 ohms) calculated as (10 - 0.7)/100 = 0.093A . If reverse bias (-10V) is applied, no current flows (0A).

Resistance Characteristics of Diodes

• Initially considered as having zero resistance post-threshold; however, all devices have finite resistance that limits current flow once above this threshold voltage. This leads to two approximations: one considering zero resistance and another accounting for bulk resistance after crossing the threshold voltage at which point it offers finite resistance known as body or bulk resistance.

Bulk Resistance Impact on Current Flow

Understanding Diode Characteristics and Behavior

Current Flow and Resistance in Diodes

• The current flowing through a 100-ohm resistor is influenced by diode resistance, which can be approximated to find more accurate values of current and voltage in the circuit.

• In many circuits, the Thevenin's equivalent resistance across the diode is significantly larger than the diode resistance, allowing us to neglect the latter for simplification.

Piecewise Linear Characteristics of Diodes

• Diode characteristics can be segmented into piecewise linear segments; it remains non-conducting until the applied voltage exceeds a threshold voltage. After this point, it offers finite resistance.

• The forward region of operation begins once the applied voltage surpasses the threshold voltage, leading to significant conduction through the diode. Conversely, below this threshold, current flow is negligible.

Reverse Region and Breakdown Voltage

• When reverse voltage is applied to a diode, it operates in what is known as the reverse region where current flow is minimal (typically micro-amperes), referred to as reverse saturation current.

• Increasing reverse voltage marginally raises current until reaching breakdown; exceeding this limit leads to undesirable operation unless using specialized diodes like Zener diodes designed for breakdown conditions.

Forward Region Characteristics

• In forward bias conditions, as voltage increases across a diode, current rises exponentially; thus it's crucial that this does not exceed maximum allowable forward current specified in datasheets.