Chapter 14: Solids Applications

FIXME: Reread section 14.3

(pg. 460)

figure 14.8: Energy level states in p-type and n-type semiconductors. Explain why the acceptor sites are above the valence band and why donor sites are below the conduction band

(pg. 460)

Explain why diodes allow current to flow in only one direction using forward and reverse bias conditions

(pg. 461)

Figure 14.13: band diagram of the npn bipolar junction transistor

(pg. 464)

FIXME: finish reading section 14.4 about transistors

(pg. 465)

inversion layer

(pg. 465)

parts of the field-effect transistor (FET): gate, source, and drain

(pg. 465)

donors

Pentavalent impurities that donate electrons to the conduction band. Examples include phosphorus and arsenic. They "donate" electrons to the conduction band and are POSITIVELY charged (see figure 14.5) (pg. 458)

pn-junction

The interface between p-type and n-type semiconductors in diodes. Notice the relationship between applied voltage and current. (i.e. current is (pretty much) only allowed to flow in one direction).(pg. 460)

Drift, diffusion, and net current in three conditions 1) No bias 2) Revers bias 3) Forward bias

replicate picture taken in class

Figure 14.3: The behavior of a hole in the valence band

Holes "want" to rise to the conduction band much like bubbles "want" want to rise to the surface of water. (pg. 457)

Figure 14.6: the locations of donor and acceptor states

FIXME: explain these. See first two paragraphs on pg. 459 (pg. 459)

npn bipolar junction transistor diagram

Figure 14.12 (pg. 464)

Figure 14.1: The difference between metals (conductors), insulators, and semiconductors

A conductor has a partially filled conduction band. An insulator has an empty conduction band with a large band gap (e.g. 5 eV or more) A semiconductor has an empty conduction band but the band gap is much smaller. Electrons may be thermally activated into the conduction band (temperature dependence) (pg. 455)

impurity semiconductor

A conductor that has a small concentration of impurities. Contrast to intrinsic semiconductors (pg. 456)

Figure 14.4: how holes are created and destroyed

A hole can be created when a photon (with appropriate energy i.e. at least the band gap) strikes the material. The electron is excited from the valence band up to the conduction band, leaving behind a hole. When the electron deexcites from the conduction band to the valence band, it release a photon and the hole and electron "annihilate" (pg. 457)

intrinsic semiconductor

A pure semiconductor (a semiconductor without impurities e.g. pure silicon or pure GaAs) Contrast with impurity semiconductor (pg. 456)

What is diode?

A single crystal semiconductor that has been doped to make one part of it p-type and the remainder n-type. They only conduct current in one direction. Think of it as a one-way valve. One use of them is to rectify an AC signal to a DC signal (pg. 460)

What is a transistor?

A three terminal device that is used as an amplifier or a switch, in which a low-power input signal is used to control a high-power output signal. It is perhaps the most important invention in the 20th century. There are two basic types of transistors: the bipolar juntion transistor (BJT) and the field-effect transistor (FET) (pg. 463)

What is a "dopant"? Effect of doping

An impurity introduced to a semiconductor. The effect of doping is to decrease the band gap between the valence band and conduction band

reverse bias condition (of diodes)

Electrons and holes are pulled away from the junction diode creating a larger depletion zone. When the electric field in the depletion zone balances the EMF, current stops flowing. In the reverse bias condition, electrons and holes are pulled AWAY from the pn-junction, the depletion zone INCREASES, and current STOPS (pg. 461)

diffusion current

Flow of electrons from the n-type side (in conduction band) to the p-type side in a diode. This type of current is dominant in the forward bias condition (contrast with drift current) but gets vanishingly small in the reverse bias condition (see figure 14.11) (pg. 462)

drift current

Flow of thermally activated electrons (i.e. electrons that have jumped from the valence band to the conduction band) from the p-type side to the n-type side in a diode. This is very small and is (virtually) the net current in the reverse bias condition (see figure 14.11) (pg. 463)

n-type semiconductors

Impurity semiconductors in which the impurities contribute electrons into the conduction band. Thus conduction is brought about by negatively charged particles. In this case the electron concentration in the conduction band n is greater than the concentration of holes in the valence band p: n > p (contrast with p-type semiconductors)(pg. 458)

p-type semiconductors

Impurity semiconductors in which the impurities contribute holes into the valence band. Thus conduction is brought about by positively charged holes. In this case p > n (contrast with n-type semiconductors) (pg. 458)

doping

Introducing impurities into an impurity semiconductor. This is how impurity semiconductors are created (pg. 457)

Why do donor states lie just below the conduction band while acceptor states lie just above the conduction band

See the first two paragraphs on pg. 459 as well as figure 14.6 (pg. 459)

Table 14.1: The difference between insulators and semiconductors

The band gap for insulators is larger than the band gap for semiconductors. The band gap for semiconductors is small enough that electrons can become exited and jump into the conductor gap (pg. 455)

The parts of an npn bipolar junction transistor: base, emitter, collector

The base is the p-type section of the semiconductor. The emiter is the n-type section of the semiconductor and is wired with a forward bias. The collector is the n-type section of the semiconductor and is wired with a reverse bias. (pg. 464)

forward bias condition (of diodes)

The condition in which we hook up the positive terminal to the p-type side and the negative terminal to the n-type side. In this condition, holes in the p-type side migrate the junction while electrons from the n-type side do likewise. When they meet, they annihilate and give off heat/light. Since the battery supplies more electrons to the n-type side and pulls off electrons from the p-type side (creating holes) current can continually flow in this condition. In the forward bias condition, electrons and holes are pulled TOWARD the pn-junction and current FLOWS. In this case the depletion zone SHRINKS (pg. 461)

depleted region of a pn-junction diode

The region near a pn-junction of a diode in which there are almost no conduction electrons or holes. In this region the ionized donor (positively charged) and ionized acceptor sites (negatively charged) form a charged bilayer that creates a very strong local electric field (pg. 460)

Semiconductor

Think of an insulator with a relatively small band gap. In this case, electrons from the lower band gap can be thermally excited into the conduction band (pg. 455)

relationship between electron concentration (n), band gap energy (Eg) , and temperature (T). Exponential decay

This predicts that the electron concentration decreases with increasing band gap energy and increases with increasing temperature (pg. 456)

acceptors

Trivalent impurities that accept electrons, creating holes. Examples include boron, aluminium, and gallium. They "accept" electrons from the valence band and create holes and are NEGATIVELY charged (pg. 459)

majority carriers

electrons in n-type and holes in p-type semiconductors (pg. 459)

hole

When an electron is thermally excited from the valence band to a conduction band, it leaves behind an unoccupied state. We call this state a hole. We associate with it a positive charge; as such, movement of these holes produces current (pg. 457)

figure 14.9: the pn-junction in equilibrium

When the p-type and n-type semiconductors are brought together, electrons from the n-type region cross the pn-junction and drop down from the conduction band to fill holes in the valence band. This creates a separation of charge resulting in a strong electric field at the pn-junction (pg. 460)

minority carriers

holes in n-type and electrons in p-type semiconductors (pg. 459)

law of mass action

the product np is always the square of the intrinsic concentration. n- electron conduction concentration p - whole concentration ni - intrinsic concentration (concentration of n and p for pure semiconductor)