Lecture 15 Semiconductor Devices:

Depletion Layer:

n-type: Assume the surface of an n-type semiconductor has been negatively charged. The free electrons near the surface will be repelled. Thus, the region near the surface has less free electrons than the interior→ Depletion layer (space-change region)

Band diagram for an n-type semiconductor with negatively charged surface.The depletion layer is a potential barrier for electrons.

1

Band diagram for a p-type semiconductor with positively charged surface.

Metal semiconductor contacts:n-type metal

1.Schottky RectifierφM>φ s2.Ohmic contact φM<φ s

2

Rectifying contacts (Schottky barrier contacts):n-type

φM>φ s

Band diagram for a metal and n-type semiconductorφ :work function

φM>φ s

If the metal and semiconductor are brought into contact, electrons flow from the semiconductors down into the metal until the Fermi energies of both sides are equal.

3

4

Look to the following drawing

Fig 9-16 on book

Thus, the metal will be charged negatively and the energy band in the semiconductor will be lowered. In equilibrium, electrons from both sides cross the potential barrier→ Diffusion Current

Contact potential:The potential barrier for the electrons diffusing from the semiconductor into the metal

φM−φs

Electron affinity (X):From the bottom of the conduction band to the ionization energy.

Drift current:When an electron hole pair is thermally created in or near the depletion layer. The excited electrons in the conduction band is swept down the barrier, and so the holes “the drift current” Note: the drift current is very small specifically for large band gap.

Total current=diffusion current + drift currentHow Schottky diodes work??

5

“metal and n-type” connected to D.C source

Reverse Bias: The metal is connected to the negative terminal, the electrons in the semiconductor will be repelled, the depletion layer, one potential barrier increases, no diffusion current (negligible)However, the drift current doesn’t depend on voltage.

Forward Bias: Metal is connected to the positive terminal of the battery, the potential barrier of semiconductor decreases “becomes narrower”

Characteristics Curve of Schottky Diodes

Ohmic Contact (Metallization)Similar to metal-metal contact, the (I-V) curve is linear (obeys Ohm’s Law)

6

Ohmic contact can occur in metal-semiconductor contact for the following cases:

- metal-n type (if φM<φ s) look to the figure (9-18) from your textbook

- metal-p type (if φM>φ s)

7

Metal p-type:Similar

- Example: Al-Si p type-

Metal n-type:Electron flows from the metal into the semiconductor and the bands of semiconductor bend downward (no barrier) electron flows in the two directions. This configuration allows the current to flow into and out of the semiconductor without power loss.

8

p-n junction (diode):Similar potential barrier to reaction is formed

9

Electrons flow from the higher level (n-type) to the p-type

The n-type loses electrons to the p-type.- The p-type loses holes to the n-type- The n-type becomes negatively charged at the

junction- The p-type becomes positively charged at the

junction

10

- An electric field is formed at the junctionThis proceeds until equilibrium and both Fermi energies are at the same level

Note: The bands move up to the p-side and down to the n-side

11

Lecture 15a: Continue semiconductor devices

Let’s continue on p-n junction

p-n junction (Diode):Semi potential barrier to reaction is formed

12

Electrons flow from the higher level (n-type) to the p-type

The n-type loses electrons to the p-type. The p-type loses holes to the n-type The n-type becomes positively charged at the

junction The p-type becomes negatively charged at the

junction.

13

An electric field is formed at the junction

This proceeds until equilibrium is reached and both Fermi energies are at the same level.

Note: The bands move up to the p-side and down to the n-side

After the potential barrier is created, (at equilibrium):At the conduction band, electrons from n-region find a potential barrier that reduces its diffusion to p- region. Electrons in p-region can easily diffuse down the potential barrier to n-region, however their number is small (they are only due to excitation). Thus, the number of electrons crossing the junction in both directions is equal.

Applying external potential to (p-n) junctionWe will obtain similar effects to Schottky diode

14

Look to the next page for more details

15

Breakdown (Avalanche and Zener Diode)

Look to this phenomena, it occurs for the p-n diode when the reverse voltage increases more than a certain value (critical value). Its break down, it occurs for avalanching or tunneling (Zener diode)

16

-Uses for Zener Diode

Photo diode (solar cell)

17

Consists of p-n junction. If light falls on or near the depleted layer, electrons are lifted from the valence band to the conduction band (electron-hole pairs are created).Let’s look to the band diagram

The electrons will move to the n-region and the holes will move to the p-region.

- we can detect these charge carriers in two ways: 1. Open Circuit (photovoltaic mode of operation)

An external potential will appear

18

2. Short Circuit the device (photo-conductive mode of operation)

An external current will flow

The p-region is made very thin (≈1nm), so the light can reach the depleted layer.Note: Defects play a serious role

19

Avalanche Photodiode:p-n photo diode operated in a high reverse bias-mode (near break down voltage)Light→ create electron-hole pairs → are accelerated through the depleted region to high velocity → ionize lattice atoms and generate more hole-electron pairs = photo current gain

Tunnel Diode:Let’s discuss first “degenerate semiconductors”At very high level doping, the Fermi level moves up into the conduction band in the n-type material and moves down to the valence in p-type material.

The photo diode is degenerate p-type and degenerate n-type semiconductor. Look to the following figure:

20

Another phenomena: Electrons can tunnel through the potential barrier in both directions. In equilibrium the net tunnel current is zero.Look to the Fermi energy in every mode of operation in the next figure and look to the resultant I-V characteristic curve of tunnel diode

21

22

Reverse Bias:Potential barrier is increased the Fermi energy in the p-area is raised. Electrons flow from p-type to n-type.

Forward Bias (small):Potential barrier is decreased. Electrons flow from n-type to p- type

Forward Bias (Medium):The area opposite (facing) the filled conduction band is forbidden the current decreases

Forward Bias (normal):Electrons in the conduction band get enough energy to climb the potential barrier of p-side

TransistorsThe most important electronic deviceBipolar transistorUnipolar transistor (field effect transistor)Bipolar transistor: two junctions

n-p-n transistorp-n-p transistor

Bipolar transistor: carriers are majority and minors current controlField effect resistor: carriers are only majority voltage control

Bipolar Junction Transistor (n-p-n)

23

24

Band diagram of an unbiased n-p-n biopolar junction transistor E: emitterB: baseC: collector

Note: The base is thin compared to the emitter and collectorAlso,Emitter→ heavily dopedBase→ lightly dopedCollector→ lightly dopedIt can work as:

Signal amplifier Switch

25

Signal Amplification:Emitter and base diode: forward biasedBase-collector diode: reverse biased

Look to the figure of:“Biasing an n-p-n transistor”

- Because of forwarding biasing the E-B diode, the barrier will be reduced, a large electron flow to the base.

Why the base is thing and lightly doped?Reverse biasing the base-collector → Causes the electrons to accelerate down to the collector

SwitchingBase voltage can stop the electron flow from the emitter to collector

26

n-p-n transistor

27