43 Questions
In reverse-bias, what is the formula for Transition Capacitance (Ct)?
$C_T = C_0 (1 + \frac{V_R}{V_k})^{\frac{1}{2}}$
What is the gen.formula for capacitance (C) in a p-n semiconductor device?
$C = \epsilon A T_d$
What is Diffusion Capacitance?
It is a capacitance associated with transport of charge carriers.
Which type of capacitance so outweighs the other in each region that we only consider the effects of one type in each region?
Transition Capacitance
Under reverse-bias, how does Transition Capacitance change with increasing reverse bias voltage?
Increases with a square root relation
What is the role of n in the formula of Transition Capacitance in reverse-bias?
$n$ adjusts the knee voltage calculation.
Which capacitance arises from charge distribution under reverse bias?
Diffusion Capacitance
Which factor plays a critical role in determining the Diffusion Capacitance in a diode under bias conditions?
Carrier lifetime within the diode
What is the relationship between the forward current (IF) and the diode voltage (VD) in the forward-bias region?
IF grows very quickly and exponentially with VD
What is the value of the diode current (ID) when the diode voltage (VD) is 0V?
ID = 0 mA
How does an increase in temperature affect the forward-bias characteristics of a silicon diode?
The characteristics shift to the left by 2.5 mV per °C increase
How does an increase in temperature affect the reverse-bias current of a silicon diode?
The reverse current doubles for every 10°C rise in temperature
What is the approximate change in diode voltage (VD) when the temperature increases from 20°C to 100°C in the forward-bias region?
200 mV
What is the approximate change in reverse current (IS) when the temperature increases from 20°C to 100°C?
256-fold increase
What is the primary factor that determines the ideality factor (n) in Shockley's equation?
The physical construction and operating conditions of the diode
If the ideality factor (n) is assumed to be 1, what does this imply about the diode's operation?
The diode is operating in the forward bias region below the knee voltage
What is the primary factor that determines the thermal voltage (VT) in Shockley's equation?
The absolute temperature (TK) and electronic charge (q)
If the reverse saturation current (Is) of a diode increases, what effect will it have on the diode current (ID) in the forward bias region?
The diode current (ID) will increase
In the forward bias region, what is the relationship between the applied voltage (VD) and the diode current (ID)?
The diode current (ID) increases exponentially with the applied voltage (VD)
What is the significance of the term 'nVT' in Shockley's equation?
It represents the thermal voltage (VT) multiplied by the ideality factor (n)
What is the expression for the diode current $I_D$ using Shockley's diode equation?
$I_D = I_S \left( e^{\frac{V_D}{nV_T}} - 1 \right)$
What is the value of the diode current $I_D$ using Shockley's diode equation, given a reverse saturation current $I_S = 0.1$ mA, ideality factor $n = 2$, and reverse bias diode potential $V_D = -10$ V at a temperature of $25.4^\circ$C?
$-100$ A
What is the value of the diode current $I_D$ using Shockley's diode equation, given a reverse saturation current $I_S = 40$ nA, ideality factor $n = 2$, and forward bias diode potential $V_D = 0.5$ V at a temperature of $17.4^\circ$C?
$872.34$ A
What is the behavior of an ideal diode in the forward-bias region?
The diode behaves like a closed switch.
What is the behavior of an ideal diode in the reverse-bias region?
The diode behaves like an open circuit.
What happens to the number of uncovered positive ions in the depletion region of the n-type material under reverse bias?
Increases due to the large number of free electrons drawn to the positive potential
What effect does widening of the depletion region have on majority carrier flow under reverse bias?
Reduces majority carrier flow to zero
What is the term used to describe the point in the reverse region where the application of too negative a voltage with the reverse polarity will result in a sharp change in the characteristics?
Breakdown potential
What is the current that exists under reverse-bias conditions known as?
Reverse saturation current
What happens to the velocity and kinetic energy of the minority carriers responsible for the reverse saturation current I_s as the voltage across the diode increases in the reverse bias region?
They increase in velocity and kinetic energy, eventually reaching a level sufficient to release additional carriers through collisions
What does applying a forward-bias potential do to the width of the depletion region?
Reduces the width of the depletion region
Which bias condition results in heavy majority flow across the junction?
Forward bias
How can the avalanche breakdown region (V_BV) be brought closer to the vertical axis?
By increasing the doping levels in the p and n-type materials
What is the term used to describe the mechanism that contributes to the sharp change in the characteristic when the breakdown voltage V_BV decreases to very low levels?
Zener breakdown
What type of current is associated with reverse-bias conditions in a diode?
$I_S$ reverse saturation current
What is the maximum reverse-bias potential that can be applied before entering the breakdown region?
Peak inverse voltage/peak reverse voltage
What is the term used to describe diodes that operate in the Zener breakdown region?
Zener diodes
What is the primary reason for the formation of the depletion region near the pn-junction?
To prevent further movement of electrons and holes
How does the width of the depletion region compare to the n and p regions in a diode under reverse bias?
It is thinner than both n and p regions
What happens to the charge carriers (electrons and holes) in the depletion region due to diffusion across the junction?
They are depleted
What does the barrier potential represent in a diode under reverse bias?
The voltage required to move electrons through the junction
How does a pn-junction behave before a voltage equal to the barrier potential is applied across it?
No electrons will begin to flow across the junction
Why do we observe a net positive charge in a donor ion and a net negative charge in an acceptor ion?
Because an electron leaves the parent atom
Study Notes
Diode Fundamentals
- A PN-junction is formed instantly upon combining a p-type and n-type material.
- The depletion region is a region near the pn-junction where charge carriers are depleted (electrons and holes) due to diffusion across the junction.
- The depletion region is formed very quickly and is very thin compared to the n region and p region.
- It also acts as a barrier to the further movement of electrons across the junction.
Depletion Region
- The depletion region barrier potential or knee voltage (Vk) is the potential difference of the electric field across the depletion region.
- The knee voltage is the amount of voltage required to move electrons through the electric field.
- A certain amount of voltage equal to the barrier potential and with the proper polarity must be applied across a pn-junction before electrons will begin to flow across the junction.
Reverse Bias
- In reverse bias, the depletion region widens, establishing a barrier that is too great for the majority carriers to overcome, effectively reducing the majority carrier flow to zero.
- The current that exists under reverse-bias conditions is called the reverse saturation current (Is).
- The widening of the depletion region reduces the majority carrier flow to zero.
Forward Bias
- A forward-bias or “on” condition is established by applying the positive potential to the p-type material and the negative potential to the n-type material.
- The application of a forward-bias potential V will “pressure” electrons in the n-type material and holes in the p-type material to recombine with the ions near the boundary and reduce the width of the depletion region.
- The reduction in the width of the depletion region results in a heavy majority flow across the junction.
Diode Characteristics
- The diode characteristic curve is a graph that shows the relationship between the voltage across the diode and the resulting current.
- The curve is divided into three regions: forward bias, reverse bias, and breakdown region.
- In the forward bias region, the current increases exponentially with the voltage.
- In the reverse bias region, the current is very small and remains almost constant until the breakdown region.
Breakdown Region
- The breakdown region is a point in the reverse region where the application of too negative a voltage with the reverse polarity will result in a sharp change in the characteristics.
- The reverse-bias potential that results in this dramatic change in characteristics is called the breakdown potential (VBV).
- Avalanche breakdown occurs when the velocity of the minority carriers responsible for the reverse saturation current Is will increase, eventually releasing additional carriers through collisions with otherwise stable atomic structures.
- Zener breakdown occurs at lower levels of voltage and is another mechanism that contributes to the sharp change in the characteristic.
Diode Equations
- Shockley's equation is a mathematical formula that describes the current-voltage characteristic of a diode: ID = IS (eVD/nVT - 1).
- The diode current (ID) is equal to the reverse saturation current (IS) multiplied by the exponential of the voltage across the diode (VD) divided by the thermal voltage (VT).
Capacitance
- There are two types of capacitance in a p-n semiconductor: transition capacitance and diffusion capacitance.
- Transition capacitance (CT) is the capacitance that occurs due to the depletion region.
- Diffusion capacitance is the capacitance that occurs due to the transport of charge carriers between two terminals of a device.
Temperature Effects
- In the forward-bias region, the characteristics of a silicon diode shift to the left at a rate of 2.5 mV per centigrade degree increase in temperature.
- In the reverse-bias region, the reverse current of a silicon diode doubles for every 10 °C rise in temperature.
Explore the behavior of the depletion region in semiconductor devices under reverse bias. Learn how the widening of the depletion region affects the majority carrier flow in n-type and p-type materials.
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