SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

7.3. TEMPORAL RESPONSE OF A SCHOTTKY DIODE 317

The general solution of the equation is

Qp(t)=iRτp+Ce−t/τp (7.2.52)

To obtain the constantC, we note that at time just beforet 2 ,

Qp(t)=iFτp=iRτp+Ce−t^2 /τp

or
C=τp(iF−iR)et^2 /τp (7.2.53)

The time dependence of the minority charge becomes

t>t 2 : Qp(t)=τp

[

iR+(iF−iR)e(t^2 −t)/τp

]

(7.2.54)

Att=t 3 , the entire excess minority charge is removed, i.e.,Qp(t 3 )=0. This gives us

iR+(iF−iR)e−(t^3 −t^2 )/τp=0

For the long diode, we get

t 3 −t 2 =τpln

iF−iR

iR

=τsd (7.2.55)

The time(t 3 −t 2 )it takes to remove the stored minority charge is called thestoragedelaytime
τsd. Until this time, the diode remains forward biased. For the short diode, the timeτpis replaced
by the transit time defined in equation 7.2.43. We have, for the short diode,

t 3 −t 2 =τTln

iF−iR

iR

=τsd (7.2.56)

Once the minority charge has been removed, the diode reverse biases in a time controlled by the
circuit resistance and the average depletion capacitance of the diode. This time, known as the
transitiontime, is
τt∼ 2. 3 RCj (7.2.57)

whereRis the resistance in the circuit andCjis the average depletion capacitance.
The discussion of the turn-off process is represented schematically in figure 7.5.

7.3 Temporal Response of a Schottky Diode ......................

In chapter 5 we have examined the Schottky diode. The key difference between the Schottky
diode and thep−ndiode is that the Schottky diode is a majority carrier device and as a result
minority carrier injection and extraction is not an issue. The small-signal equivalent circuit of a
Schottky diode is shown in figure 7.6. One has the parallel combination of the resistance

Rd=

dV

dI

(7.3.1)