Heterojunction bipolar transistor
The Heterojunction Bipolar Transistor (English, HBT or HJBT , dt. »Bipolar transistor with heterojunction« or »heterojunction bipolar transistor«) is a bipolar transistor (BJT), the emitter or collector material of which is chosen differently than the base material. This creates the eponymous heterostructure . It therefore corresponds to the bipolar design of a high electron mobility transistor (HEMT). Due to its heterostructure, the HBT has a very high transit frequency and is therefore mainly used in high-frequency circuits such as transmission amplifiers.
The idea of using different materials in a transistor instead of just one homogeneous semiconductor material goes back to William Shockley from 1951. The theoretical elaboration and functional description of the heterojunction bipolar transistor was developed in 1957 by Herbert Kroemer , who received the Nobel Prize in Physics in 2000 for his work on heterostructures .
Layout and function
The main difference between BJT and HBT is the use of different semiconductor materials for the emitter-base junction and the base-collector junction, which creates two heterojunctions. In the case of an NPN-HBT, this limits the injection of holes from the base into the emitter area, since the potential barrier in the valence band is higher than in the conduction band . This means that significantly higher doping can occur in the area of the base of the HBT than of the BJT, which increases the charge carrier mobility in the base of the HBT and at the same time the gain factor remains roughly the same. This essential property is measured in the field of semiconductor technology in the form of the Kroemer factor .
The semiconductor materials used for the substrate, consisting of the emitter and collector zones, are predominantly silicon , gallium arsenide or indium phosphide . In the epitaxial layer the base zone primarily come alloys such as silicon germanium , and depending on the application, rare different compound semiconductor such as aluminum gallium arsenide , indium gallium arsenide , gallium nitride or indium gallium nitride is used. With the frequently used silicon germanium, the amount of germanium is gradually reduced in the course of the base zone to the collector, whereby the band gap at the collector becomes narrower, as shown in the adjacent diagram, and the transit frequency can be increased further.
With the heterojunction bipolar transistor, switching frequencies of over 600 GHz can be achieved. The record for individual laboratory samples is a transit frequency of 710 GHz, based on indium phosphide in combination with indium gallium arsenide .
This type of transistor has found widespread use, for example, in the high-frequency part of mobile radio devices . Further applications are in the field of optoelectronics in the form of fast phototransistors for the reception of the optical signals in fiber optics . HBTs are usually manufactured using the epitaxial process .
literature
- Patent US4683487 : Heterojunction bipolar transistor. Published on Jul 28, 1987 , Inventors: Kiichi Ueyanagi, Susumu Takahashi, Toshiyuki Usagawa, Yasunari Umemoto, Toshihisa Tsukada.
Individual evidence
- ↑ Patent US2569347A : Circuit element utilizing semiconductive material. Published September 25, 1951 , Applicant: Bell Labs, Inventor: William Shockley.
- ^ Herbert Kroemer: Theory of a Wide-Gap Emitter for Transistors . In: Proceedings of the IRE . tape 45 , no. November 11 , 1957, p. 1535-1537 , doi : 10.1109 / JRPROC.1957.278348 .
- ↑ Didier Decoster, Joseph Harari (ed.): Optoelectronic Sensors . John Wiley & Sons, New York, NY 2013, ISBN 978-1-118-62292-6 .
- ↑ Walid Hafez, William Snodgrass, Milton Feng: 12.5 nm base pseudomorphic heterojunction bipolar transistors achieving f T = 710GHz and f MAX = 340GHz . In: Applied Physics Letters . tape 87 , no. 25 , December 14, 2005, pp. 252109 , doi : 10.1063 / 1.2149510 .