1964 no: P-N Junction Diode Switching 11 Fig. 4-Current and voltage switching characteristics of a diode Fig 5-Hole density distributions during the switching Fig. 6-Storage time 4, of a diode as a function of Ip/IR iRR 4=232+R (16) Differentiation of this with respect to time t gives 1+_8 d (12) Overdriven Switching Operation Then(1) becomes In normal switching operation, we have derived the expression for the storage time t, given by(9). It can be di ir=(TR+RC+-iR (13)seen that, as Is is inereased keeping Ir constant, t, de- creases and becomes zero when with the boundary condition given by( 8); i.e IRE-IE Let us define I we can solve for in(t) for t>t, We get Ro as the critical reverse current beyond which t, becomes zero, b.e., 1+ (17 i(6-1s(-(x+Z(-)·10 When i,>IRo, viz., overdriven switching, the charge Defining t, as the time that in takes to decay from I2 stored in the base region cannot support the reverse to 10 per cent of Ir as shown in Fig. 4, i.e current. Thus, the reverse current immediately starts to iR(n=0.1IN lo=I, (18) t, can be obtained from(14) as shown in Fig. 7, and assume that to is very gmall so tr (15) that the charge stored in the base region does not decay appreciably during this interval of time. Then the all Authorized icensed use limited to: IEEE Xplore. Downloaded on December 15, 2008 at 03: 47 from IEEE Xplore. Restrictions appl1964 Kuno: P-N Junction Diode Switching 11 owt "(t) Fig. &Current and voltage switching characteristics of a diode. (cl t = tS (a) t > tg Fig. 5-HoIe density distributions during the switching. Fig. 6-Storage time t, of a diode as a function of IF/IR. Vj = - V, + iRR. Differentiation of this with respect to time t gives Then (1) becomes with the boundary condition given by (8); i.e., &(is) = IRTR we can solve for iR(t) for t > t,. We get 7 (12) Overdriven Switching Operation In normal switching operation, we have derived the expression for the storage time t, given by (9). It can be (13) seen that, as I, is increased keeping I, constant, ts de￾creases and becomes zero when TF TR 1, = --I,. Let us define IRo as the critical reverse current beyond which t, becomes zero, Le., I20 = - I,. TP TR (17) (14) When i, > IRO, vix., overdriven switching, the charge Defining tf as the time that iR takes to decay from I, stored in the base region cannot support the reverse to 10 per cent of I, as shown in Fig. 4, i.e., current. Thus, the reverse current immediately starts to decay to IRo. Let us define to such that iR(tf) = 0.1 I,, i,(t3 = IRO, (18) tf can be obtained from (14). as shown in Fig. 7, and assume that to is very small so (15) that the charge stored in the base region does not decay appreciably during this interval of time.3 Then the all 1+- I TF J or thus to, is very small compared with rg in most cases. This assumption is justifiable since the time constant RCi, Authorized licensed use limited to: IEEE Xplore. Downloaded on December 15, 2008 at 03:47 from IEEE Xplore. Restrictions apply