Schottky Barrier Photodiodes
Schottky barrier diodes are similar to asymmetrical p+-n junctions
Metal - Semiconductor Contact (Schottky Barrier)
Contact formation:
Band diagrams of the metal and semiconductor BEFORE the contact:
Φm is the work function of electrons in the metal;
ΦS is the work function of electrons in the semiconductor;
χS is the electron affinity in the semiconductor
Rectifying Metal – n-type Semiconductor contact ( φm > φS)
Since φm > φS, the electron will
mostly transfer from the
semiconductor into the metal
in the equilibrium
and under the forward bias
Depletion region at the metal-semiconductor interface
Current flow mechanism in Schottky contact
In low doped semiconductors the major mechanism is the thermionic emission.
The current thorugh a Schottky contact comprises the electron flow from
the semiconductor to the metal and vice versa.
• In equilibrium, these two components are equal.
• Under forward bias, electron flow from semiconductor into metal is
enhanced due to a reduced potential barrier Vd(V). The flow from metal
to semiconductor remains unchanged, since φb remains unchanged. This
results in a large net current.
• Under the reverse bias, due to increased barrier, electron flow from
semiconductor to metal is almost negligible, and therefore the nearly
constant small reverse current is due to the flow of electrons from metal
to semiconductor over an unchanged barrier φb.
Schottky Barrier Photodiode
• Very simple device structure;
• No p-n junction needed; no optical loss in the p-layer;
• Abrupt metal-semiconductor interface - very small thickness of the active region
is achievable - very high speed of response;
Metal
-
Semiconductor
hν
+
Reverse bias.
Fundamental mode (band-to-band)
of operation
Schottky Barrier Photodiode
• The spectral range can be extended toward longer wavelengths due to
photoexcitation in the metal:
φb < hν < EG
Metal
-
Semiconductor
hν
+
III. Metal - Semiconductor -Metal (MSM) Photodetectors
The photodetectors discussed before have VERTICAL design.
For integrated circuits applications and for other HIGH-SPEED applications
PLANAR design is preferable.
Other issues with regular (Schottky or pin photodetectors) issues:
ƒ Require two different contact types (p- and n- or Schottky and ohmic)
ƒ Slow diffusion current components
ƒ Absorption non-uniformity at short wavelengths.
Metal- semiconductor-metal photodetector concept
+
-
Schottky
Schottky
S/C
The device consists of two (or more) identical Schottky contacts deposited on
the top surface of semiconductor layer.
An external voltage applied between two electrodes biases one of them in the
forward and another one in the reverse direction.
Schottky detector
MSM detector
light
electron
metal
s/c
hole
At zero bias Schottky
photodiode produces a
photocurrent
light
electron
metal
Semiconductor
metal
hole
At zero bias MSM structure is symmetrical;
The electric field in the center is zero.
Photoexcited electrons are "TRAPPED"
in the "potential well"
The net photocurrent is ZERO
MSM detector at moderate bias
light
light
electron
metal
Semiconductor
metal
electron
+
metal
Semiconductor
metal
hole
hole
The potential barrier at the forward biased contact decreases.
There is an electric field between the Schottky electrodes.
The majority of photo-electrons are "TRAPPED" in the "potential well"
Photo-holes are not trapped but still cannot leave the active region
due to the charge of the "trapped" electrons.
The net photocurrent is very small
MSM detector at punch-through bias
-
light
electron
+
metal
Semiconductor
metal
hole
below punch-through
at punch-through
Potential barrier for photo-electrons disappears
Flat-band (“punch-through”) conditions for MSM
photodetector
Consider reverse-biased (left) electrode.
As the bias increases, the depletion region width,
W, increases:
At a flat-band bais, VFB , the depletion region width W is equal to the
electrode spacing, L:
W=L
L2
MSM diode capacitance
The capacitance is low due to the PLANAR device geometry
Punch-through voltage
⎛ πL ⎞
⎟⎟ and k ' = (1 − k 2 )1/ 2
k = tan2 ⎜⎜
⎝ 4( L + W ) ⎠
⎛ π L ⎞
⎟⎟ and
k = tan 2 ⎜⎜
4
(
L
W
)
+
⎠
⎝
k ' = (1 − k 2 )1 / 2
Large area multi-finger MSM diode
W
For N fingers,
MSM capacitance calculation:
complete elliptic integral of the first kind
3.5
3
2.5
2
K( k)
M(k)
L( k)
1.5
1
0.5
M(k)=K(k)/K(k’)
0
0
0.2
0.4
0.6
k
0.8
1
Advantages of MSM photodetectors
ƒ Strong electric field in the active area
Æ pure drift photocurrent , no diffusion component
Æ very fast photoresponse, determined by saturation
velocity, vS
ƒ No need for Ohmic contacts Æ the material can be low-doped.
ƒ Dark current is very low (two back-to-back Schottky contacts)
ƒ Very low capacitance Æ very small RC time – constant
ƒ Planar layout IC-compatible