Semiconductor Devices: Characteristics and Mechanisms

Posted on Feb 16, 2025 in Technology

Formation of Depletion Layer

A depletion layer, also known as a depletion region, is formed in a semiconductor material when charge carriers diffuse away or are forced away by an electric field. This process is caused by the diffusion of holes and electrons across a p-n junction diode:

  1. Diffusion: Holes diffuse from the p-side to the n-side, and electrons diffuse from the n-side to the p-side.
  2. Neutralization: Some charge carriers combine with opposite charges to neutralize each other.
  3. Potential Difference: This creates an excess of positively charged ions in the n-region and an excess of negatively charged ions in the p-region.
  4. Electric Field: This sets up a potential difference, called the barrier potential, and an internal electric field across the junction.
  5. Depletion Layer: The layer on either side of the junction that is free from mobile charge carriers becomes the depletion layer.

Extrinsic Semiconductors

Extrinsic semiconductors are required because they have higher conductivity than intrinsic semiconductors, making them more suitable for use in electronic devices.

  • Explanation: Intrinsic semiconductors are pure materials with low conductivity at room temperature, making them unsuitable for electronic devices. Extrinsic semiconductors are created by adding impurities, called dopants, to improve their conductivity. The process of adding dopants is called doping.
  • Uses: Extrinsic semiconductors are used to make electronic devices such as diodes, transistors, integrated circuits, semiconductor lasers, LEDs, and photovoltaic cells.
  • Doping: The size of the dopant and semiconductor atoms must be the same so that the lattice structure of the semiconductor is not changed.

Differences Between BJT and FET

  • Control: BJTs are current-controlled devices, while FETs are voltage-controlled devices.
  • Charge Carriers: BJTs use both electrons and holes, while FETs use either electrons or holes.
  • Terminals: BJTs have three terminals: base, emitter, and collector, while FETs have three terminals: source, drain, and gate.
  • Type: BJTs are bipolar transistors, while FETs are unipolar transistors.
  • Production: BJTs were the first type of transistor to be mass-produced commercially.
  • Noise: BJTs are less noisy than FETs.
  • Temperature: FETs are more temperature stable than BJTs.

Intrinsic Semiconductors

An intrinsic semiconductor is a pure semiconductor, or one that is free of impurities, where the number of electrons is equal to the number of holes. They are also known as undoped or i-type semiconductors.

Silicon is an example of an intrinsic semiconductor:

  • Structure: A silicon atom has 14 electrons, with 4 valence electrons in the outermost orbital.
  • Covalent Bonds: In a silicon crystal lattice, each line connecting the atoms represents an electron being shared between the two atoms.
  • Electrical Conductivity: Intrinsic semiconductors have low electrical conductivity at room temperature, and their conductivity depends on temperature. At absolute zero, they act as a perfect insulator.

V-I Characteristics of Common Emitter Transistor

The V-I characteristics are broken down into two key regions: Input Characteristics and Output Characteristics.

Input Characteristics

These describe the relationship between the base current (IB) and the base-emitter voltage (VBE) for a constant collector-emitter voltage (VCE).

  • Forward Active Region: In this region, the base-emitter junction is forward biased, meaning that the VBE is typically around 0.6V to 0.7V for silicon transistors. As VBE increases, the base current (IB) increases exponentially.

Output Characteristics

These describe the relationship between the collector current (IC) and the collector-emitter voltage (VCE) for different levels of base current (IB).

  • Cut-off Region: In this region, the base-emitter junction is not forward biased enough to allow current to flow from the base to the emitter.
  • As a result, both IC and IB are approximately zero.
  • Active Region: In this region, the base-emitter junction is forward biased, and the collector-emitter junction is reverse biased. The collector current (IC) is proportional to the base current (IB) and is relatively independent of the collector-emitter voltage (VCE).

Diode Equivalent Circuit Models

Equivalent circuit models of a diode are essential for understanding and analyzing how a diode behaves in different circuits. These models simplify the complex behavior of a diode into manageable components.

Types of Equivalent Circuit Models

Ideal Diode Model

  • Components: A perfect switch.
  • Behavior:
    • Conducts (short circuit) when forward biased (V > 0V).
    • Blocks (open circuit) when reverse biased (V < 0V).
  • Applications: Used in theoretical analysis where exact behavior is less critical.
  • Limitation: Ignores real-world characteristics like threshold voltage and resistance.

Piecewise Linear Model

  • Components: A series combination of:
    • A voltage source (Vf) representing the threshold voltage.
    • A resistor (rd) for the dynamic resistance in forward bias.
  • Behavior:
    • V = Vf + I ⋅ rd in forward bias.
    • Open circuit in reverse bias.
  • Applications: Provides a better approximation for forward-biased operation.
  • Limitation: Less accurate for dynamic or reverse-biased conditions.

Small-Signal Model

  • Components: A resistor (rd) in parallel with a small capacitance (Cd).
  • Behavior:
    • Captures the diode’s behavior for small AC signals superimposed on the DC bias.
    • rd = nVT/ID, where VT is the thermal voltage, n is the ideality factor, and ID is the DC current.
  • Applications: Used in high-frequency and AC signal analysis.
  • Limitation: Applicable only to small signal variations around the operating point.

Detailed Model

  • Components:
    • Ideal diode.
    • Threshold voltage (Vf).
    • Dynamic resistance (rd).
    • Reverse leakage current (Is).
    • Junction capacitance (Cj).
  • Behavior:
    • Provides a comprehensive representation, including reverse bias characteristics and capacitance effects.
  • Applications: Used in precise simulations and high-frequency applications.
  • Limitation: More complex compared to simpler models.

Factors Affecting Built-in Potential

The built-in potential barrier and the depletion layer width of a semiconductor depend on the following factors:

  • Doping Concentrations: The doping concentrations of the p-type and n-type regions.
  • Temperature: The temperature of the semiconductor.
  • Impurities: The number of impurities added to the semiconductor.

Explanation

  • Doping Concentrations: Lower doping concentrations result in larger depletion widths and lower built-in potentials.
  • Temperature: As temperature increases, the density of charge carriers increases. This means that less width is required to store the required charge, so the depletion width decreases.
  • Impurities: The number of impurities added to the semiconductor affects the width of the depletion region.

Diodes in Breakdown Region

Normal diodes are not used in the breakdown region because they can be permanently damaged by the sudden increase in current.

  • Breakdown Region: The region where the reverse current rapidly increases due to a breakdown phenomenon.
  • Avalanche Breakdown: Occurs when a diode is subjected to a high reverse voltage.
  • Damage to Normal Diodes: The sudden surge in current from avalanche breakdown can permanently damage a normal diode.
  • Zener Diodes: Designed to withstand avalanche breakdown and can handle the sudden current spike. They are used in the breakdown region and are considered special-purpose semiconductor diodes.
  • Uses of Zener Diodes: Used as voltage regulators.

They are used in applications that require a stable voltage reference.

Avalanche and Zener Breakdown Mechanisms

Avalanche and Zener breakdown are both types of breakdown that can occur in diodes, but they have different mechanisms and applications:

  • Zener Breakdown: Occurs when a diode is reverse biased, creating a strong electric field that pulls valence electrons into the conduction band. This generates a large number of free charge carriers, resulting in a large current. Zener breakdown is used in Zener diodes for voltage regulation and protection.
  • Avalanche Breakdown: Occurs when a high reverse voltage is applied to a diode, creating an electric field that frees electrons from their covalent bonds. These free electrons collide with other atoms, generating more free electrons, which increases the current. Avalanche breakdown is used in applications that require signal amplification or detection.

Zener Diode as a Voltage Regulator

A Zener diode is designed to operate in the reverse breakdown region, which makes it particularly useful as a voltage regulator.

Working Principle

  1. Reverse Bias Operation: When a Zener diode is reverse-biased, it remains non-conductive until the reverse voltage reaches the Zener breakdown voltage (VZ).
  2. Zener Breakdown: Once the reverse voltage exceeds VZ, the diode enters the breakdown region and starts conducting. The voltage across the diode remains nearly constant at VZ.
  3. Regulation: This constant voltage characteristic makes the Zener diode ideal for maintaining a stable output voltage despite variations in the input voltage or load conditions.

Pinch-Off Voltage

The pinch-off voltage is a critical parameter in field-effect transistors (FETs). It is defined as the drain-to-source voltage (VDS) at which the channel conduction stops or is significantly reduced.

When VDS Exceeds Pinch-off Voltage

  1. Saturation Region: The FET enters the saturation region. The drain current (ID) remains relatively constant.
  2. Pinch-off Point: At VDS = VP, the channel near the drain becomes pinched off.
  3. Constant Current: Further increases in VDS do not significantly affect ID.
  4. Heat Dissipation: Operating beyond the pinch-off voltage can lead to increased power dissipation and heating.

N-Channel Enhancement Type MOSFET

An n-channel enhancement type MOSFET works by creating a conductive channel between the source and drain terminals by applying a positive voltage to the gate. This attracts electrons from the p-type substrate, inverting the region under the gate to form an n-type channel, allowing current to flow when a positive voltage is applied between drain and source. It acts like a switch that turns “on” when the gate voltage reaches a certain threshold level and turns “off” when the gate voltage is below the threshold level.

  • Structure: The device consists of a p-type substrate with n-type source and drain regions, separated by a thin oxide layer with a metal gate on top.

JFET Drain and Transfer Characteristics

Drain Characteristics

  • Definition: The relationship between the drain current (ID) and the drain-source voltage (VDS).
  • Factors: The drain current depends on the gate-to-source voltage (VGS).
  • Characteristics: The drain current is maximum when VGS = 0V and decreases as VGS increases.
  • Pinch-off Voltage: The drain current is zero when VGS = VP.

Transfer Characteristics

  • Definition: The graph between the drain current for zero bias (IDSS) and the gate-to-source voltage (VGS).
  • Information: Shows how the drain current varies with the gate voltage.
  • Control Ability: Larger values indicate higher amplification capabilities.

N-Channel Depletion Type MOSFET

The V-I characteristics of an n-channel depletion-type MOSFET can be explained through its characteristics in different regions:

V-I Characteristics

  1. Cut-off Region: When the gate-source voltage (VGS) is sufficiently negative, the MOSFET is in the cut-off region, and no current flows from drain to source (ID ≈ 0).
  2. Ohmic (Linear) Region: When a small positive gate-source voltage (VGS) is applied, the MOSFET operates in the linear region, where the drain current (ID) increases linearly with an increase in the drain-source voltage (VDS).
  3. Active (Saturation) Region: When VDS is increased beyond a certain point (and VGS is positive), the MOSFET enters the saturation region. Here, the drain current (ID) becomes relatively independent of VDS and is controlled by VGS.
  4. Depletion Region: For negative values of VGS, the channel narrows but does not completely close. The MOSFET can still conduct, but the current is reduced compared to when VGS is zero or positive.