E6A: Semiconductor Materials and Devices

Semiconductors are the foundation of all modern electronics. Unlike conductors that freely pass current or insulators that block it, semiconductors sit in between — and their behavior can be precisely controlled by introducing small amounts of impurity atoms. This process, called doping, is what makes transistors, diodes, and integrated circuits possible.

This lesson covers semiconductor materials, how doping creates N-type and P-type regions, how bipolar junction transistors (BJTs) and field-effect transistors (FETs) work, and how to identify schematic symbols for common semiconductor devices.

N-Type and P-Type Semiconductors

Pure silicon and germanium are intrinsic semiconductors — they conduct electricity poorly at room temperature because their valence electrons are tightly bound. Adding impurity atoms (doping) changes this dramatically.

N-type semiconductor is created by adding donor impurity atoms (such as phosphorus or arsenic) that have one extra valence electron beyond what the crystal structure needs. These extra electrons are free to move, so N-type material has an excess of free electrons as majority carriers.

P-type semiconductor is created by adding acceptor impurity atoms (such as boron or gallium) that have one fewer valence electron than the crystal structure needs. This leaves "holes" — positive charge carriers — as the majority carriers. An impurity atom that adds holes to the crystal structure is called an acceptor impurity.

PN Junction Behavior

When P-type and N-type materials are joined, a PN junction forms at the boundary. At this junction, free electrons from the N side and holes from the P side recombine, creating a charge-depleted region called the depletion region.

When the junction is reverse biased — with the positive terminal connected to the N side and negative to the P side — the applied voltage pulls electrons toward the positive terminal and holes toward the negative terminal. This widens the depletion region, and essentially no current flows. The junction acts as an open switch.

When the junction is forward biased, the depletion region narrows and current flows freely once the threshold voltage is reached.

Special Semiconductor Materials

Silicon is the most common semiconductor material, but other materials are used for specific applications. Gallium arsenide (GaAs) has much higher electron mobility than silicon, which allows it to operate efficiently at microwave frequencies. GaAs is the preferred semiconductor material for microwave circuits including amplifiers and oscillators at GHz frequencies where silicon is too slow.

Bipolar Junction Transistors (BJT)

A bipolar junction transistor consists of three doped semiconductor regions: emitter, base, and collector. The two junction types are NPN and PNP, depending on the doping arrangement.

A key parameter of any BJT is its beta (β), which is the ratio of collector current change to base current change: β = ΔIC / ΔIB. Beta describes how well the transistor amplifies base current into collector current. A beta of 100 means a 1 mA change in base current produces a 100 mA change in collector current.

For a silicon NPN transistor to be biased on (conducting), the base-to-emitter voltage (VBE) must be approximately 0.6 to 0.7 volts. This forward-biases the base-emitter junction and allows collector current to flow. Values much lower than 0.6 V leave the transistor off; values much higher can damage it.

The alpha cutoff frequency is the frequency at which the grounded-base current gain of a BJT drops to 0.7 of the gain measured at 1 kHz. Above this frequency, the transistor's usefulness as an amplifier degrades significantly.

Field-Effect Transistors (FET)

Field-effect transistors control current flow using an electric field applied to the gate terminal, rather than by injecting current into the base. Because the gate is separated from the channel by a dielectric layer (MOSFET) or a reverse-biased junction (JFET), the gate draws essentially no DC current.

This gives FETs a much higher DC input impedance at the gate than bipolar transistors. A BJT base draws significant base current; an FET gate draws virtually none. This makes FETs ideal as high-impedance input stages, voltage-controlled devices, and switches.

A depletion-mode FET has current flowing between source and drain even with zero gate voltage. The gate voltage is used to reduce (deplete) the channel and decrease current. This is in contrast to enhancement-mode FETs, which require a gate voltage to turn on.

Schematic Symbols: Figure E6-1

Figure E6-1 shows schematic symbols for several types of FETs and MOSFETs. Two exam questions ask you to identify specific symbols from this figure.

An N-channel dual-gate MOSFET has two separate gate terminals and an N-type channel. Its symbol includes two gate lines entering the device on the same side — look for symbol 4 in Figure E6-1.

A P-channel junction FET (JFET) uses a reverse-biased junction to control channel current, and its arrow in the symbol points inward (toward the channel) indicating P-channel. Look for symbol 1 in Figure E6-1.

MOSFET Gate Protection

MOSFET gates are isolated by an extremely thin oxide layer, which gives them their very high input impedance. The downside is that this thin oxide is easily destroyed by static electricity — even a small electrostatic discharge can punch through and permanently damage the gate.

Connecting Zener diodes between the MOSFET gate and its source or drain provides protection against static damage. The Zener diodes clamp any voltage spike that exceeds safe limits, diverting the energy before it can damage the gate oxide. This is standard practice in MOSFET circuit design and is why handling MOSFETs requires ESD precautions.

E6A Practice Questions