Physics and Engineering of Radiation Detection

300 Chapter 5. Solid State Detectors

Just like a semiconductor diode, a semiconductor-metal diode can be produced,
which is generally referred to as Schottky diode. It is created by bringing a very thin
metal layer in contact with a semiconductor material (see Fig.5.1.32). The process
produces a junction near to the surface of the semiconductor.
The detectors made from Schottky diodes are generally used to detect photons.
To ensure high transparency of the metal layer to the photons, the layer is kept
as thin as possible. Since the active detection material essentially starts at the
surface, therefore the signal loss is minimum and the charge collection efficiency is
very high with reduced charge transit time. This type of detector works very well
as a photodiode as it can operate at very high frequencies (in GHz) without causing
high frequency roll off typical of pn junction diodes.

Metal Ohmic Contact

(a)

Forbidden Gap

Valence Band

EF

+

−

(b)

E

Conduction Band

Semiconductor

Figure 5.1.32: Sketch of a Schottky diode (a) with

its energy band structure (b). The metal layer

is made very thin to allow the photons to pass

through it when used as a photodetector. Typi-

cal metallic surface is gold on p type silicon. The

ohmic contact could be made by depositing alu-

minum on silicon surface.

K.3 HeterojunctionDiode

It is sometimes desirable to build diodes that have multiple junctions of different
types to optimize the generation of output signal with respect to the parasitic ab-
sorption of incident radiation. for example in photon detection where incident light
level may be quite low, a considerable fraction of photons may get absorbed in the
material before reaching the depletion region. The trick in this case would be to use
a material for the surface layer that has wider band gap than the incident radiation.
In this case most of the radiation would pass the surface layer without being ab-
sorbed and reach the main depletion region. Heterojunction diodes are mainly used
for photon detection.