SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

7.2. MODULATION AND SWITCHING OF AP-NDIODE: AC RESPONSE 307

δp(x)

T = 0

Increasing

time

x

(a)

How fast can minority charge

be removed?

Electron-hole recombination

• τ~ 10–6 sec for
  indirect gap materials


• τ ~10-9 sec for
  direct gap materials

Impurity enhanced

recombination

• τ can approach
  a few picoseconds
  problems with non-ideal
  behavior

Short devices

• τ dominated by diffusion
  time
  problems with high
  reserve current

(b)

δn(x) Minority charge

injection

Device response:

How fast is excess minority

charge removed?

Figure 7.2: (a) The minority hole distribution in a forward-biasedp-ndiode. If the diode is to be
switched, the excess holes have to be extracted. (b) A schematic of what controls device response
of minority-carrier-based devices. Three approaches used to speed up the device response are
described.

where we showed earlier (see equation 4.2.22 through equation 4.2.24)

Wn=

Na

Na+Nd

W;Wp=

Nd

Na+Nd

W (7.2.4)

Thus

|Q|=

eA NaNd

Nd+Na

W=A

[

2 e(Vbi−V)

NdNa

Nd+Na

] 1 / 2

(7.2.5)