Transistors. Bipolar Junction transistors Principle of operation Characteristics. Field effect transistors Principle of operation Characteristics

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1 Transistors ipolar Junction transistors Principle of operation haracteristics Field effect transistors Principle of operation haracteristics

2 ntroduction Radio based on vacuum tubes Fundamental building block of electronics in computers, cellular phone, and more Semi-conductor device Use small voltage or current to control large voltage/current Fast response transistor used in many elementary electronic functions including: Amplification, Switch, Feedback system, regulation, Signal modulation, Oscillators. ntegrated circuit contains thousands of transistor in very small areas Nobel price was awarded to William radford Shockley, John ardeen and Walter Houser rattain for their researches on semiconductors and their discovery of the transistor effect P. Piot, PHYS 375 Spring st pocket radio based on transistors

3 Transistor types Two types of transistor (based on two different physical mechanisms: Field effect transistor ipolar Junction transistors. To 1 st order they act as current source FET ~ voltage-controlled current source JT ~ current-controlled current source N = A OUT N = G OUT V N V N urrent source controlled by a current A = current gain urrent source controlled by a voltage G = transconductance.

4 Transistors Transistor PNP Transistor NPN diode «E» E E diode «E» oupling between diodes diode P + N P emitter base collector N + P N diode Two coupled PN junctions (or diodes) «transistor effect»

5 ipolar Junction Transistor (JT) Going back to the p-n junction e- Electrons moved into the p-type semiconductor Locally (at the junction interface) there is a recombination hole-electron This leave positive ions in the n-type semiconductor and negative ions in the p- type semiconductor e+

6 JT: N-P-N transistor n each of the N-layers, conduction can take place (motion of free electrons in conduction band) n the P-type layer, conduction can take place (movement of free holes in valence band) n absence of externally applied E-field, depletion zones form at both P-N junction, so no charge move from on layer to the other.

7 JT: N-P-N transistor Now voltage is applied between collector and base parts of the transistor, with polarity such to increase force pulling N type electron and P-type holes apart Effect is to widen the depletion zone between ollector and ased No current flow base-collector diode junction is reversed biased.

8 JT: N-P-N transistor Now relatively small voltage is across to the emitter-based junction such to forward-biased the junction Electron from emitter flow toward the based current flow across emitter/base junction. Elkectron in the experience attractive force from positively biased collector Emitter/ollector current with magnitude depending on Emitter-base voltage

9 urrent flow in a bipolar junction Kirchoff current law imposes Let s define the parameter and the current gain We have α T is the common base forward short circuit current gain β F is the forward common emitter current gain (20 to 50) An ideal junction would have α T =1, real transistors have 0.95<α Τ <0.99, a value close to unity for thin or weakly doped bases For a NPN JT, V >V E while V <V E for a PNP

10 Operating mode for a NPN transistor Active mode : V E 0. 7V ~0.3 V < V E < V c β F ut off mode: 0 V V 0 E Saturated mode : V E 0. 8V V E 0. 2V c β F ~0.7V h FE ~0.8V ~0.2V E E E E Active mode ut off mode Saturated mode V = voltage source for and E. V E n<vcc!

11 Operating mode for a PNP transistor Active mode V E 0. 7V ~ 0.3V < V E < V ( < 0) c β T ut off mode 0 V 0 E V Sat, mode V E 0. 8V V E 0. 2V c β T E ~0.7V E h FE E ~0.8V E ~0.2V Active mode ut off mode Saturated mode

12 haracteristics of a bipolar junction Parameters choices The various operating currents and voltages ( E,, V E,V E, ) of a transistor are related to each other V E So different equivalent characteristics exist. For common base configuration, V EE R E V E E V R V characteristics : E (V E,V ), (V, E ) For common emitter configuration, characteristics : (V E, V E ), (V E, )

13 haracteristics E (V E, V ) : E (ma) V =0, ~ characteristics for a PN junction V exp E E s 1 VT 1! Small influence of (resp. V ) V E (V) (V, E ) : c (ma) E (ma) V E P. Piot, PHYS 375 Spring V (V) E

14 Field Effect transistor (FET) Three terminals : S, D et G, (sometime four: substrat) V GS A current ( D ) can flow from source S to drain D via a channel (area located close to the gate): S G D substrat (Si) V DS D channel The current flowing though the gate ( G ) is small. => S = D! D, at constant V DS is controlled by the gate voltage source (V GS ) electric field effect n-channel FET : current induced by electrons, from S to D p-channel FET: current carried by holes, from S to D

15 Field Effect transistor (FET) N-type channel P-type channel

16 ( ) Typical D V DS characteristics V GS Pinch-off ohmic breakdown

17 Typical characteristics D (ma) V DS sat = VGS + VP V GS =0 transistor Reverse biased V DS > V DSsat DSS V GS =-1V V GS (V) V GSoff V P V DS (V) Pinch-off regime for V DS > V DSsat : D V 1 GS DSS V GS off 2 = k ( V V ) 2 GS GSoff k = DSS 2 VGS off Linear (Ohmic) regime for V DS < V DSsat : D 2k V ( ) DS VGS VGS VDS off 2

18 Differences between FET and JT G << very high input impedance (sometime > Ω) Simpler circuits linear regime slope = f(v GS ) variable resistance (nothing equivalent for JT) V DSsat > V Esat : higher residual voltage in saturated regime. Saturation regime (active mode) D is controlled by a voltage d transconductance g m = d (instead of β F ) dv From manufacturing higher dispersion in g m value compared to β F gs Different characteristics in active mode: JT: with V E constant, = or =α E FET: with V DS constant, D = f(v GS ) = nonlinear relationship depends on considered FET types.

19 Differences between FET and JT FET JT

20 Load line to find operating point of a transistor The operating point of a transistor is determined by its characteristics and by Kirchhoff s law applied to the considered circuit. Example : How to find,, V E, V E? +V Load line R c V th = Rth + VE Vth V = E Rth R th V th V = R + VE V V = E R

21 transistor V = th V R th E V EQ V, dès que V th > 0.7V (transistor in active or saturated mode) Q Q V EQ V E (V) c (ma) VEsat VEQ V Q V Esat Q V EQ V = Q VE R O V E (V) O c V V R Esat c V R Q is the operating point of the transistor c