Physics of Bipolar Transistor

Transcription

1 Physics of Bipolar Transistor Motivations - In many electronic applications, amplifier is the most fundamental building block. Ex Audio amplifier: amplifies electric signal to drive a speaker RF Power amplifier: amplifies high-frequency signal to transmit RF signal for long distance - What is the device requirement for an amplifier? 1. The device needs at least 3 terminals: Vin and Vout with one shared reference 2. The device should act as a dependent source : Vout = f(vin The Transistor is such a device: a key active device in all modern electronics Bipolar Junction Transistor and its operation - Two PN junctions connected back-to-back with thin base Bipolar Junction Transistor (BJT. Called NPN-type. (Will learn PNP later N P N C P N+ B Top-View EN+ C B E N+ 100~200 μm N P N+ Cross Section 7 1

2 - Step-by-step analysis on operation 1. Set VBE>0 and VBC<0 Base-Emitter is forward biased & Base-Collector is reverse biased 2. We want the current I mainly from electrons Dope Emitter heavily (n+. With n+, electron is the dominant carrier. 3. (a: With VBE>0 (=forward bias, large number of electrons diffuses from E to B (and small number of holes diffuse from B to E. We will check this later. From PN junction theory, we know that I E = I S exp ( V BE 4. (b: Because Base is very thin, most of electrons entering B reaches B-C interface. 5. (c: Because Base-Collector is reversed biased, there exists a strong electric field from C to B in the depletion region. The electrons that entered B-C interface are swept by this E-field. Conclusion: Most of the electrons entering Base end up arriving at Collector I C I E - Collector Current: Because I C I E and I E = I S exp ( V BE, we can say that I C = I S exp ( V BE - I S = A 2 E q D n n i : A E = cross-section area of BJT. W B =effective base width N B W B 2

3 - We assumed VBE>0 (Forward and VCE<0 (Reverse Called Forward-Active Mode Main mode of operation in BJT - In forward-active mode, voltage-between two terminals (B & E controls the current through the third terminal (C BJT operates as voltage-controlled current source can build an amplifier because 1 it has 3 terminals and 2 it is a dependent source! - Base and Emitter Currents: - Base current IB has two components I1 : Small number of holes (# of holes in B << # of electrons in E due to n+ doping diffuses by forward-biased B-E junction This hole current I1 scales as I 1 exp ( V BE I2: Small fraction of electrons recombine with holes in B The higher IE is, the higher recombination I2 is I2 also proportional to IE I 2 exp ( V BE - IB is much smaller than IE and IC because of n+/p junction in base-emitter - Common modeling approach: I C = β I B, β 1 (β: current gain I C = β I B, β 1 (β: current gain - Also from KCL, I E = I B + I C - Can also write as: I E = (β + 1 I B or I C = β β+1 I E = α I E, α 1 3

4 BJT Large-Signal Model 1. I C = I S exp ( V BE 2. I C = β I B (or I B = I C β 3. I E = I B + I C I C I C V BE V CE - IC increases exponentially with VBE if VCE constant and Q1 stays in forward-active - IC does not change with VCE if VBE constant and Q1 stays in forward-active 4

5 BJT Small-Signal Model - Transconductance (gm : a metric that quantifies the goodness as voltage-controlled current source. g m = I C V BE = di C = I V BE S exp( dv BE - Small-Signal Modeling: Recall that small-signal model is for the change in V & I - With 3 terminals, we should consider VBE and VCE and how they impact currents = I C 1. VBE: IC will increase and so will IB because I B = I C β Suppose when VBE=VBE0, I C = I C0 = I S exp ( V BE0 Now, VBE=VBE0+ VBE, then I C + I C = I S exp ( V BE0+ V BE = I S exp ( V BE0 exp ( V BE I C0 (1 + V BE = I V C0 + g m V BE T I C = g m V BE Also, recall that I B = I C β I B = I C β = g m β V BE = V BE r π, where r π = β g m - Small-Signal Model of NPN BJT for VBE: B C E 5

6 2. VCE: What we have learned so far I C = I S exp ( V BE - Ideally, IC is a function of only VBE: VCE change no change on IC and hence IB! - In reality, VCE does change IC due to Early Effect - Early Effect: When VCE increases C/B reverse bias depletion region width Effective base width WB - Recall that I C = I S exp ( V BE, where I V S = A 2 E q D n n i IS IC T VCE>0 IC>0 N B W B - To include this effect in large-signal model, we add another factor in the model: 6

7 I C = I S exp ( V BE (1 + V CE V A, VA : Early Voltage - Including Early Effect in small-signal model: di C dv CE = I S V A exp ( V BE I C V A VCE= V CE = r o I C, where r o = V A I C 3. Complete Large-Signal and Small-Signal Model of NPN BJT - Large-Signal Model B C E - Small-Signal Model B C E 7

8 Other modes of operation: Saturation and Cutoff - If VBE>0 but C/B junction is also forward biased Saturation mode - Normally, we don t want the BJT to operate in this mode - If B/C is forward-based, there is extra current path from Base to Collector due to forward-based PN junction b/c B and C. (DBC in the figure IC - If VBE<0 cutoff mode : IC=0 Example 1: (a Verify that Q1 is in forward-active mode (b Find how much Vout will change when V1 changes by +1mV V out 1.8V V 1 0.8V + - Q 1 I S = β=100 V A = T=300 8

9 PNP Bipolar Junction Transistor - Opposite polarity of E, B, and C from NPN, but same principle of operation Emitter Base Collector Majority Carrier Forward Active Mode NPN PNP - Circuit Symbol Comparison: NPN vs. PNP vs NPN PNP - Large-Signal Model of PNP BJT - i I C = I S exp ( V EB (1 + V EC V A ii I C = β I B (or I B = I C iii I E = I B + I C β - Small-Signal Model of PNP BJT : Exactly same as that of NPN. Even the direction of small-signal current is the same at all terminals 9

10 Example 2: When Vin=0 5mV, how will Vout change? 10