science

Heterojunction bipolar transistor

Heterojunction bipolar transistor is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Heterojunction bipolar transistor rather than just read about it. In short: A heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) that uses different semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it can handle signals of very high frequencies, up to several hundred GHz.

Key takeaways

Heterojunction bipolar transistor belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Heterojunction bipolar transistor to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Heterojunction bipolar transistor from memory before moving on to harder problems.

Reference excerpt

A heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) that uses different semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it can handle signals of very high frequencies, up to several hundred GHz. It is commonly used in modern ultrafast circuits, mostly radio frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones. The idea of employing a heterojunction is as old as the conventional BJT, dating back to a patent from 1951. Detailed theory of heterojunction bipolar transistor was developed by Herbert Kroemer in 1957.

Materials

The principal difference between the BJT and HBT is in the use of differing semiconductor materials for the emitter-base junction and the base-collector junction, creating a heterojunction. The effect is to limit the injection of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. The efficiency of the heterojunction is measured by the Kroemer factor. Kroemer was awarded a Nobel Prize in 2000 for his work in this field at the University of California, Santa Barbara. Materials used for the substrate include silicon, gallium arsenide, and indium phosphide, while silicon / silicon-germanium alloys, aluminum gallium arsenide / gallium arsenide, and indium phosphide / indium gallium arsenide are used for the epitaxial layers. Wide-bandgap semiconductors such as gallium nitride and indium gallium nitride are especially promising. In SiGe graded heterostructure transistors, the amount of germanium in the base is graded, making the bandgap narrower at the collector than at the emitter. That tapering of the bandgap leads to a field-assisted transport in the base, which speeds transport through the base and increases frequency response.

Fabrication Due to the need to manufacture HBT devices with extremely high-doped thin base layers, molecular beam epitaxy is principally employed. In addition to base, emitter and collector layers, highly doped layers are deposited on either side of collector and emitter to facilitate an ohmic contact, which are placed on the contact layers after exposure by photolithography and etching. The contact layer underneath the collector, named subcollector, is an active part of the transistor. Other techniques are used depending on the material system. IBM and others use ultra-high vacuum chemical vapor deposition (UHVCVD) for SiGe; other techniques used include MOVPE for III-V systems. Normally the epitaxial layers are lattice matched (which restricts the choice of bandgap etc.). If they are near-lattice-matched the device is pseudomorphic, and if the layers are unmatched (often separated by a thin buffer layer) it is metamorphic.

Limits A pseudomorphic heterojunction bipolar transistor developed at the University of Illinois at Urbana-Champaign, built from indium phosphide and indium gallium arsenide and designed with compositionally graded collector, base and emitter, was demonstrated to cut off at a speed of 710 GHz. Besides being record breakers in terms of speed, HBTs made of InP/InGaAs are ideal for monolithic optoelectronic integrated circuits. A PIN-type photo detector is formed by the base-collector-subcollector layers. The bandgap of InGaAs works well for detecting 1550 nm-wavelength infrared laser signals used in optical communication systems. Biasing the HBT to obtain an active device, a photo transistor with high internal gain is obtained. Among other HBT applications are mixed signal circuits such as analog-to-digital and digital-to-analog converters.

See also High-electron-mobility transistor (HEMT) MESFET

References

External links NCSR HBT at the Wayback Machine (archived April 4, 2008) HBT Optoelectronic Circuits developed in the Technion (15Mb, 230p) New Material Structure Produces World's Fastest Transistor 604 GHz Early 2005

Worked examples

Example 1 — a first encounter with Heterojunction bipolar transistor

Start with the simplest possible case. Write down what Heterojunction bipolar transistor claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Heterojunction bipolar transistor before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Heterojunction bipolar transistor ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Heterojunction bipolar transistor

In research: Heterojunction bipolar transistor appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Heterojunction bipolar transistor in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Heterojunction bipolar transistor is common in secondary-school and first-year university syllabi. It links to neighbouring topics Microwave technology, Terahertz technology, Transistor types, so understanding it makes those chapters shorter.
In everyday life: Look for Heterojunction bipolar transistor outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Heterojunction bipolar transistor” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Heterojunction bipolar transistor in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Heterojunction bipolar transistor means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Heterojunction bipolar transistor out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Heterojunction bipolar transistor in simple terms?

Why does Heterojunction bipolar transistor matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Heterojunction bipolar transistor?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Heterojunction bipolar transistor.