Semiconductor devices 1

Question 1

In a N doped Crystal, what are the minority and majority carriers?

Answer 1

Electrons are majority carriers, Holes are minority carriers

Question 2

In a P doped Crystal, what are the minority and majority carriers?

Answer 2

Holes are majority carriers, Electrons are minority carriers

Question 3

Are N doped crystals negatively charged?

Answer 3

No, Doped semiconductors are electrically neutral. The terms n- and p-type doped do only
refer to the majority charge carriers. Each positive or negative charge carrier belongs
to a fixed negative or positive charged dopant.

Question 4

In a PN junction, are there charge carriers in the area near the junction?

Answer 4

There are no free charge carriers, since the free electrons of the
n-conductor, and the holes of the p-doped crystal in the vicinity of the interface recombine with each other, which means that the electrons fill the holes.

Question 5

In the area near the PN junction on either side, how does the charge react?

Answer 5

The regions near the interface are loaded due to the loss of free charge carriers (positive charge in the n-crystal, negative charge in the p-crystal).

(N crystal has lost electrons so becomes positive, P crystal has gained electrons so becomes negative)

Question 6

What electronic component does a PN junction form?

Answer 6

A p-n junction represents an electrical component with the function to allow an electric
current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition): a diode.

Question 7

What is the depletion zone in a PN junction?

Answer 7

The depletion zone is the area with no charge carriers

Question 8

What happens when a positive voltage is applied to the N type and a negative voltage is applied to the P type region in a PN junction?

Answer 8

The electric field inside the semiconductor and the field of the voltage source are in the
same direction. Thus the electric field at the p-n junction is reinforced. The oppositely
charged free carriers are attracted by the poles of the voltage source, thus the barrier
layer is increased and a current flow is inhibited

(Current does not flow)

Question 9

What happens when a positive voltage is applied to the P type and a negative voltage is applied to the N type region in a PN junction?

Answer 9

If the external voltage is applied in the reverse direction, the external and internal electric field are in the opposite direction and the inner field is weakened. If the inner field
is completely eliminated from the outer field, a constantly flow of free charge carriers from the power source to the interface is possible and the carriers can recombine
continuously: there is electric current.

Question 10

What is Oxidation used for in MOSFET manufacturing

Answer 10

Oxidation is used as an insulating barrier between different layers, to isolate different components laterally.

Question 11

What is nitride used for in MOSFET manufacturing

Answer 11

Nitiride is used as a mask

Question 12

Explain how photoresists and masks work.

Answer 12

A photoresist is put on top of a mask with a certain pattern. Using photolithography, photoresist areas that have parts under them that don’t need protection are removed. This exposes the mask.
The whole device is etched, removing unwanted parts of layers.
The photoresist and mask ca be etched off later.

Question 13

What is polycrystalline silicon used for in a MOSFET?

Answer 13

polycrystalline silicon is used for the gate electrode

Question 14

What is Ion implantation?

Answer 14

Atoms of dopant are bombarded into the device to introduce dopants eg. phosphorus (N type) Boron (P type)

Question 15

Does current flow with no gate voltage applied in Enhancement-mode?

Answer 15

No. Enhancement-mode mosfets are “Self-Locking” eg. no current flows without a positive gate voltage applied

Question 16

What happens when a positive voltage is applied to a Enhancement-mode MOSFET?

Answer 16

A positive voltage applied to the gate attracts electrons in the substrate, while holes are
pushed away. Thus forming a conducting n-channel beneath the gate electrode and inbetween source and drain, respectively. The insulating silicon dioxide layer prevents a
current flow between the substrate and the gate.

Question 17

What is a Depletion-mode MOSFET?

Answer 17

With a light n-type doping between the source and drain, a conductivity is even possible without a gate voltage (a voltage between source and drain is sufficient). So-called
depletion FETs, or self-conducting transistors block the current flow only, if the voltage applied to the gate electrode is lower than the voltage at the source port. If the gate
voltage is decreased, the electrons that are located beneath the gate, are pushed away -
the conductive electron-channel is lost.

Question 18

How is a finFET different to traditional MOSFETS?

Answer 18

In contrast to planar MOSFETs the channel between source and drain is built as a
three dimensional bar on top of the silicon substrate, called fin. The gate electrode is
then wrapped around the channel, so that there can be formed several gate electrodes
on each side which leads to reduced leakage effects and an enhanced drive current.