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Subelement E6
CIRCUIT COMPONENTS
Section E6A
Semiconductor materials and devices: semiconductor materials; germanium, silicon, P-type, N-type; transistor types: NPN, PNP, junction, field-effect transistors: enhancement mode; depletion mode; MOS; CMOS; N-channel; P-channel
In what application is gallium arsenide used as a semiconductor material?
• In high-current rectifier circuits
• In high-power audio circuits
In microwave circuits
• In very low-frequency RF circuits

Gallium Arsenide (GaAs) semiconductors really shine at higher frequencies. They have less noise as compared to silicon, reduced sensitivity to heating, higher electron mobility, and higher saturated electron velocity. This makes them usable for frequencies up to 250GHz.

Unrelated Memory Trick: The genus of chickens is "Gallus", so just think Gallium is microwaved chicken.

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Which of the following semiconductor materials contains excess free electrons?
N-type
• P-type
• Bipolar
• Insulated gate

N-Type material contains an excess of free electrons. Electrons hold a negative charge, so think of "N-Type" as "Negative" as a way to remember.

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Why does a PN-junction diode not conduct current when reverse biased?
• Only P-type semiconductor material can conduct current
• Only N-type semiconductor material can conduct current
Holes in P-type material and electrons in the N-type material are separated by the applied voltage, widening the depletion region
• Excess holes in P-type material combine with the electrons in N-type material, converting the entire diode into an insulator

When you "forward bias" a diode, electrons flow from the N-Type material to the holes in the P-Type material, which allows current to flow.

When reverse biasing a diode, There are no electrons to flow to the holes in the P type material because the applied voltage widens the depletion region to separate the P type material and the N type material. This is why current does not flow when you reverse bias a diode.

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What is the name given to an impurity atom that adds holes to a semiconductor crystal structure?
• Insulator impurity
• N-type impurity
Acceptor impurity
• Donor impurity

The correct answer is Acceptor impurity, because an acceptor impurity creates holes in the semiconductor crystal lattice where electrons could fit.

A donor impurity is incorrect as it would add extra electrons to the semiconductor crystal. An N-type impurity would also be incorrect, because this is another way of describing a donor impurity.

There's no such thing as an insulator impurity, so that one is a pure distractor.

Remember: acceptors provide holes while donors provide electrons.

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How does DC input impedance at the gate of a field-effect transistor compare with the DC input impedance of a bipolar transistor?
• They are both low impedance
• An FET has lower input impedance
An FET has higher input impedance
• They are both high impedance

The base-emitter junction of a bipolar transistor draws a small, but non-zero amount of current. This lowers the effective input impedance of a bipolar transistor circuit.

Field-effect transistors (FETs) on the other hand, are essentially charge-based devices. While an ideal FET draws no current, real FET circuits draw minuscule amounts of current. This drives up in the input impedance significantly, making FET's ideal for applications where high input impedance is required.

Memory trick: bipolar >>> low; therefore FET must have high impedance.

Alternate memory trick: A fete (FET) is a celebration, think high spirits.

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What is the beta of a bipolar junction transistor?
• The frequency at which the current gain is reduced to 0.707
The change in collector current with respect to base current
• The breakdown voltage of the base to collector junction
• The switching speed

The Beta of a transistor is a published ratio of the change in collector current that results from a change in base current. A typical value for a small transistor is 100-150, while "high gain" transistors may have higher beta values. Power transistors, on the other hand, tend to have a lower value of beta. Beta is also called the "common emitter current gain."

It is widely considered bad practice to design a transistor circuit that is dependent on beta for biasing or proper function. For a bipolar junction transistor, beta changes as a function of temperature. Unfortunately, it increases which can lead to a condition known as "thermal runaway".

A mathematical identity is given in Wikipedia:

$\beta=\frac{I_C}{I_B}=\frac{\text{collector current}}{\text{base current}}$

This is roughly representative of the correct answer in which the beta, or $\beta$, is the change in collector current with respect to the base current.

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Which of the following indicates that a silicon NPN junction transistor is biased on?
• Base-to-emitter resistance of approximately 6 to 7 ohms
• Base-to-emitter resistance of approximately 0.6 to 0.7 ohms
• Base-to-emitter voltage of approximately 6 to 7 volts
Base-to-emitter voltage of approximately 0.6 to 0.7 volts

When measuring the P-N forward biased junction, the forward voltage should be around 0.7V over a range of currents.

This is just a property of silicon NPN (bipolar) junction transistors that you have to remember applies to silicon NPN transistors and diodes. Other types may have other voltage drops.

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What term indicates the frequency at which the grounded-base current gain of a transistor has decreased to 0.7 of the gain obtainable at 1 kHz?
• Corner frequency
• Alpha rejection frequency
• Beta cutoff frequency
Alpha cutoff frequency

The performance of a transistor amplifier is relatively constant up to a point. When the frequency exceeds this point, the performance of the transistor degrades as frequency increases. The Beta Cutoff Frequency is the frequency at which the current gain falls to unity. The Alpha Cutoff Frequency is the frequency at which the current gain falls to 0.707 of the low frequency current gain.

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What is a depletion-mode FET?
An FET that exhibits a current flow between source and drain when no gate voltage is applied
• An FET that has no current flow between source and drain when no gate voltage is applied
• Any FET without a channel
• Any FET for which holes are the majority carriers

There are essentially two flavors of field-effect transistors (FETs):

• enhancement mode
• depletion mode.

Most Junction FETs (JFET, or just FET) are depletion mode, while most many MOSFETS are enhancement mode devices.

Depletion mode FETs will exhibit "normally on" behavior. That is to say that current will flow between the source and drain even when the voltage between the gate and the source ($V_\text{gs}$) is zero.

To stop the flow of current you need to "deplete" the gate -- pull it below the device's specific threshold voltage ($V_\text{th}$). When $V_\text{gs} < V_\text{th}$, the gate is "depleted" and current flow will stop.

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In Figure E6-1, what is the schematic symbol for an N-channel dual-gate MOSFET?
• 2
4
• 5
• 6

FET Schematic Symbols List

1 - P JFET
2 - N channel MOSFET
3 - P channel MOSFET
4 - Dual Gate N channel MOSFET
5 - Dual Gate P channel MOSFET
6 - N JFET

Additional Hint: Like their transistor counterparts, the direction of the arrow as in NPN (Not Pointing iN).

For MOSFETs, the arrow is either pointing iN, or out. In this case it would be the arrow pointing iN, so symbol 4.

Also, it's a dual-gate, so it's got to be one of the symbols with two gates (G2, G1), so it could only be 4 or 5.

Memory trick: for these schematic questions, all N type symbols are at eveN numbers (2,4,6,8)

-KE0IPR Edited by Norm K6YXH 5/13/2019 to explain that the N means iN, and that dual-gate means it could only be 4 or 5. Also made the N bold font.

W1IKW - "Not pointing In" is wrong for Mosfets. If that were true, the correct answer would be 5, whereas 4 is actually the correct answer.

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In Figure E6-1, what is the schematic symbol for a P-channel junction FET?
1
• 2
• 3
• 6

FET Schematic Symbols List

1. P JFET
2. N channel MOSFET
3. P channel MOSFET
4. Dual Gate N channel MOSFET
5. Dual Gate P channel MOSFET
6. N JFET

Memory trick: for these schematic questions, all N type symbols are at eveN numbers (2,4,6,8).

Hint: Like their transistor counterparts the direction of the arrow as in NPN - Not Pointing iN. In this case it would be the arrow pointing out.

-KE0IPR

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Why do many MOSFET devices have internally connected Zener diodes on the gates?
• To provide a voltage reference for the correct amount of reverse-bias gate voltage
• To protect the substrate from excessive voltages
• To keep the gate voltage within specifications and prevent the device from overheating