Bipolar Junction Transistor


In general, semiconductors are inorganic or organic materials that can control their conduction depending on chemical structure, temperature, illumination, and the presence of dopants. The name semiconductor comes from the fact that these materials have electrical conductivity between a metal, like copper, gold, etc., and an insulator, like glass. They have an energy gap of less than 4eV (about 1eV). In solid-state physics, this energy gap or band gap is an energy range between the valence band and conduction band where electron states are forbidden. In contrast to conductors, semiconductors’ electrons must obtain energy (e.g., from ionizing radiation) to cross the band gap and reach the conduction band. The properties of semiconductors are determined by the energy gap between valence and conduction bands.

Bipolar Junction Transistor

A bipolar junction transistor (BJT) is a three-terminal electronic device that can amplify or switch electronic signals. It is made by joining three layers of semiconductor material together: an n-type layer, a p-type layer, and another n-type layer (for an NPN transistor) or a p-type layer (for a PNP transistor).

The three regions of the BJT are called the emitter, base, and collector. The base is located between the emitter and the collector and is made very thin to allow for easy flow of charge carriers from the emitter to the collector.

The BJT operates by controlling the flow of charge carriers (electrons or holes) from the emitter to the collector using a small current at the base. When a small current is applied to the base, it changes the voltage across the base-emitter junction, which allows a larger current to flow from the emitter to the collector.

BJTs can be used as amplifiers or switches, depending on how they are configured in a circuit. In an amplifier circuit, a small input signal is applied to the base, and the BJT amplifies it to a larger output signal at the collector. In a switch circuit, the BJT is either fully on or fully off, depending on the voltage applied to the base.

BJTs are widely used in electronic applications, such as audio amplifiers, radio receivers, and digital logic circuits. They are preferred in low-voltage, low-power applications, where their high current gain and fast switching speeds make them a popular choice.

Types of Semiconductors

Semiconductors can be classified into two basic types based on their electronic properties:

  1. Intrinsic Semiconductors: These are pure semiconductors that are made up of a single element (e.g., Silicon, Germanium) and have no intentional doping with impurities. Intrinsic semiconductors have a specific number of electrons in their valence band and conduction band. They conduct electricity when they are heated, and some electrons gain sufficient energy to break free from their bonds and become free electrons in the conduction band.
  2. Extrinsic Semiconductors: These are impure semiconductors that are intentionally doped with impurities to change their electronic properties. Extrinsic semiconductors can be further classified into two types:
    1. p-type semiconductors: In p-type semiconductors, impurity atoms such as boron are introduced into the semiconductor material. These impurities have fewer valence electrons than the semiconductor material, which results in “holes” (absence of electrons) being created in the valence band. These holes can conduct current like positive charge carriers, which gives the material its p-type designation.
    2. n-type semiconductors: In n-type semiconductors, impurity atoms such as phosphorus are introduced into the semiconductor material. These impurities have more valence electrons than the semiconductor material, which creates excess electrons in the conduction band. These excess electrons can conduct current like negative charge carriers, which gives the material its n-type designation.

Here is a table with 3 intrinsic semiconductors and 2 p-type and n-type semiconductors, along with 4 key properties:

SemiconductorTypeBand Gap (eV)Electron Mobility (cm²/Vs)Hole Mobility (cm²/Vs)Thermal Conductivity (W/mK)
Silicon (Si)Intrinsic1.121500450150
Germanium (Ge)Intrinsic0.673900190060
Gallium Arsenide (GaAs)Intrinsic1.43850040046
Boron-doped Silicon (p-Si)p-type1.1215001800150
Phosphorus-doped Silicon (n-Si)n-type1.1215004500150
Aluminum-doped Gallium Arsenide (p-GaAs)p-type1.43850020046
Silicon-doped Gallium Arsenide (n-GaAs)n-type1.43850080046

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