Power Transistors: BJT, MOSFET, and IGBT

Power Transistors

• Power transistors handle high currents, in the hundreds of Amperes, but operate at low voltages.
• These transistors have limited ability to block reverse voltages.
• They have low current gain, needing continuous base drive during on-state conditions, without forced commutations.
• Current limit protection can be provided through the base drive circuit.
• Power transistors serve primarily as static switches in power electronic converters, particularly for DC voltage applications like inverters and choppers.

Transistor Classification

• Power transistors function as fully controlled semiconductor switches, turning on when a voltage or current signal is applied to the control terminal.
• Types of power transistors:
• Power BJT (Bipolar Junction Transistor)
• Power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)
• Power IGBT (Insulated Gate Bipolar Transistor)

BJT - Bipolar Junction Transistor

• Three-Terminal Device: The collector (C) and emitter (E) are part of the power circuit, while the base (B) controls switching.
• Current-Driven Operation: The base current determines the ON/OFF state of the transistor.
• No base current results in an OFF state, acting as an open switch.
• Sufficient base current leads to an ON state, operating as a closed switch in saturation mode.
• Fast Switching is achievable with a turn-on time of less than 1µs and a turn-off time of less than 2µs.
• Control and Efficiency: BJTs are fully controllable for both turn-on and turn-off.
• Low Current Gain: The current gain, represented as IC/IB, is approximately 10.
• Operation within Safe Operating Area (SOA): They operate within a defined SOA, with a maximum current around 50A.
• High-Frequency Applications: BJTs can function at frequencies up to 100kHz.
• Equations:
• IE = IB + IC
• IC = β * IB
• VCE = VCB + VBE

Power BJT Construction

• Conflicting Design Requirements: Like power diodes, power BJTs need to balance high off-state blocking voltage with high on-state current density.
• Structure: Power BJTs consist of a 4-layer vertical structure with alternating n-type and p-type semiconductor layers.
• Collector Drift Region: A unique n-layer enhances voltage blocking capability.
• Vertical Structure Advantage: Maximizes cross-sectional area for current flow, reducing on-state resistance and power loss.
• Doping and Thickness:
• The drift region thickness controls the breakdown voltage.
• A thin base improves gain but reduces breakdown voltage.
• A wide base reduces gain (β), which typically ranges from 5-20.
• Emitter Doping: High emitter doping enhances current gain (β) by reducing base drive current.
• Emitter and Base Design:
• Narrow finger-shaped emitters and bases prevent current crowding and secondary breakdown.
• A multiple emitter structure reduces parasitic resistance in the base current path.

Geometry of Power BJTs

• Wide base width results in low beta values.
• Lightly doped collector drift region ensures large breakdown voltage.

Steady-State Characteristic

• Cutoff Region: Base current is approximately zero, and no collector current flows.
• The transistor is OFF, acting as an open switch.
• Quasi-Saturation & Hard Saturation: Base drive is applied, and collector current flows.
• The transistor is ON, acting as a closed switch.
• Quasi-saturation occurs due to a lightly doped drift collector region.
• Active Region (Amplification Mode): Not used in power applications but used in small-signal BJTs.
• Breakdown Voltages:
• BVCEO: Max Collector-Emitter breakdown voltage with base open.
• BVCBO: Max Collector-Base breakdown voltage with emitter open.
• BVSUS: Max Collector-Emitter voltage when carrying significant current.

Breakdown Mechanisms

• Primary Breakdown (Avalanche Breakdown):
• Caused by excess carrier injection in the base, increasing saturation current.
• Leads to impact ionization, reducing breakdown voltage.
• Causes high power dissipation and should be avoided.
• Second Breakdown (Thermal Runaway):
• Caused by localized hot spots in the transistor due to uneven current flow.
• Leads to localized thermal damage, potentially destroying the device.
• Results in a sharp drop in VCE and high IC.
• Prevented by maintaining uniform current distribution.
• Design Considerations: A wider base width reduces gain (β) but increases BVCEO, and avoiding hot spots prevents second breakdown.

Quasi-Saturation

• Beta decreases in quasi-saturation because the effective base width (virtual base) width increases.

Turn-Off Waveforms

• Controlled Base Current: Base current needs a controlled transition with a controlled value of -diB/dt from positive to negative values to reduce turn-off times and switching losses.
• Uncontrolled Base Current: Results in excessive switching losses due to collector current tailing.

Special Adjustments

• The base thickness balances high amplification and breakdown voltage requirements.
• The base thickness in a power BJT is generally larger than in a low power BJT.
• The thicker base result in a smaller current gain (β), typically about 5 to 10.
• Darlington pair BJTs are used to increase current gain, with βDarlington = β1 * β2 + β1 + β2.

Power BJT Merits

• High switching frequencies because of low turn-on and turn-off times.
• Small turn-on losses.
• Controlled turn-on and turn-off characteristics due to base drive control.
• No commutation circuit is needed.
• Low on-state or saturation voltage.
• High off-state voltage capability.

Demerits

• Current controlled device, requires base losses.
• Base current must be present during the closing period.
• Reverse base current is required during turn-off.
• Low current gain in the saturation region.
• Sensitive to temperature.
• Charge storage limits switching frequencies.
• Drive circuit is complex.
• Paralleling cannot be used because of negative temperature coefficients.

Applications

• DC to AC converters
• D.C motor inverters
• PWM inverters
• Switching mode power supplies
• Power control circuit
• Relays

FET's

• Use field effect for their operation and are made by diffusing two areas of p-type into n-type semiconductor (vice versa).
• When a p-region is connected to a gate terminal, the source and drain regions are n-type.
• In BJT, the B-E junction is forward bias but in JFET gate-source is always reverse bias
• exhibiting small reverse current in the gate, with IG = 0 and Rin = ∞ (Ideal).
• With the gate reverse biased (n-channel), electrons pass through a narrow channel from source to drain between two depletion layers.
• The more negative the gate voltage, the tighter the channel becomes, so JFET acts as a voltage controlled rather than current controlled device.

MOSFET

• Stands for metal oxide semiconductor field effect transistor.
• Two types: depletion and enhancement.

D-MOSFET Operation

• Structure: Contains a highly doped p-type substrate with two heavily doped n-type regions (source & drain).
• Insulation: A thin SiO2 layer separates the gate from the n-channel, giving very high input impedance (10^10 to 10^15 Ω).
• Operation:
• With VGS = 0V, applying VDS allows current flow.
• Negative VGS reduces electrons in the n-channel, decreasing drain current (ID).
• Positive VGS attracts more electrons to the channel, increasing ID.
• It is a conduction channel that is physically implanted.

Enhancement Type MOSFET

• Structure: Consists of a p-type substrate with two n-doped regions (source & drain) but no physical n-channel between them.
• Insulation: A SiO2 layer separates the gate and the substrate and n-regions, preventing direct current flow.
• Operation:
• With VGS = 0V, applying VDS results in no current flow because there is no n-channel.
• Positive VGS repels holes in the p-substrate, accumulating free electrons at the SiO2 interface, form an inversion layer (n-channel.
• The voltage at which significant current begins to flow is the threshold voltage (VT).
• As VGS increases, free carrier density rises, increasing drain current (ID).
• With constant VGS, increasing VDS leads to ID saturation, like in a JFET.

Power MOSFET'S

• Type: High-voltage N-channel, enhancement-mode, double-diffused Power MOSFET.
• Advantages:
• High input impedance due to insulated gate.
• Majority carrier device, which avoids issues like minority carrier storage time, thermal runaway, and second breakdown.
• Fast switching, which result in high-frequency operation.
• Limitations:
• High on-state resistance, leading to conduction losses at high currents.
• More suitable for low-power applications.
• Operation:
• Turns ON when VGS is applied.
• Turns OFF when VGS is removed.
• Gate voltage controls conduction.
• Applications: Used in choppers and inverters due to easy gate control and high-speed switching.

Operation

• A power MOSFET consists of thousands of MOSFET cells connected in parallel on a single silicon chip.
• Built on an n+ substrate with a high-resistivity n- layer (epitaxially grown), that determines voltage blocking capability.
• p- regions are diffused into the n- layer, followed by n+ regions inside the p- regions.
• SiO2 layer is etched to fit metallic source and gate terminals. With VGS = 0V, the n+ and p- junctions are reverse biased, preventing current flow.
• With a positive VGS, an electric field attracts electrons into the channel, allowing current flow from drain to source.
• Conduction occurs through majority carriers.
• Key Features:
• The drift region increases the ON-state voltage drop.
• Drift region thickness determines the breakdown voltage.
• Parasitic BJT is shorted at the emitter-base junction, preventing unwanted conduction.

Construction

• Power MOSFETs inherently contain a parasitic BJT.
• Body region is the Base
• Source is the Emitter
• Drain is the Collector
• A parasitic BJT turns ON when the base-to-emitter junction is forward biased, causing destructive latch-up.
• Use this: To keep the parasitic BJT OFF, short the body (base) and source (emitter) together.

Switching Characteristics

• Power MOSFETs do not suffer from minority carrier storage time effects since they are the majority carrier device. Gate insulated from parasitic effects.
• Switching ON and OFF rely the input gate charging

Capacitance Model Switching Characteristics

• The turn on time, td, is the time required to charge the input capacitance to the threshold voltage level, VT.
• The rise time, tr, is the gate charging time from this threshold level to the gate to source peak voltage, Vgsp.
• The turn off delay time, tdoff, is the time required for the input capacitance to discharge from overdriving the voltage V1 to the pinch off region.
• The fall time, tf, is the time required for the input capacitance to discharge from pinch off region to the threshold voltage.

Power MOSFET Advantage

• Low gate signal power requirement.
• No gate current flows into the gate after the gate oxide capacitance is charged.
• Electrons flow from drain to source when the channel opens, thus it has fast switching speeds.
• The channel depth is proportional to the gate voltage, thus eliminating storage time.

Disadvantages

• Gate-Substrate Connection: Gate-source potential is electrically connected to the substrate.
• Depletion Region: Depletion region extends outward from the n+ drain without gate bias.
• Channel length should be at least as long as the minimum depletion width.
• Wider channels lower resistance but consume more silicon real estate and cause higher gate capacitance.
• Narrower channels cause higher resistance but faster switching due to lower capacitance.

Applications

• Switch mode power supplies (SMPS): Hard switching above 200kHz
• Switch mode power supplies (SMPS): zero-voltage switched (ZVS) below 1000 watts
• Battery charging
• High frequency applications (>200kHz)
• Wide line or load variations
• Long duty cycles
• Low-voltage applications (<250V)
• Low output power (< 500W)

Comparison

• Power MOSFET has Lower switching losses, higher On-Resistance & Conduction Losses
• Also Preferred for High-Frequency Applications, doesn't have a Temperature Coefficient, and Secondary Breakdown Does not occur
• Power BJT has Higher switching losses, On-Resistance & Conduction Losses, as well as Secondary Breakdown
• Power BJT is Not suitable for High-Frequency Applications, has a Temperature Coefficient

IGBTs

• Preferred for fast switching, peak current handling, ease of drive, and wide Safe Operating Area (SOA).
• Majority carrier device but has higher conduction losses at high voltage ratings.
• As voltage rating increases (>250V), switching losses rise due to higher Qrr and Trr.

(Insulated-Gate Bipolar Transistors):

• Combines BJT's low conduction losses with MOSFET's fast switching & voltage control.
• Superior conduction characteristics but slower switching than MOSFETs.
• Free from thermal runaway and second breakdown issues.
• Widely used in power electronics (inverters, converters, power supplies).

BJTs:

• Lower conduction losses at high voltages but slower switching speeds.
• Current-driven device, prone to thermal runaway and second breakdown

IGBT Schematic and Circuit Configuration

• Can be constructed with the equivalent circuit of two transistors and MOSFET.
• The input side is a MOSFET with a Gate terminal, the output side is a BJT with Collector and Emitter.
• Collector and Emitter are conduction terminals, gate is the control terminal.

Physical Structure of IGBT

• Composed of four semiconductor layers (p+, n−, p, n+).
• Consist of a pnp transistor - p+ as collector, n- as base, p as emitter, an npn transistor which is creating a Darlington-like structure.
• Gate, Collector, and Emitter are three-Terminal Device.
• The metal gate terminal is insulated by SiO2
• J2: Between p+ and n- layers.
• J1: Between n- and p layers.

Characteristics

• Constructed differently by adding an injection layer.
• This injection layer results in holes injected into the N-layer, thus creating a carrier overflow.
• The N-layer that is connected as such reduces the ON-state voltage.
• It is regarded as a power switch for high-voltage (greater than 500 V) and high-power (greater than 500 W) applications.

Cross-sectional View

• Structural Similarity - Identical to Power MOSFET, but IGBT contain p+ substrate.
• Types:
• Punch-Through (PT) Uses N+ buffer layer
• Lower reverse blocking capability
• Faster turn-off from buffer layer.
• Non-Punch-Through (NPT) (without N+ buffer layer) - Higher reverse blocking capability, known as symmetric IGBT.
• Junctions:
• J1: Influences reverse blocking
• J2: Blocking junction, N-layer holds depletion layer.

IGBT Equivalent Circuit

• The Gate-Emitter and Collector-Emitter are positively biased in Turn-On Process.
• Turn-On Process consists of a N-channel MOSFET conducts, thus an allowing drain current flow
• Current Flow in IGBT - pnp transistor (QPNP) turns the IGBT ON as current flows into base
• Conductivity Modulation - Due to low DC gain (a) of QPNP, most emitter current flows as base current Collector bypasses because of RBE resistor
• An IGBT is turned off when the gate is turned off

Operation Characteristics

• The collector is negative voltage which is reverse biased - Reverse Blocking. The depletion layer then spreads into the negative zone
• The gate-emitter is the shorted section while the collector is the reverse bias - Forward Blocking. With all that N- drift region upholds to all existing voltages
• Latches happens when IGBT behaves like a thyristor which leads to uncontrollable conduction
• Higher temperatures increase risk for latch, resulting is a higher current value

Bias Characteristics

• When a gate is applied or when the gate is turned on, that triggers base turn on to perform conduction
• Thus, results a regenerative reaction in the n region (n drift ), thus it reduces on-state lost (like a BJT)
• With resistance less the injecting, IGBT maintains on-state
• The length of time is short, creating minority carriers in N-epitiy layer

TURN-ON

• There a a creation where junction is the zone between p+ and N on the body.
• After the body goes to positive bias, and it it results in the creation of a gate, N-channel occurs, flux increases to range 0.7v, thus modulating/decreasing resistance.

IGBT TURN-OFF

• During a -ve bias, channel will be inhibited and no holes will happen to insert in the N- region.
• With MOSFETs that can decrease current flux by increasing, thus collectors decreases, resulting to that carriers decrease by layers. This whole act can be strictly linked to density of said charges.

Applications

• Suited for high power applications because the ratings is 300 - 1200V, 20-500A
• Commonly used in DC and AC motors, UPS, High frequency welders, and many more Other Applications:
• Motor Control - Less than 20khz,
• UPS: Constant load
• Low light and welding

Transistor Devices Comparison

• The most notable advantage with BJT is its price
• MOSFET is ideal device for input drive of about voltage 3-10
• IGBT is a very useful application because of over 1k value