EE105 Fall 2014 Microelectronic Devices and Circuits. NPN Bipolar Junction Transistor (BJT)

Transcription

1 EE105 Fall 2014 Microelectronic Devices and Circuits Prof. Ming C. Wu 511 utardja Dai Hall (DH) 1 NPN Bipolar Junction Transistor (BJT) Forward Bias Reverse Bias Hole Flow Electron Flow N P N Emitter (E) Base (B) Collector (C) 3 layers of semiconductors Emitter (N-type), Base (P-type), Collector (N-type) B-E junction forward-biased B-C junction reverse-biased Two important criteria Emitter more heavily doped than base Much more electrons injected into base than holes into emitter Base very thin Most electrons travel through base and reach collector Note: BJT is NOT 2 back-to-back junction diodes. Current gain mall base current controls large collector current 2 1

2 Physical Construction of BJT The transistor is usually planar All contacts are on the front side Emitter and base are created by ion implantation To facilitate electrons travel to collector contact, an n+ buried layer is usually employed to reduce collector resistance. Typical doping N E > cm -3 N B ~ cm -3 N C ~ cm -3 3 npn Transistor Forward Characteristics Collector current: i C ) " V T %, V T kt q (26mV at room temp) I : BJT saturation current (note: different from pn junction's I ) Base current: i B Emitter current:! i E i B + i C 1 $ # +1&i C " % : current gain (typical value: ~ 100) : typical value 0.95 to

3 npn Transistor Reverse Characteristics (Low Gain, Used Rarely in This Mode) β R is the reverse common-emitter current gain. β R is typically small, even smaller than 1, because the doping profile is optimized for forward active gain. Collector current i C is given by Emitter current i E is ) i E i R exp v + % ( 1. + % (. i B i R β R +,, β +, R # & V $ T ' # exp v % ( 1 / % ( / & V $ T ', -. Base current i B is given by -./ i C i B i E α R α R is the reverse common-base current gain α R $ exp v ' -, & ) 1/ + % V T (. β R β R +1 5 npn Transistor Complete Transport Model - Valid for Any Bias i C exp v $! # & # &- I ) " V T % " V T %, β R ) " V T %, i E exp v $! # & # &-+ I ) " V T % " V T %, ) " V T %, i B I ) " V T %, β R ) " V T %, First term in both emitter and collector current expressions gives current transported completely across base region. ymmetry exists between base-emitter and base-collector voltages in establishing dominant current in bipolar transistor. 6 3

4 pnp Transistor tructure Voltages v EB and v CB are positive when they forward bias their respective pn junctions. Collector current and base current exit transistor terminals and emitter current enters the device. 7 pnp Transistor Forward Characteristics Collector current i C equals the forward transport current is i C i F EB ) " V T %, Base current i B is given by i B Emitter current i E is given by i E + i B 8 4

5 Transport Model Circuit Representations In npn transistor (expressions are analogous for pnp transistors), the total current traversing base is modeled by a current source given by: ( " i T i F i R exp v % " $ ' exp v % + $ '- ) # V T & # V T &, Diode currents correspond directly to the two components of base current. i B I I ) " V T %, β R ) " V T %, 9 i-v Characteristics Collect current increases exponentially with V Flat part of the i-v curves is called forward active region. The non-flat part is called saturation region Please note aturation in BJT and MOFET refers to opposite regions aturation Forward Active Collect current increases linearly with I B 10 5

6 Operation Regions of Bipolar Transistors Base-Emitter Junction Base-Collector Junction Reverse Bias Forward Bias Forward Bias Reverse Bias Forward-Active Region (Good Amplifier) Cutoff Region (Open witch) aturation Region (Closed witch) Reverse-Active Region (Poor Amplifier) Binary Logic tates 11 Example 1 Problem: Estimate transistor terminal currents and base-emitter voltage Given data: I A, 0.95, V V B - V C -5 V, I E 100 µa Assumption: BJT in forward-active Analysis: I C I E 0.95( 100µA) 95 µa I B I C 95µA 19 5 µa " V V T ln I % C $ ' V # & I 12 6

7 Example 2 Problem: Estimate terminal currents and voltages Given data: I A, 0.95, V C +5 V, I B 100 µa Assumptions: BJT in forward active Analysis: ( ) 1.90 ma ( ) 2.00 ma I C I B µA I E ( +1)I B µA " V V T ln I % " C $ ' 0.025V ln 1.9mA % $ ' V # & # 0.1fA & I V V B V C V V C V 13 implified Circuit Model Forward-Active Region Current in base-emitter diode is amplified by common-emitter current gain and appears at collector; base and collector currents are exponentially related to base-emitter voltage. Base-emitter diode is replaced by constant voltage drop model (V 0.7 V) since it is forward-biased in forward-active region. dc base and emitter voltages differ by 0.7-V diode voltage drop in forward-active region. 14 7

8 implified Forward-Active Region Model Example 3 Problem: Find transistor Q-point Given data: 50, β R 1 Assumptions: Forward-active region of operation, V 0.7 V Analysis: V +8200I E V EE 0 I E V 1.01 ma 8200 Ω I B I E mA 19.8 µa 51 I C I B µA V CE V CC I C R C V V CE mA 4.3K ( ) ma ( ) ( ) V Forward-active region is correct. 15 implified Circuit Model aturation Region In the saturation region, both junctions are forward-biased, and the transistor operates with a small voltage between collector and emitter. v CEAT is the saturation voltage for the npn BJT.! I C exp V $ # & I! exp V $ # & I B I! exp V $ # &+ I! exp V $ # & " V T % α R " V T % " V T % β R " V T % ( I 1+ C +! 1 $ β V CEAT V V V T ln R +1 - ( )I B # & " α R % 1 I - for I B I C C - ) I B, - implified Model No simplified expressions exist for terminal currents other than i C + i B i E. 16 8