Bipolar Transistors deal Transistor Bipolar Transistor Terminals Reading: (45 th edition) 816, 2633 P Bipolar Transistor Physics Large Signal Model Early Effect Small Signal Model Modern Electronics: F3 Bipolar transistor 1
deal Transistor characteristics out current source controlled by a voltage (V in ) in out ncreasing V in V in kv in V out Two main transistor types: Bipolar Transistor (Power Amplifiers, HighSpeed AD/DA) V out Field Effect Transistors (99.999% mainly integrated circuits / digital) out independent of V out! in independent of V out! Modern Electronics: F3 Bipolar transistor 2
Bipolar Transistor npn ollector Sign convention: V xy =V x V y V B Base B V E B base current collector current E Emitter urrent = ( B ) V BE E Emitter Modern Electronics: F3 Bipolar transistor 3
Bipolar Transistor npn in active mode Emitter P Base ollector npn V BE >0 Forward biased P junction V B <0 Reverse biased P junction Modern Electronics: F3 Bipolar transistor 4
Bipolar transistor: band structure n=n po exp(v BE /kt) n 0 E B P E Fn ev BE B Forward biased emitterbase junction injects electrons Reverse biased basecollector junction sweeps away electrons (independent of V B ) Modern Electronics: F3 Bipolar transistor 5
Operating modes Unbiased P utoff EBJ: reverse / BJ: reverse emitter base collector Active EBJ: forward / BJ: reverse Saturation EBJ: forward / BJ: forward Modern Electronics: F3 Bipolar transistor 6
Electron diffusion currents active mode ncreasing V BE ncreasing V E V E Modern Electronics: F3 Bipolar transistor 7
urrents in active mode E B1 B2 = 0 E Emitter P Base ollector B2 B1 B V BE >0 V B <0 On whiteboard: calculate, B1, B1 and the gain β F Modern Electronics: F3 Bipolar transistor 8
(A) Saturation Large Signal Model active mode base B B S e b F V V BE T b F B emitter collector V BE >0.6V V B <0.4V V E <V BR β F : current gain Kollektorström (A) 0.1 0.08 0.06 0.04 0.02 0 Active ncreasing V BE ( B ) cutoff V BE < 0.6V 0 1 2 3 4 V ce (V) Modern Electronics: F3 Bipolar transistor 9
Example typical Si P Transistor W B =0.5µm Emitter P Base ollector Area: 200µm 2 D =1 10 19 cm 3 D p =1 cm 2 /s (hole diffusion constant) L p =0.5µm (diffusion length) A =2 10 17 cm 3 D n =15 cm 2 /s (electron diffusion constant) τ b =1 µs (minority carrier lifetime) alculate S, b F Modern Electronics: F3 Bipolar transistor 10
n p (x), p n (x) 2 min excersise Early effect deally should not increase with V E, however the width of a reverse biased pnjunction depends on applied voltage! 1. How does the basecollector depletion region change for increasing V E? 2. Sketch the minority carrier distribution in the base with V E applied? 3. How does change with V E in this case? emitter P base collector x Modern Electronics: F3 Bipolar transistor 11
(A) Large Signal Model Early Effect base B collector 0.12 0.1 b F B emitter Kollektorström (A) 0.08 0.06 0.04 B S b V 1 V F S E A e V V BE T V 1 V b E F A e V V BE T 0.02 0 V A Early voltage 15100V 0 1 2 3 4 V ce (V) Modern Electronics: F3 Bipolar transistor 12
(A) Small Signals Taylor Expansion b (t)= B i b (t) B c (t)= i c (t) V BE v be (t) = V be (t) v be (t) E t V BE V E i c (t) = c (t) V be (t)=v BE v be (t) t Kollektorström (A) 0.12 0.1 0.08 0.06 0.04 0.02 0 0 1 2 3 4 V ce (V) V ce (V) 1 st order Taylor expansion linearization f ( x x) f ( x ) df ( x) dx 0 0 xx V v V BE be V E vce V E vce Modern Electronics: F3 Bipolar transistor 13 BE g 1 r 0 m v be 0 x...
capacitances: µ, je, b emitter base collector b n V BE dv BE dq e V BE W B n p0 je µ W B x je, µ : depletion region (junction) capacitances (included only in more advanced model). b : diffusion (base charging) capacitance (only forward biased pnjunction). hange in V BE give change in charge in base (Q e ) > capacitance apacitances become important for highfrequencies b dq dv e BE... WB 2D 2 τ F : base transit time = average time for a carrier to cross base n g m F g m Modern Electronics: F3 Bipolar transistor 14
(simple) small Signal Model Active Mode B E v 1 g m v 1 r 0 r b Transconductance controls the current source g m V T remember: V T =kt/q nput resistance B change with V BE r b F g m Output resistance Early effect r 0 V A VA g V m T Diffusion (base charging) capacitance forward biased baseemitter junction b g F m Modern Electronics: F3 Bipolar transistor 15
Example low f model A BJT is biased so that =5mA. Low frequency > capacitances are open circuit Parameters: β F =500 V A =100 V V T =25.9 mv alculate B and the corresponding small signal model. B v 1 r g m v 1 r 0 Modern Electronics: F3 Bipolar transistor 16
Small Signal Model more advanced Add junction capacitances µ (basecollector junction) and je (baseemitter junction). π = je b. Add series resistances in emitter (r ex ), base (r b ) and collector (r c ). r b µ r c B v 1 r je b g m v 1 r 0 v be r ex E r b, r ex, r c, je and µ : Depends on exact transistor geometry. Modern Electronics: F3 Bipolar transistor 17
Summary BJTs P or PP. P is faster due to higher electron mobility Active mode (used for amplifiers): Emitterbase junction is forward biased (injects minority carriers into base) ollectorbase junction is reverse biased (removes minority carriers from base) Early effect: increasing V E extends the collectorbase depletion region narrowing the base resulting in a higher diffusion current. Small signal model: nput resistance (r π ) due change in base current ( B ) when changing V BE. Output resistance (r 0 ) due to Early effect. Base charging capacitance ( b ) due to change in charge in base region when changing V BE. an add more capacitances and resistances to get more accurate (and complicated) model. Modern Electronics: F3 Bipolar transistor 18
Stateofthe Art: SiGe Bipolar Transistor Emitter Base ollector V BR ~ 1.5V V ce,sat ~ 0.3V b F ~ 600 f t =300 GHz Modern Electronics: F3 Bipolar transistor 19