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