Lecture 19 - p-n Junction (cont.) October 18, Ideal p-n junction out of equilibrium (cont.) 2. pn junction diode: parasitics, dynamics

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1 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-1 Lecture 19 - p-n Junction (cont.) October 18, 2002 Contents: 1. Ideal p-n junction out of equilibrium (cont.) 2. pn junction diode: parasitics, dynamics Reading assignment: del Alamo, Ch. 7, 7.2 (7.2.4), 7.3

2 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-2 Key questions What happens to the majority carriers in a pn junction under bias? What are the main practical issues in synthesizing pn junction diodes? How does a pn junction switch? What dominates its dynamic behavior?

3 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Ideal p-n junction out of equilibrium (cont.) Minority carrier storage Quasi-neutrality in QNR s demands n p. Consequences: compensating holes + Q hp n, p Q en compensating electrons NA p(x) +Q hp -Q en n(x) ND ni 2 NA injected electrons Q ep n(x) -Q ep +Q hn + Q hn p(x) injected holes -w p -x p 0 x n wn ni 2 ND x In n-qnr, quasi-neutrality implies: Q hn Q en Q n Also, if V Q hn Q en with: Q hn Q en Q n Q hn supplied from p-contact, Q en supplied from n-contact.

4 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-4 Looks like a capacitor diffusion capacitance. Diffusion capacitance (per unit area): with: C d = dq n dv + dq p dv Q n = q w n x n p (x)dx Q p = q x p w p n (x)dx If both sides are long : Q n = ql h p (x n )=τ h J h (x n ) Q p = ql e n ( x p )=τ e J e ( x p ) and C d q kt [τ hj h (x n )+τ e J e ( x p )] = q kt (τ hj hs + τ e J es ) exp qv kt

5 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-5 If both sides are short and S = : Q n = q 1 2 p (x n )(w n x n )=τ tn J h (x n ) Q p = q 1 2 n ( x p )(w p x p )=τ tp J e ( x p ) with τ tn and τ tp are the minority carrier transit times through QNR s: τ tn = (w n x n ) 2 2D h τ tp = (w p x p ) 2 2D e Then: C d q kt [τ tnj h (x n )+τ tp J e ( x p )] = q kt (τ tnj hs + τ tp J es ) exp qv kt Similar result to long diode!

6 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-6 General expression: C d = q kt n,p dominant minority carrier time constant injected minority carrier current density C d grows exponentially in forward bias, negligible in reverse bias: C d exp qv kt [check that C d exp qv kt and not exp qv kt 1] Total diode capacitance: C C Cd Cj 0 0 V

7 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture p-n junction diode pn junctions present in most semiconductor devices (BJTs and MOSFETs). pn junction diodes used in rectifying circuits, detectors in communications applications, bias shifters, input protection devices against electrostatic discharge. For integrated p-n diodes, no special process steps available. Typical cross section of p-n diode implemented in BJT process: n n+ p+ n+ p

8 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-8 Parasitics Series resistance: Accounts for ohmic drop in QNR s (neglected so far). Reduces internal diode voltage I I = I s [exp q(v IR s) kt 1] ideal with series resistance I log I IRs 1/Rs 0 0 V 0 V linear scale semilogarithmic scale Higher V F required to deliver desired I F more power dissipation, potential process control problems RC time constant degraded.

9 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 19-9 Minority carrier boundary conditions: At top surface: depending on relative depth of emitter, effective diode area changes top surface=ohmic contact area (A c, S = ) + peripheral SiO 2 -covered area (A p, S =0) peripheral area Ap contact area Ac n p+ W p Je (periphery) Je (area) if W p L e, volume recombination only A eff A c + A p if W p L e, surface recombination only A eff A c At bottom surface: high-low junction, characterized by S hl.

10 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Isolation: Parasitic substrate p-n junction needs to be reverse biased to avoid turning it on. Even reverse biased, substrate contributes parasitic capacitance. Additional danger: bipolar effect between diode and substrate minority carriers can be extracted by substrate current diverted away from main body of diode. VDD VDD n p+ n+ hole recombination in cathode desired current path parasitic current path p hole extraction by substrate VSS desired result VSS actual result Hard to make integrated pn diodes in CMOS process (unless they hang directly from the power rail that is connected to the substrate). Easier in bipolar since n + buried layer and collector plug can prevent minority carrier injection.

11 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Dynamics Fundamental difference between p-n and Schottky diodes: minority carrier storage slows down p-n diode. Consider simple voltage switching: I V + V Vf Vr Vf 0 t - -Vr Vf-IfRs V j 0 t -Vr I If(Vf) 0 Vf+Vr Ir(pk)=If- Rs Vf+Vr If(pk)= Rs RsC RsC trr slower decay τ t t As in Schottky diode, delays associated with R s C j. Additionally, delays associated with minority carrier storage.

12 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Switch-off transient Evolution of excess minority carrier concentration (long diode): excess minority carrier concentration t=0 - t= x switch-off At t =0, V f I f ; minority carrier stored charge: Q = τ t I f with τ t, dominant minority carrier time constant. At t =0 +, external V abruptly changes from V f to V r ; internal V cannot change abruptly need to get rid of minority carriers!

13 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture t=0 - t=0 + I I R s R s - I f (V f ) V f -I f R s +V r V f -I f R s C V f V f -I f R s C V r - - I(0 )=I f (V f ) I(0 + )= 1 R s (V f I f R s +V r ) V r R s Two phases to discharge: Phase I - From V j = V f I f R s to V j 0. Since Q exp qv f kt,asq discharges, V f cannot change much. Discharge proceeds nearly at constant current I r (pk) V r R s Time to discharge (reverse recovery time): t rr Q I r (pk) τ t I f (V f )R s V r Phase II - From V j 0toV j = V r. Charge depletion capacitance R s C j time constant (as in Schottky diode).

14 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Switch-on transient t=0 - t=0 + I I R s R s + V f +V r V r C V r V r C V f + + I(0 )= I s I(0 + )= V f + V r R s Evolution of excess minority carrier concentration (long diode): excess minority carrier concentration t= t=0 - x switch-on First, R s C j -type charge up of junction capacitance. Then, minority carrier charge injection takes τ t.

15 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Example of SPICE simulations: IS= 6e 17, N= 1, EG= 1.12, RS= 8, CJO= 1.2e 13, VJ= 0.6, M= 0.41, XTI= 3.814, and TT= 4e 9 V f =+0.89 V, V r = 5 V ps 0.0 current (A) E-10 2E-10 3E-10 4E-10 5E-10 time (s) parameter SPICE hand calculation τ rr 41 ps 46 ps τ on 2.6 ns 4 ns I rr 730 ma 736 ma I f (pk)? 736 ma

16 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Key conclusions Diffusion capacitance arises from minority carrier storage in QNR s and quasi-neutrality: C d = q kt Σ n,p(dominant minority carrier time constant minority carrier current density) C d J exp qv kt C d dominates in forward bias, C j dominates in reverse bias. Difficult to implemented integrated diodes in CMOS process: parasitic bipolar transistor. Dynamics of p-n diode dominated by minority carrier storage. Reverse recovery time is time required to eliminate minority carrier stored charge in switch-off transient.

17 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Self study Equivalent circuit models for pn diode