Bipolar Junction Transistor
Transistor Construction
Transistors are used to amplify or switch electronic signals. There are mainly two types of transistors:
- Junction transistor
- Point contact transistor
Junction transistors are often used because of their small size and ruggedness. The junction transistors are again classified into two types:
- NPN transistor
- PNP transistor
Both npn and pnp transistors have three terminals: they are emitter, base and collector.
The transistor can be constructed by using one of the five basic techniques. Based on these techniques, they are classified as:
- Grown junction transistor
- Alloy junction transistor
- Diffusion junction transistor
- Epitaxial junction transistor
- Point contact junction transistor
Grown junction transistor
The grown junction transistor was developed on June 23, 1948, by William Shockley. It was developed six months after the first bipolar point-contact transistor.
The Czochralski technique is used to form the two p-n junctions of a grown junction transistor.
The NPN grown junction transistor is made up of a single crystal of semiconductor material (silicon or germanium). The impurity concentration of this semiconductor material is changed during the crystal drawing operation by adding n-type or p-type atoms as required.
Alloy junction transistor
The alloy transistor or alloy junction transistor is a germanium BJT (bipolar junction transistor) developed at General Electric and RCA in 1951 as an improvement over the earlier grown junction transistor. The alloy junction technique is also called the fused technique.
The alloy junction transistor is made up of a thin wafer of n-type germanium material forming the base, with two dots of indium (acceptor atoms) attached to opposite sides of the n-type material.
The whole structure is raised to a high temperature, above the melting point of indium but below that of germanium.
A tiny portion of indium dissolves and enters into the wafer of n-type material. Thus, the p-type material is created at two sides of the n-type wafer.
The upper p-type material is the emitter, the lower p-type material is the collector, and the center thin n-type material is the base.
The collector (lower p-type material) is made larger than the emitter (upper p-type material) to withstand the heavy current.
Diffusion junction transistor
The diffusion junction transistor was developed by Bell Laboratories in 1954.
The diffusion junction transistor is a transistor which is formed when the n-type silicon wafer called substrate is exposed to p-type and n-type gaseous impurities.
Diffusion is the process by which charged particles flows from a higher concentration region to a lower concentration region. The diffusion junction transistors use this diffusion technique to form the transistor.
In this technique, an n-type substrate is placed in gaseous acceptor impurities and heated. The acceptor impurities diffuse into the n-type substrate (collector) to form a p-type layer (base) on it.
Thus, a p-type layer (base) is created on the n-type layer (collector).
The entire system is exposed to the gaseous donor impurities and again heated. The donor impurities diffuse into the p-type layer (base) to form an n-type layer (emitter) on it.
Thus, an n-type layer (emitter) is created on the p-type layer (base).
At last, a thin layer of silicon dioxide is grown over the entire surface and photoetched, so that aluminum contacts can be made for the emitter and base leads.
Epitaxial junction transistor
The term Epitaxy is a Greek word composed of two parts, namely ‘epi’ which means ‘on’ and ‘taxy’ which means ‘ordered arrangement’. Thus, Epitaxy refers to ordered arrangement on some materials.
In this technique, a very thin layer of p-type semiconductor or n-type semiconductor is grown on a heavily doped substrate of the same material. If the substrate is n-type, a thin n-type semiconductor layer is grown on the substrate. In a similar way, if the substrate is p-type, a thin p-type semiconductor layer is grown on the substrate.
This single n-type or p-type semiconductor layer forms the collector on which the base and emitter regions may be diffused.
The most commonly used epitaxial techniques are grown diffused types, diffused alloy types, and alloy emitter epitaxial types base transistors.
Point contact junction transistor
The point contact transistor was the first type of transistor ever constructed. It was developed by Walter Brattain, John Bardeen, and William Shockley at Bell Laboratories in December 1947.
The point contact transistor consists of a block of Germanium semiconductor, with two very closely spaced gold contacts held against it by a spring. A small strip of gold foil is attached over the point of a plastic triangle.
The germanium material has an excess of electrons. When an electrical signal passes through the gold foil, it injects holes into the n-type germanium. This creates a thin p-type semiconductor layer over the n-type semiconductor layer.
A small current applied to one of the two contacts had an influence on the current flows between the other contact and the base upon which the block of germanium was mounted.
A small change in the first contact current causes a greater change in the second contact current. Thus, it acts as an amplifier.
The first contact is the emitter and the second contact is the collector. The low current input terminal is the emitter while the high current output terminals are the base and collector.
