Bipolar Junction Transistor (BJT)

Transcription

1 Bipolar Junction Transistor (BJT) - three terminal device - output port controlled by current flow into input port Structure - three layer sandwich of n-type and p-type material - npn and pnp transistors for npn - inner n-type region => emitter (E) - outer n-type region => collector (C) - central p-type region => base (B) for pnp => reverse the region type

2 Structure of BJT (con t) NPN PNP

3 Figure 5.25 Physical structure of NPN bipolar junction transistor *cross-section view of BJT

4 * top view of BJT

5 - physical operation similar to junction diode - normal operation base-emitter => forward biased collector-base => reverse biased - in npn BJT - forward biased B-E causes electrons to be injected from emitter to base - because base is thin, electrons travel completely across base and arrive at reversed biased B-C junction - upon arrival at B-C junction, electrons are pulled across B-C depletion region and flow through collector region and out collector contact - forward biased B-E also causes holes to be injected from base to emitter. These holes become part of emitter current but not part of collector current

6 - emitter current similar to diode current I E = I EO (e V BE /ηv T - 1) I EO = emitter saturation current - electron current at collector is a fraction (α F ) of the total emitter current I C = α F I E And since I E = I C + I B I B = (1 - α F )I E (KCL) Furthermore I I C B = α I F F (1 - α )I F E α F = = (1 - α ) F β F - α F and β F very dependent on doping levels and device geometry typical values β F => 10 to 1000 α F => 0.9 to 0.999

7 BJT Input Port - Base Emitter Output Port - Collector Emitter Figure 5.26 Circuit symbol and voltage-current characteristics of an idealized NPN bipolar junction transistor - collector current I C is plotted as a function of collector - emitter voltage V CE. - I B is a controlling variable which gives a family of curves - base current given by I B = I BO (e V BE /ηv T - 1) - base - emitter has turn-on voltage V F which is V for silicon

8 Regions of Operation Cutoff - V BE < 0.6V - I B ~ 0 - I C ~ 0 Constant Current Flat portion of curves. I C is controlled by base current I B I C = β F I B sometimes referred to as active region Saturation Region - if V CE becomes small - base - collector no longer reverse biased and electrons no longer collected at B - C junction - collector current falls towards 0

9 Find operating point for V BB = 1V pnp BJT - three layer sandwich of p-type, n-type and p-type material - identical function as npn - holes are primarily current carrier - V BE must be negative to turn on (i.e.: forward bias B-E junction

10 Constant Current Regions of BJT's and FET s - horizontal constant current lines only true for ideal devices - actual devices have slight upward slope - slope is due to depletion width modulation in BJT - slope is due to channel length modulation in FET - for BJT slope typically ma/v slope increases with increasing I B BJT FET Figure 5.31 Upward slope of BJT and FET v-i characteristics in the constant current region

11 - if constant current lines are extrapolated into negative quadrant they all cross the V CE axis at a common point called the early voltage - early voltage is typically V and is a positive number. The symbol for the early voltage is V A *see next slide for diagram BJT => slope = di dv C CE = I (0) C V I = I (0) + di C C dv A C CE V = CE β F V I A B = 1 r O Similarly for FET = I (0) (1 + V C V CE A = I (1 + V CE ) β F B ) V A I = k(v - V ) (1 + V DS GS TR V 2 GS A )

12 Figure 5.32 Extrapolation of constant current (active) region of BJT v-i characteristics into the negative quadrant. Lines converge at -V A, where V A is the Early voltage

13 Phototransistor - similar in structure to BJT. Made of npn sandwich of semiconductors - base region exposed to external light via transparent window in transistor package - window often covered by a small lens that concentrates on base-emitter junction Figure 5.34 Schematic representation of an NPN phototransistor - focused light causes excess hole - electron pairs in the base - emitter depletion region - excess electrons are injected into the base region and collected at the B-C junction - the number of injected electrons is proportional to the light intensity - the output port is identical to a BJT

14 Phototransistor (con t) Figure 5.35 Circuit symbol and v-i characteristic of the phototransistor In constant current region, ratio of collector current to light intensity is called photoconductance β I I C = β I λ I Light intensity λ I is expressed in lumens (lm) - in some phototransistors a base terminal is also provided so that the transistor can be operated with a combination of incident light and base current - similar type of device is photodiode which requires more surrounding circuitry but usually operates faster

15 Phototransistor (con t) Figure 5.36 Fiber optic cable and light source drive a phototransistor circuit

16 Phototransistor Select R L to maximize the amplitude of the decoded signal β I = 0.2 ma/lm, λ I = 40 lm

17 Vacuum tube triode - first three terminal device - largely surpassed by transistor - still used in high power radio frequency (RF) applications - handle large voltages (Kvolts) and power (Kwatts) Temperature Dependence of Devices - all electronic devices are sensitive to changes in temperature - causes significant changes in V-I characteristics or variations in device parameters - most temperature dependencies in semiconductors are related to two effects: a) changes in the number of thermally generated hole-electron pairs b) changes in the thermal voltages V T which is a result of energy changes required to move electrons/holes