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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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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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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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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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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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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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.

Silly hint: point SEVEN goes with SILICON. (There's a similar question on the General exam where they ask about a silicon transistor and the correct answer is 0.7)

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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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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.

Memory Aid: Deplete by flowing down the drain.

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

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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.

Dumb memory hint: Zener - Z - Zap! - Static Shock, "Protect the gate from static."

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