Page 71 - Photonics Essentials an introduction with experiments

Electrical Response Time of Diodes

Electrical Response Time of Diodes 65

4.4 Drift
When the minority carrier reaches the depletion region (#2 and #4 in
Fig. 4.1). Then it moves across the depletion region under the action
of the electric field. The carrier velocity is proportional to the electric
field until a velocity saturation is reached. The saturation electric
3
field is about 3 × 10 V/cm for common semiconductor materials such
as GaAs, Ge, InGaAs, and Si. The electric field in the depletion region
at 0 bias is much larger than the saturation field. This fact means
that the carriers drift across the depletion region at constant velocity
regardless of the reverse bias voltage. For electrons in Si, this is about
7
10 cm/sec, whereas for GaAs and InGaAs it is about a factor of two
larger. The saturation velocity for holes in all semiconductors is about
6
10 cm/sec. The typical value for the depletion width is 1 m = 10 –4
cm. Therefore, the drift time for carriers to cross the depletion region
of this size is a few tens of picoseconds for either electrons or holes in
all photodiode materials.
Unlike the diffusion time, the drift time is linearly dependent on
the drift distance. This feature can be used to improve the response
time of indirect band gap photodiodes (i.e., Si or Ge) by replacing dif-
fusion current with drift current. This will be discussed in more detail
shortly. If we refer to the example above, the effect of replacing all the
diffusion by drift current would shorten the intrinsic response time
from 2 × 10 –6 sec to 5 × 10 –9 sec. If the diode were built on a p-type
substrate, then electrons would be the minority carriers. A drift-dom-
inated response time would be closer to 10 –9 sec.
To summarize so far, the response time for diffusion depends on the
carrier mobility and the diffusion length. It does not depend on the
size of the diode or on the bias voltage. The diffusion time can be quite
short in photodiodes made from materials in which electrons or holes
have very high mobility. For example, in Fig. 42, we show the time re-
sponse of an InGaAs photodiode in which electrons have a mobility of
2
–1
about 10,000 cm -V –1 sec , a factor of 10 greater than that for elec-
trons in silicon.
The response time due to drift current depends on the thickness of
the depletion region and on the saturated drift velocity. The saturated
drift velocity is approximately one order of magnitude higher for elec-
trons than for holes. The velocity is independent of the electric field.
The response time due to drift current does not depend on the size of
the diode, but it can depend on the bias voltage, because an increase
in the bias voltage will make the depletion region wider.
The speed of transport by diffusion cannot be compared directly to
the speed of transport by drift current, since these two mechanisms
do not have the same dependence on distance.

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