In recent years, MOSFETs have benefited from huge investments in technology and promotion alike, overshadowing bipolar devices to the degree that many designers view bipolars as old technology. Meanwhile, developments in bipolar transistors continue to position the technology to rival or exceed MOSFET performance in many applications. It's therefore important to review the characteristics and benefits of each technology to extract the best system performance. The first characteristic that designers most often consider is on-state resistance for a given breakdown voltage. Trench MOSFETs give lower on-resistance by allowing greater channel density. The results are spectacular - especially at low breakdown voltages - but current flow remains concentrated in narrow channel regions. Higher voltage MOSFETs also suffer from the high resistance of the lightly doped drain region so on-resistance typically increases proportionally with breakdown voltage:
RDS(on) proportional to BV2.6
Under the correct drive conditions, it's worth noting that bipolar transistors have matched or bettered MOSFETs in terms of die-area-specific on-state resistance (see Figure 1). By optimizing process technology and chip layout, voltage biasing and current flow is evenly distributed across the chip area to maximize silicon efficiency. Furthermore, bipolar transistors benefit from conductivity modulation of the resistive collector region when operated as a saturated switch, significantly reducing the RCE(sat). MOSFETs don't possess any similar conductivity modulation mechanism, which accounts for one of the bipolar transistor's advantages. As is demonstrated in the straight-line plot in Figure 2, the relationship between breakdown voltage and collector-emitter saturation voltage for the Zetex third generation transistor series yields:
RCE(sat) proportional to BV2
Figure 1. Specific on-resistance (20V devices)
Figure 2. Breakdown voltage versus RCE(sat) for the Zetex third generation transistor series
The difference in the exponent value in these formulae emphasizes the specific-area-resistance advantage that the bipolar transistors hold over MOSFETs as breakdown voltages increase. For example, the 450V-rated FMMT459 NPN transistor has a current capability of 150mA and a typical RCE(sat) of just 1.4´´ that allows SOT-23 packaging. A similarly rated MOSFET has such high specific on-resistance and poor current capabilities that it requires packages such as D-PAK to accommodate the necessary silicon area. Also, don't forget that bipolar transistors block voltage in two directions, as specified by their BVEBO or BVECO characteristic. When a bidirectional capability is required, this attribute can eliminate the need for a series diode or back-to-back MOSFET pair and their attendant conduction losses. (See application example 1 below.)
How switch resistance changes with temperature is another important factor in determining the current capability of a power switch. Because bipolar-transistor gain rises with temperature and reduces the VBE component of VCE(sat), the rise in bipolar RCE(sat) is generally half that of a MOSFET's equivalent RDS(on). This characteristic leads to bipolars running cooler at high current densities, and/or higher continuous currents for comparable die area.
Drive requirements are arguably where bipolars and MOSFETs differ most. Care must be taken to check the drive conditions when comparing the two technologies. For example, bipolar transistors require sufficient base current to achieve their lowest RCE(sat) values, and the base drive loss must be taken into account in power dissipation calculations. High-gain devices minimize such losses, and the fact that bipolar transistors require less than 1 volt to fully turn on and show better temperature stability can be useful in low-voltage or battery-powered applications. Alternatively, a MOSFET requires gate current only to charge and discharge its gate capacitances so under dc drive conditions the drive current is negligible. However, the gate drive voltage is critical to achieve the lowest RDS(on), and on-resistance dramatically increases as drive voltage approaches the gate threshold voltage. For these reasons, the highest practical values of drive current and voltage were chosen to give the fairest comparison among device types in Figure 1.
Being majority carrier devices, MOSFETs can switch at over 1MHz provided that they have a sufficiently high-current drive circuit to charge and discharge their parasitic capacitances. Ironically perhaps, bipolars are often employed as MOSFET pre-drivers, exploiting their high-current capabilities and fast switching speed when operating in the linear region (see application example 2 below). However, when bipolars are operated as a saturated switch, the supply and removal of stored charge during each switching cycle extends turn-off times and limits their practical switching speed to a few hundreds of kHz.
MOSFET's are inherently sensitive to ESD, suffering catastrophic failure when the electrostatic charge causes the gate voltage to exceed its rupture voltage. With good assembly housekeeping, it's possible to minimize but not obviate the potential for ESD failures. Bipolar transistors are comparatively rugged, and have no difficulty passing standard human-body model ESD tests.
Finally, many of the factors discussed here impact total circuit cost. By understanding the relative strengths and weaknesses of each technology the performance-cost relationship can be maximized. A summary of the key parametric differences of the competing technologies appears in Table 1.
|| Bipolar Transistor
| 'On' resistance
|| Excellent - down to half that of the best MOSFET, depending on drive current available
|| Good at full enhancement; Moderate at low gate drive
| Blocking voltage
|| Bi-directional blocking capability, BVces, BVcev or BVcbo may be appropriate for some applications
|| Mono-directional, may require a series Schottky diode or back-to-back MOSFET pair in some Applications
| Pulse Current
| Drive voltage
|| Less than 1 V
|| I.8V to I0V, depending on the Optimization
| Temperature stability
Vbe: approx 2mV/°C
RCE(sat) approx 0.4%/°C
Vth: approx 4 to 6mV/°C
Rds(on) approx 0.6%/°C
| Drive Power
|| DC: excellent; High frequency: moderate
|| Linear switch: Very fast
Saturated switch: Moderate
| ESD sensitivity
|| Very rugged
| Price per area of silicon
Table I. Parametric differences between MOSFETs and bipolar transistors
Application example 1: Linear mode battery charging
Linear chargers are simple in design, small, and emit no EMI making them suitable for low noise environments. They use an external pass element to drop the voltage from the input supply to the battery voltage thus power dissipation is high. A typical linear charger circuit is shown below, featuring the ZXT13P20. Power losses in the transistor are dominated by the collector-emitter losses:
PD(CE) = ICHG x (VIN - VDCD - VSENSE) (W)
where VSENSE = ICHG x RSENSE (V)
and the selection criteria usually include current capability, current gain, cost, and package dissipation. Bipolar PNP transistors are advantageous in this application because of their bidirectional blocking capability, whereas a MOSFET requires a series Schottky diode to prevent current flowing from the battery to the supply, through its body diode.
Figure 3. A typical linear charger circuit diagram
Application example 2: MOSFET gate drivers
High-current low RDS(on) MOSFETs can exhibit gate capacitances that require amps of drive current for successful high-frequency operation. Typically, the pre-driver devices supply the MOSFET via a resistor, so the gate voltage follows a characteristic RC time-constant. This time must be short enough to traverse the linear region without incurring excessive losses, but not so short as to cause EMI problems.
The average gate current during switching can be calculated:
IG = Q/t
IG is the average gate current
Q is the total gate charge, (QGS + QGD)
t is the switching transient time (ton or toff)
For example, a typical 100V, 35m&Omega ´´MOSFET requires approximately 50nC, so the gate requires 2.5 Amps to switch the device in 20ns.
The choice of gate-driver solutions includes dedicated IC drivers, logic ICs, discrete MOSFETs, and bipolar transistors. The selection criteria usually include switching speed and current capability, current gain, cost, and size. Of the options, bipolar transistors are very suitable as they feature fast switching in linear mode, high pulse-current capability, and high current
density—hence small size and cost.
One of the most popular and cost-effective drive circuits is a bipolar, non-inverting totem-pole driver:
Figure 4. Totem Pole driver stage for power MOSFET
In the example above if the MOSFET is required to switch at 1MHz from a 5V drive the worst
case power dissipation in each driver transistor is approximately:
PD = ((Vdrive * I * t * f) /´2) + (VBE * (IC / ´hFE) * Duty Cycle)
= ((5 * 2.5 * 2E-8 * 1E6) /´2) + (0.8 * 8.3 E-3 *2E-8 * 1E6)
Assuming the base current is supplied from Vdrive the losses per transistor are approximately:
PD = ((Vdrive * I * t * f) / ´2) + (Vdrive * (IC / ´hFE)* 2E-8 * 1E6) * Duty Cycle)
= ((5 * 2.5 * 2E-8 * 1E6) / ´2) + ((5 * 8.3 E-3*2E-8 * 1E6) * 0.02)
With both devices dissipating just 251.6mW, small surface-mount bipolar transistors are ideal -- preferably co-packaged as complimentary dual devices.