4.5 - 4.5 Modern BJT Structures Modern BJT devices are...

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ECE3080, Chapter 4.5 1 June 27, 2005 ECE 3080: Chapter 4.5 – BJT O. Brand, 1 of 16 4.5 Modern BJT Structures Modern BJT devices are especially developed for high-frequency and high-speed applications . The main goal for new BJT device development is, thus, improvement of the high-frequency and switching behavior. 4.5.1 Polysilicon Emitter BJT 4.5.2 Heterojunction Devices – Heterojunction – 2-D Electron Gas – Si-Ge Base Transistor – Heterojunction Bipolar Transistor Pierret, Chapter 11.3, Pages 426-432 Bhattacharya, Chapter 4.4, Pages 194-202 Neamen, Chapter 9.3 and 10.8, Pages 349-359, 430-435 June 27, 2005 ECE 3080: Chapter 4.5 – BJT O. Brand, 2 of 16 4.5.1 Polysilicon Emitter BJT Assumption for ideal BJT: Emitter width W E » diffusion length L E Reality in modern BJTs: Small emitter width W E « L E in order to improve device speed and reduce parasitic resistance How does a small emitter affects the BJT characteristics? – W E « L E influences the minority carrier distribution in the emitter…. – resulting in an increased minority carrier gradient in the emitter (initial slope ~W E -1 instead of ~L E -1 ) – resulting in an increased (unwanted) B-E junction current (from base to emitter) – resulting in a decreased emitter efficiency resulting in a decreased current gain α 0 and β 0
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ECE3080, Chapter 4.5 2 June 27, 2005 ECE 3080: Chapter 4.5 – BJT O. Brand, 3 of 16 Polysilicon Emitter BJT (cont.) Solution: use emitter consisting of single-crystalline (next to base) and poly-crystalline part polysilicon emitter Polysilicon has lower carrier mobility µ than single-crystalline Si because of increased scattering on grain boundaries µ E,poly < µ E,cryst D E,poly < D E,cryst Continuity of diffusion current at poly-Si/Si interface requires: (b) Poly-Si/Cryst-Si Emitter (a) Single Crystalline Si Emitter
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This note was uploaded on 08/28/2010 for the course ECE 3080 taught by Professor Staff during the Spring '08 term at Georgia Tech.

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4.5 - 4.5 Modern BJT Structures Modern BJT devices are...

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