Transistor (BJT)

Introduction BJT (Bipolar Junction Transistor) Vaccum tubes It comes because it is most advantageous in amplification Why it is called transistor? Transistor = Transfer + Resistor Why it is called BJT? Types of BJT

Introduction(cont.) npnpnp npn B C pnp E B C Cross Section B C E Schematic Symbol B C E Collector doping is usually ~ 10 9Collector doping is usually ~ 10 9 Base doping is slightly higher ~ – 10 11Base doping is slightly higher ~ – Emitter doping is much higher ~ 10 17Emitter doping is much higher ~ 10 17

Junction Transistor Sandwich structure. Base is always in between E & C. B is lightly doped. E & C are heavily doped. E is more heavily doped than C and area of C is more than E. Two PN junctions. E B C BE CB E B C BE CB N-P-N P-N-P

Unbiased transistor No external supply is applied. Penetration of depletion region (less in E & C, more in B)

Transistor Biasing ModeBE junctionBC junction cutoffreverse biased reverse biased linear(active)forward biased reverse biased saturation forward biased forward biased Inverse active reverse biased forward biased

Transistor Biasing (cont.) Transistor biasing in active region. EB junction is forward biased and CB junction is reversed biased.

Transistor operation in active region (NPN) E B C N P N v

9 Large current

Transistor operation (cont.) Electrons will flow from E to B. Now electrons have three options 1.Recombine with holes (I B ) 2.Diffuse through base and out of the base connection. 3.Remaining e- will go in C (I c ).

Transistor operation (cont.) E B C N P N Electrons emitted Electrons collected Recombination current Emitter current Collector current I E =I B +I C

Transistor current Emitter current, Base current, Collector current. I E = I B + I C. (I E ≈ I C ) I E = I PE + I NE (for NPN I NE for PNP I PE ). I B = I PE - I PC. Reverse saturation current (I CBO ) : It is the reverse sat. current when EB junction is open. I C = I PC + I CBO.

Parameters relating to current components

Transistor as an amplifier Discussion of an amplification effect With E.g. for common-base configuration transistor:

Transistor construction technologies Grown type. Alloy type. Electrochemically etched type. Diffusion type Epitaxial type.

Transistor configuration Made one of three terminal common to i/p and o/p. Depending on which terminal is made common. There are three possibilities 1. Common base configuration (CB). 2. Common emitter configuration (CE). 3. Common collector configuration (CC).

Common Base configuration (CB) V ee V cc ReRe RcRc IEIE IBIB ICIC V ee V cc ReRe RcRc IEIE IBIB ICIC Common base configuration for NPN transistor Common base configuration for PNP transistor

Common Base configuration (CB) Input Output Current relations in CB configuration 1. I c = I C(INJ) + I CBO 2. I C(INJ) (practically) 3. I CBO (with emitter open) I CB= collector to base current I O = emitter is open

Common Base configuration (CB) 4. current amplification factor/current gain (α dc ) α dc =I C(inj) /I E So I c = (α dc * I E ) + I CBO E xpression for I B : I B = (1- α)I E.

Transistor char in CB configuration 1.Input char.2. Output char.3. Transfer char. I/P char : graph of I/P current versus I/P voltage. O/P char : graph of O/P current versus O/P voltage. Transfer char: graph of O/P current versus I/P current

CB I/P char. I/P current is emitter current(I E ) and I/P voltage is emitter to base voltage(V BE ). 1. Its identical to VI char of diode in FB. 2. Up to cut-in V 3. I/P resistance 4. Effect of V CB on I/P VI char (Early effect) V CB = 4V V CB = 8V I E (mA) V BE

Early effect / Base width modulation Effect on β and α. E BC E B C Increase V CB Total base width = width of depletion region at CB junction + width of region which contains free charge carriers

Output char of transistor in CB ICICICIC Active Region Saturation Region Cutoff Region I B = 0 IEIEIEIE V CB Operating region 1.Cutoff region 2.Active region 3.Saturation region

Output char of transistor in CB Cut off region : region below the curve I E =0 Active region : I C ≈ I E (Const. current source) Dynamic O/P resistance Saturation region Current controlled current source.

Breakdown voltage and punch-through effect Increasing V CB causes CB junction to breakdown. Reach through / Punch through effect V CB ICIC E BC

Potential variation through transistor Without biasing With external bias

Transfer characteristic Linear rela.tionship ICIC IEIE

Common Emitter (CE) configuration V cc V BE V CE RBRB RCRC V BB V BE V CE RCRC RBRB Common emitter config. for NPN transistor Common emitter config. for PNP transistor

CE Configuration Input Output Current relation I E = I B + I C I C = α * I E + I CBO I C = I B (α/1- α) + I CBO / (1- α) But (α/1- α) = β So I C = I B * β + (β+1) I CBO I C = I B * β + I CEO

CE Configuration Reverse leakage current in CE configuration (I CEO ) Thermal instability, so thermal stabilizing circuit is required. Relation between α and β. α = β/(1+ β) and β = α /(1- α)

CE characteristics (Input) Same as conventional PN junction diode Dynamic i/p resistance Base current reduces as V CE increases. V CE = 8V V CE = 4V I B (μA) V BE

CE characteristics (output) 1. Cut off region 2. Active region 3. Saturation region 4.Dynamic O/P resistance 5. Definition of β 6. Maximum VCE and breakdown

Transfer characteristic Why CE o/p char is more sloping than CB o/p char??? ICIC IBIB V CE = 2V V CE = 5V

Typical transistor junction voltage values Cutoff region Short-circuited base Open-circuited base Cut-in voltage Saturation voltage VoltageSi transistorGe transistor V BE (Cutoff) V BE (Cut in) V BE (active) V BE (sat.) V CE (sat.)0.20.1

Standard test for regions Saturation region 1. find I C, I B then check I B >= I C / β 2. measure V CB, positive for PNP and negative for NPN. Active region measure V BE = 0.7 and measure V CB is negative (reverse biased) Cutoff V BE is < 0.5 and V CB is negative

Common collector configuration V cc V BC V EC RBRB RERE V BB V BC V ECs RERE RBRB

Practical way to draw CC config. V CC O/P voltage I/P voltage

Current relation I E = I B + I C I C = α * I E + I CBO I E = (β+1) * I B Current gain γ = I E /I B Maximum use of CC is for impedance matching (I/P is high and O/P is low).

I/P char. And O/P char. V EC = 2V V EC = 1V I B (μA) V BC IEIEIEIE Active Region Saturation Region Cutoff Region I B = 0 IBIBIBIB V EC

Transfer char. IE IBIB V CE = 2V V CE = 5V

Comparison of configurations parameterCBCECC Common terminal between i/p and o/p BaseEmitterCollector I/P currentIEIE IBIB IBIB O/P currentICIC ICIC IEIE Current gainα = I C /I E β= I C /I B Γ= I E /I B I/P voltageV EB V BE V BC O/P voltageV CB V CE V EC

Analytic expression for transistor char.

Base spreading resistance E B C V CE V EB V CB V CB = V C + r bb * I B VEVE VCVC

Ebers – moll model VEVE VCVC IEIE IBIB ICIC αI ICαI IC αN IEαN IE

Ebers – moll model (cont.) I = I C + α N I E I C = - α N I E + I (I is diode current) VCVC αN IEαN IE ICIC I

Why can’t we construct a transistor by connecting back to back diodes? VEVE VCVC

Photo transistor Dark current 20 mW/(cm* cm) 40 mW/(cm* cm) 60 mW/(cm* cm) 80 mW/(cm* cm)

Photo transistor (cont.) Advantages: Photo current multiplied by β High sensitivity Good switching speed No memory effect.

Phototransistor (cont.) Disadvantages Not so fast as conventional transistor because of photo- conducting material Poor linearity Temperature sensitive device External voltage source is needed for operation