Read Sections 9.0-9.2 of the text, on metal-semiconductor junctions. Write a sho

ID: 2265745 • Letter: R

Question

Read Sections 9.0-9.2 of the text, on metal-semiconductor junctions. Write a short summary of the material, on the order of ½ - 1 page. To get full credit, you must touch on the following details:

1) (1 point) Metal-semiconductor diodes

a) What is a Schottky diode?

b) What determines Vbi for a Schottky diode? What are typical values?

c) Describe the important differences in reverse current for Schottky diodes vs. pn junctions, including the ideal formulae.

d) Describe the important differences in frequency response between Schottky diodes and pn junctions. (Just a qualitative description is sufficient.)

(2) (0.5 point) Ohmic contacts

a) What distinguishes a rectifying contact from an ohmic contact?

b) How are ohmic contacts achieved on semiconductors; in other words, what do you have to do to make sure your ohmic contact is not a Schottky diode?

Explanation / Answer

a) Schottky Diode :

The Schottky diode also known as hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action. The detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.

When sufficient forward voltage is applied, a current flows in the forward direction. A silicon diode has a typical forward voltage of 600–700 mV, while the Schottky's forward voltage is 150–450 mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency.

A metal–semiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier (instead of a semiconductor–semiconductor junction as in conventional diodes). Typical metals used are molybdenum, platinum, chromium or tungsten, and certain silicides (e.g., palladium silicide and platinum silicide), whereas the semiconductor would typically be n-type silicon. The metal side acts as the anode, and n-type semiconductor acts as the cathode of the diode. This Schottky barrier results in both very fast switching and low forward voltage drop.

The choice of the combination of the metal and semiconductor determines the forward voltage of the diode. Both n- and p-type semiconductors can develop Schottky barriers. However, the p-type typically has a much lower forward voltage.

As the reverse leakage current increases dramatically with lowering the forward voltage, it can not be too low, so the usually employed range is about 0.5–0.7 V, and p-type semiconductors are employed only rarely. Titanium silicide and other refractory silicides, which are able to withstand the temperatures needed for source/drain annealing in CMOS processes, usually have too low a forward voltage to be useful, so processes using these silicides therefore usually do not offer Schottky diodes.

The Schottky diode is used in many applications as a result of its characteristics that differ appreciable from several aspects of the more widely used standard PN junction diode.

Comparison of Characteristics of Schottky Diode and PN Diode

Characteristic

Schottky Diode

PN Junction Diode

Forward current mechanism

Majority carrier transport.

Due to diffusion currents, i.e. minority carrier transport.

Reverse current

Results from majority carriers that overcome the barrier. This is less temperature dependent than for standard PN junction.

Results from the minority carriers diffusing through the depletion layer. It has a strong temperature dependence.

Turn on voltage

Small - around 0.2 V.

Comparatively large - around 0.7 V.

Switching speed

Fast - as a result of the use of majority carriers because no recombination is required.

Limited by the recombination time of the injected minority carriers.