BJT vs FET Comparison and SCR & Zener Diode Characteristics
Unit 4
1.8 Compare BJT and FET?
Comparison Between BJT and FET
| BJT (Bipolar Junction Transistor) | FET (Field Effect Transistor) |
|---|---|
| Current controlled (base current controls collector current) | Voltage controlled (gate voltage controls drain current) |
| Both electrons and holes contribute to current flow | Only majority carriers (electrons in N-channel, holes in P-channel) |
| Moderate input impedance | High input impedance (due to reverse bias on gate) |
| Relatively high power consumption | Relatively low power consumption |
| High current gain (β) | Moderate current gain (limited by transconductance, gm) |
| More sensitive to temperature changes | Less sensitive to temperature changes |
| Higher (base current even with no bias) | Lower (gate current negligible due to reverse bias) |
| Requires some base current for operation | Requires no gate current for operation |
| More linear relationship between input and output | Less linear, especially at the edges of the operating region |
| Relatively simpler and cheaper to manufacture | Can be more complex to manufacture |
| Amplifiers, switches, voltage regulators, logic gates | Amplifiers, switches, analog signal processing, constant current sources |
| Current-mode devices | Voltage-mode devices |
| Common Emitter (CE), Common Base (CB), Common Collector (CC) | Common Source (CS), Common Drain (CD), Common Gate (CG) |
Unit 5
1. Explain the characteristics and operation of SCR?
Silicon Controlled Rectifier (SCR): The Silicon Controlled Rectifier (SCR), also known as a thyristor, is a three-terminal power semiconductor device used for switching and controlling high-power currents.
Operation
1. Forward Blocking Mode
When the anode is positive with respect to the cathode and the gate is open, the SCR is in its forward blocking mode. Only a small leakage current flows through the device.
2. Forward Conduction Mode
When a small gate current is applied to the gate terminal, the SCR switches to the forward conduction mode. This gate current forward-biases junction J2, allowing a large current to flow from the anode to the cathode.
3. Reverse Blocking Mode
When the anode is negative with respect to the cathode, the SCR is in its reverse blocking mode. Junctions J1 and J3 are reverse-biased, preventing current flow. Similar to the forward blocking mode, only a small leakage current flows through the device.
4. Turning Off (Commutation)
To turn off the SCR, the anode-to-cathode current must be reduced below the holding current. This can be achieved by interrupting the current path or applying a reverse voltage across the SCR.
V-I Characteristics
The voltage-current (V-I) characteristic curve of an SCR illustrates its operation in the different modes:
- Forward Blocking Region: The SCR blocks the forward voltage until the applied voltage exceeds the forward breakover voltage or a gate current is applied.
- Forward Conduction Region: Once triggered by the gate current, the SCR enters the forward conduction region, allowing large current flow with a small voltage drop across it. The SCR stays in this region as long as the current remains above the holding current.
- Reverse Blocking Region: The SCR blocks reverse voltage, similar to a regular diode in reverse bias.
2. Explain the characteristics and operation of Zener diode?
A. A Zener diode is a special type of diode designed to reliably allow current to flow backwards when a certain set reverse voltage, known as the Zener voltage, is reached.
Structure and Symbol
• Structure: Zener diodes are constructed similarly to regular diodes but are heavily doped to have a sharp breakdown voltage.
• Symbol: The symbol of a Zener diode is similar to a regular diode with bent edges on the bar representing the anode.
Structure and Symbol
• Structure: Zener diodes are constructed similarly to regular diodes but are heavily doped to have a sharp breakdown voltage.
• Symbol: The symbol of a Zener diode is similar to a regular diode with bent edges on the bar representing the anode.
Characteristics
- Reverse Breakdown Voltage (VZ): The Zener voltage is the voltage at which the diode conducts in reverse bias.
- Sharp Breakdown: Zener diodes have a sharp breakdown characteristic, meaning they switch from a high-resistance to a low-resistance state quickly as the reverse voltage reaches VZ.
- Zener Impedance (ZZ): Zener diodes exhibit a small dynamic resistance in the breakdown region, known as Zener impedance. Lower ZZ values are preferred for better voltage regulation.
- Temperature Coefficient: The Zener voltage can vary slightly with temperature. Zener diodes below 5.6 V typically have a negative temperature coefficient, while those above 5.6 V have a positive coefficient.
- Power Dissipation: Zener diodes can dissipate power, given by P = VZ × IZ, where IZ is the current through the diode. They are rated for maximum power dissipation to avoid damage.
V-I Characteristics
The voltage-current (V-I) characteristic curve of a Zener diode shows:
- Forward Region: Similar to a regular diode, with a small forward voltage drop and exponential increase in current.
- Reverse Region (Pre-Breakdown): Minimal current flow with increasing reverse voltage, exhibiting high resistance.
- Breakdown Region: At the Zener voltage (VZ), the diode conducts significantly in the reverse direction.