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Subelement E6

CIRCUIT COMPONENTS

Section E6A

Semiconductor materials and devices: semiconductor materials; bipolar junction transistors; operation and types of field-effect transistors

In what application is gallium arsenide used as a semiconductor material?

  • In high-current rectifier circuits
  • In high-power audio circuits
  • Correct Answer
    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?

  • Correct Answer
    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
  • Correct Answer
    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
  • Correct Answer
    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 "aDD" holes while Donors "provide" electrons.

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How does DC input impedance at the gate of a field-effect transistor (FET) compare with that of a bipolar transistor?

  • They are both low impedance
  • An FET has lower input impedance
  • Correct Answer
    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.

Best memory trick ever: Boba FET's jetpack enables him to fly higher.

HINT: FET trumps DC

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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
  • Correct Answer
    The change in collector current with respect to the change in 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.

SILLY HINT: What is the fate of a bipolar disfunction transition? - Change in current mood with respect to base mood.

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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 ohms to 7 ohms
  • Base-to-emitter resistance of approximately 0.6 ohms to 0.7 ohms
  • Base-to-emitter voltage of approximately 6 volts to 7 volts
  • Correct Answer
    Base-to-emitter voltage of approximately 0.6 volts 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 is the term for the frequency at which the grounded-base current gain of a bipolar junction transistor has decreased to 0.7 of the gain obtainable at 1 kHz?

  • Corner frequency
  • Alpha rejection frequency
  • Beta cutoff frequency
  • Correct Answer
    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 field-effect transistor (FET)?

  • Correct Answer
    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
  • An FET that exhibits very high electron mobility due to a lack of holes in the N-type material
  • An 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

HINT: "When the going gets tough, the tough get going." (You can relate this to "depletion" & "current")

(\(V_\text{th}\)). When \(V_\text{gs} < V_\text{th}\), the gate is "depleted" and current flow will stop.

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In Figure E6-1, which is the schematic symbol for an N-channel dual-gate MOSFET?

  • 2
  • Correct Answer
    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

With a MOSFET, N Channel - think "pointing IN", so symbol 4.

For MOSFETs, the arrow is either pointing iN, or out. In this case it would be the arrow pointing iN. It is a dual gate, so it has to be one with two gates (G1 and G2), so with those two things we know it has to be symbol 4.

See https://en.wikipedia.org/wiki/MOSFET#Circuit_symbols

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

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In Figure E6-1, which is the schematic symbol for a P-channel junction FET?

  • Correct Answer
    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.

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What is the purpose of connecting Zener diodes between a MOSFET gate and its source or drain?

  • 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
  • Correct Answer
    To protect the gate from static damage

MOSFETs, unlike other CMOS-based devices, contain built-in ElectroStatic/Voltage protection in the form of Zener diodes. A typical lightning strike has been known to blow (puncture) such devices without such protection.

Note: The correct answer has "To reduce the chance.." A distraction says "Protects..." (which implies complete protection) which of course does not exist.

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