Depletion Mode MOSFET

Fig 5.1 Depletion Mode N Channel MOSFET

The depletion mode MOSFET shown as a N channel device (P channel is also available) in Fig 5.1 is more usually made as a discrete component, i.e. a single transistor rather than IC form. In this device a thin layer of N type silicon is deposited just below the gate−insulating layer, and forms a conducting channel between source and drain.

Therefore when the gate source voltage V_GS is zero, current (in the form of free electrons) can flow between source and drain. Note that the gate is totally insulated from the channel by the layer of silicon dioxide. Now that a conducting channel is present the gate does not need to cover the full width between source and drain. Because the gate is totally insulated from the rest of the transistor this device, like other IGFETs, has a very high input resistance.

Fig. 5.2 Operation of a Depletion Mode MOSFET

In the N channel device, shown in Fig. 5.2 the gate is made negative with respect to the source, which has the effect of creating a depletion area, free from charge carriers, beneath the gate. This restricts the depth of the conducting channel, so increasing channel resistance and reducing current flow through the device.

Depletion mode MOSFETS are also available in which the gate extends the full width of the channel (from source to drain). In this case it is also possible to operate the transistor in enhancement mode. This is done by making the gate positive instead of negative. The positive voltage on the gate attracts more free electrons into the conducing channel, while at the same time repelling holes down into the P type substrate. The more positive the gate potential, the deeper, and lower resistance is the channel. Increasing positive bias therefore increases current flow. This useful depletion/enhancement version has the disadvantage that, as the gate area is increased, the gate capacitance is also larger than true depletion types. This can present difficulties at higher frequencies.

Fig. 5.3 Circuit Symbols for Depletion Mode MOSFETs (IGFETs)

Notice the solid bar between source and drain, indicating the presence of a conducting channel.
Note: Making the gate more negative reduces conduction between source & drain In N channel devices, but increases conduction between source & drain In P channel devices.

Applications of FETs

Although FETs have a lower gain than bipolar transistors, their very high input impedance makes them suitable for applications where input signals may be severely reduced if applied to a bipolar transistor base that needs base current to operate. The planar technology used to make FETs is the same as that used to make integrated circuits, so most of the transistors found in I / Cs are of this type. A useful feature of FETs is that they tend to produce less background noise than Bipolar types and so are useful in the initial stages of systems such as amplifiers; radios etc. where signal levels are very small and could be swamped by excessive background noise.

High Power FETs

FETs used in high power output stages are often seen referred to as VMOS, DMOS or TMOS. These transistors are basically the same as other IGFETs but have specialised constructions that allow them to pass currents as large as 10A. They are also able to switch on and off very quickly (in nano seconds). This allows them to be used in such circuits as switch mode power supplies where very fast switching is essential.

The MOSFET

The Metal Oxide FET - MOSFET

As well as the Junction Field Effect Transistor (JFET), there is another type of Field Effect Transistor available whose Gate input is electrically insulated from the main current carrying channel and is therefore called an Insulated Gate Field Effect Transistor or IGFET. The most common type of insulated gate FET which is used in many different types of electronic circuits is called the Metal Oxide Semiconductor Field Effect Transistor or MOSFET for short.

The IGFET or MOSFET is a voltage controlled field effect transistor that differs from a JFET in that it has a "Metal Oxide" Gate electrode which is electrically insulated from the main semiconductor N-channel or P-channel by a thin layer of insulating material usually silicon dioxide (commonly known as glass). This insulated metal gate electrode can be thought of as one plate of a capacitor. The isolation of the controlling Gate makes the input resistance of the MOSFET extremely high in the Mega-ohms ( MΩ ) region thereby making it almost infinite.

As the Gate terminal is isolated from the main current carrying channel "NO current flows into the gate" and just like the JFET, the MOSFET also acts like a voltage controlled resistor were the current flowing through the main channel between the Drain and Source is proportional to the input voltage. Also like the JFET, this very high input resistance can easily accumulate large amounts of static charge resulting in the MOSFET becoming easily damaged unless carefully handled or protected.

Like the previous JFET tutorial, MOSFETs are three terminal devices with a Gate, Drain and Source and both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available. The main difference this time is that MOSFETs are available in two basic forms:

• 1. Depletion Type   -   the transistor requires the Gate-Source voltage, ( V_GS ) to switch the device "OFF". The depletion mode MOSFET is equivalent to a "Normally Closed" switch.
•  
• 2. Enhancement Type   -   the transistor requires a Gate-Source voltage, ( V_GS ) to switch the device "ON". The enhancement mode MOSFET is equivalent to a "Normally Open" switch.

The symbols and basic construction for both configurations of MOSFETs are shown below.

The four MOSFET symbols above show an additional terminal called the Substrate and is not normally used as either an input or an output connection but instead it is used for grounding the substrate. It connects to the main semiconductive channel through a diode junction to the body or metal tab of the MOSFET. In discrete type MOSFETs, this substrate lead is connected internally to the source terminal. When this is the case, as in enhancement types it is omitted from the symbol. The line between the drain and source connections represents the semiconductive channel. If this is a solid unbroken line then this represents a "Depletion" (normally closed) type MOSFET and if the channel line is shown dotted or broken it is an "Enhancement" (normally open) type MOSFET. The direction of the arrow indicates either a P-channel or an N-channel device.

Basic MOSFET Structure and Symbol

The construction of the Metal Oxide Semiconductor FET is very different to that of the Junction FET. Both the Depletion and Enhancement type MOSFETs use an electrical field produced by a gate voltage to alter the flow of charge carriers, electrons for N-channel or holes for P-channel, through the semiconductive drain-source channel. The gate electrode is placed on top of a very thin insulating layer and there are a pair of small N-type regions just under the drain and source electrodes.

We saw in the previous tutorial, that the gate of a JFET must be biased in such a way as to forward-bias the PN-junction but with a insulated gate MOSFET device no such limitations apply so it is possible to bias the gate of a MOSFET in either polarity, +ve or -ve. This makes MOSFETs especially valuable as electronic switches or to make logic gates because with no bias they are normally non-conducting and this high gate input resistance means that very little or no control current is needed as MOSFETs are voltage controlled devices. Both the P-channel and the N-channel MOSFETs are available in two basic forms, the Enhancement type and the Depletion type.

Depletion-mode MOSFET

The Depletion-mode MOSFET, which is less common than the enhancement types is normally switched "ON" without the application of a gate bias voltage making it a "normally-closed" device. However, a gate to source voltage ( V_GS ) will switch the device "OFF". Similar to the JFET types. For an N-channel MOSFET, a "positive" gate voltage widens the channel, increasing the flow of the drain current and decreasing the drain current as the gate voltage goes more negative. In other words, for an N-channel depletion mode MOSFET: +V_GS means more electrons and more current. While a -V_GS means less electrons and less current. The opposite is also true for the P-channel types. Then the depletion mode MOSFET is equivalent to a "normally-closed" switch.

Depletion-mode N-Channel MOSFET and circuit Symbols

The depletion-mode MOSFET is constructed in a similar way to their JFET transistor counterparts were the drain-source channel is inherently conductive with the electrons and holes already present within the N-type or P-type channel. This doping of the channel produces a conducting path of low resistance between the Drain and Source with zero Gate bias.

Enhancement-mode MOSFET

The more common Enhancement-mode MOSFET is the reverse of the depletion-mode type. Here the conducting channel is lightly doped or even undoped making it non-conductive. This results in the device being normally "OFF" when the gate bias voltage is equal to zero.

A drain current will only flow when a gate voltage ( V_GS ) is applied to the gate terminal greater than the threshold voltage ( V_TH ) level in which conductance takes place making it a transconductance device. This positive +ve gate voltage pushes away the holes within the channel attracting electrons towards the oxide layer and thereby increasing the thickness of the channel allowing current to flow. This is why this kind of transistor is called an enhancement mode device as the gate voltage enhances the channel.

Increasing this positive gate voltage will cause the channel resistance to decrease further causing an increase in the drain current, I_D through the channel. In other words, for an N-channel enhancement mode MOSFET: +V_GS turns the transistor "ON", while a zero or -V_GS turns the transistor "OFF". Then, the enhancement-mode MOSFET is equivalent to a "normally-open" switch.

Enhancement-mode N-Channel MOSFET and circuit Symbols

Enhancement-mode MOSFETs make excellent electronics switches due to their low "ON" resistance and extremely high "OFF" resistance as well as their infinitely high gate resistance. Enhancement-mode MOSFETs are used in integrated circuits to produce CMOS type Logic Gates and power switching circuits in the form of as PMOS (P-channel) and NMOS (N-channel) gates. CMOS actually stands for Complementary MOS meaning that the logic device has both PMOS and NMOS within its design.

The MOSFET Amplifier

Just like the previous Junction Field Effect transistor, MOSFETs can be used to make single stage class A amplifier circuits with the Enhancement mode N-channel MOSFET common source amplifier being the most popular circuit. The depletion mode MOSFET amplifiers are very similar to the JFET amplifiers, except that the MOSFET has a much higher input impedance. This high input impedance is controlled by the gate biasing resistive network formed by R1 and R2. Also, the output signal for the enhancement mode common source MOSFET amplifier is inverted because when V_G is low the transistor is switched "OFF" and V_D (Vout) is high. When V_G is high the transistor is switched "ON" and V_D (Vout) is low as shown.

Enhancement-mode N-Channel MOSFET Amplifier

The DC biasing of this common source (CS) MOSFET amplifier circuit is virtually identical to the JFET amplifier. The MOSFET circuit is biased in class A mode by the voltage divider network formed by resistors R1 and R2. The AC input resistance is given as R_IN = R_G = 1MΩ.

Metal Oxide Semiconductor Field Effect Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The MOSFETs ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then MOSFETs have the ability to operate within three different regions:

• 1. Cut-off Region   -  with V_GS < V_threshold   the gate-source voltage is lower than the threshold voltage so the transistor is switched "fully-OFF" and I_DS = 0, the transistor acts as an open circuit
•  
• 2. Linear (Ohmic) Region   -  with V_GS > V_threshold   and V_DS > V_GS the transistor is in its constant resistance region and acts like a variable resistor whose value is determined by the gate voltage, V_GS
•  
• 3. Saturation Region   -  with V_GS > V_threshold the transistor is in its constant current region and is switched "fully-ON". The current I_DS = maximum as the transistor acts as a closed circuit

MOSFET Summary

The Metal Oxide Semiconductor FET, MOSFET has an extremely high input gate resistance with the current flowing through the channel between the source and drain being controlled by the gate voltage. Because of this high input impedance and gain, MOSFETs can be easily damaged by static electricity if not carefully protected or handled. MOSFETs are ideal for use as electronic switches or as common-source amplifiers as their power consumption is very small. Typical applications for MOSFETs are in Microprocessors, Memories, Calculators and Logic CMOS Gates etc.

Also, notice that a dotted or broken line within the symbol indicates a normally "OFF" enhancement type showing that "NO" current can flow through the channel when zero gate-source voltage V_GS is applied. A continuous unbroken line within the symbol indicates a normally "ON" Depletion type showing that current "CAN" flow through the channel with zero gate voltage. For P-channel types the symbols are exactly the same for both types except that the arrow points outwards. This can be summarised in the following switching table.

MOSFET type	VGS = +ve	VGS = 0	VGS = -ve
N-Channel Depletion	ON	ON	OFF
N-Channel Enhancement	ON	OFF	OFF
P-Channel Depletion	OFF	ON	ON
P-Channel Enhancement	OFF	OFF	ON

So for N-channel enhancement type MOSFETs, a positive gate voltage turns "ON" the transistor and with zero gate voltage, the transistor will be "OFF". For a P-channel enhancement type MOSFET, a negative gate voltage will turn "ON" the transistor and with zero gate voltage, the transistor will be "OFF". The voltage point at which the MOSFET starts to pass current through the channel is determined by the threshold voltage V_TH of the device and is typical around 0.5V to 0.7V for an N-channel device and -0.5V to -0.8V for a P-channel device.

In the next tutorial about Field Effect Transistors instead of using the transistor as an amplifying device, we will look at the operation of the transistor in its saturation and cut-off regions when used as a solid-state switch. Field effect transistor switches are used in many applications to switch a DC current "ON" or "OFF" such as LED’s which require only a few milliamps at low DC voltages, or motors which require higher currents at higher voltages.

Parallel and Serial Capacitors

How do you hook up multiple capacitors? What happens when you connect them in serial and parallel circuits? How can you increase the total voltage rating? Will serial or parallel store more total energy?

Parallel Capacitors

Capacitors connected in parallel will add their capacitance together.
C_total = C₁ + C₂ + ... + C_n
A parallel circuit is the most convenient way to increase the total storage of electric charge.
The total voltage rating does not change. Every capacitor will 'see' the same voltage. They all must be rated for at least the voltage of your power supply. Conversely, you must not apply more voltage than the lowest voltage rating among the parallel capacitors.

Series Capacitors

Capacitors connected in series will have a lower total capacitance than any single one in the circuit.
If you have only two capacitors in series this equation can be simplified to:
If you have two identical capacitors in series this is further simplified to:
This series circuit offers a higher total voltage rating. The voltage drop across each capacitor adds up to the total applied voltage.
Caution: If the capacitors are different, the voltage will divide itself such that smaller capacitors hog more of the voltage! This is because they all get the same charging current, and voltage is inversely proportional to capacitance.
Worse yet, if one capacitor is slightly leaky, it will gradually transfer its voltage to the others, possibly exceeding their voltage rating in turn. And if one of them punches through its dielectric barrier, it can then damage others in a cascading fashion. This is why series capacitors are generally avoided in power circuits.

Resistor Network for Series Capacitors

But the series network is just too attractive when you have limited money and scavenged parts. How can you build in some safety?
When you connect capacitors in series, any variance in values causes each one to charge at a different rate and to a different voltage. The variance can be quite large for electrolytics. On top of that, once the bank is charged, each capacitor's leakage current also causes a *different* voltage across each capacitor.
If you charge a series bank up all the way, some caps are always undercharged and some overcharged (not good). To help them share voltage equally, you add balancing resistors. Basically they act like a big voltage divider and counteract the effects of variance in capacitance and leakage current. And if there is no leakage current, the capacitors must eventually become charged according to the voltage divider values.
Use this equation from p.13 of this excellent guide provided by Cornell Dubilier, "Aluminum Electrolytic Capacitor Application Guide" to calculate balancing resistors:

For 2 capacitors in series:	R = (2Vm - Vb) / (0.0015 C Vb)
For N > 2 capacitors:	R = (NVm - Vb) / (0.0015 C Vb)

where

R = resistance in megohms Vm = max voltage you'll permit on either capacitor Vb = max voltage across the entire bank of two (or N) capacitors N = number of caps in series C = capacitance in microfarads

Example: Suppose you have two identical 1000uf capacitors, and connect them in series to double the voltage rating and halve the total capacitance. Let's also assume they are rated for 100 wvdc (working voltage) and 125v maximum surge. Solve the equation, using V_m = 125, and V_b = 200.
Solution: R = (2x125 - 200) / (0.0015 x 1000 x 200) = 50/300 = 0.167 M = 167 K ohms
Some related consequences in this example are...

• The resistors in this example will put a load of I = 200v/(2*167K) = 0.6mA on the charging system. No problem here, this is most likely negligible to a high-power coilgun charging system.
• The balancing resistors will trickle down the cap's charge with an RC time constant of 2min 47sec, which imeans you should keep the charger connected right up until you fire. They act as a safety 'bleeder' resistor, which ensures that the caps don't remain charged the next day (or week or month!). Also, it prevents the capacitors charging themselves up to a few volts due to the dielectric memory effect.
• Each resistor will generate heat (P = I²R) in this example of 60mW nominal, 120 mW worst case. So you should use a 1-watt resistor, or larger. Okay, a 1/2-watt resistor would work too but it may get quite warm.
• The term "0.0015 C V_b" is an estimate (in microamps) of the difference in leakage currents in two capacitors in series at the rated temperature.
• A bad capacitor with leakage current more than about 1 mA is going to over-charge the other cap. So be careful, and periodically check the actual voltages!
• A bad capacitor with an internal short is going to over-charge the other capacitors (unless it fails open) regardless of what resistors you use.

Total Energy of Series vs. Parallel

Let's see whether a series or parallel circuit can store more total energy.
Recall that energy in a single capacitor is proportional to the square of the voltage. It is tempting to use series capacitors to gain an energy boost by using 'voltage squared' to our advantage. But let's take a closer look...
Assume you have two identical capacitors, of capacitance C and voltage rating V. (The capacitors wouldn't have to be identical but the results are valid for the general case, and the math is much easier this way.) Let's calculate stored energy E for both circuits.

1. Stored energy in two parallel capacitors, charged to voltage V:
   
2. Stored energy in two series capacitors, charged to voltage 2V:
   

There is no difference! Both circuits store the same amount of energy. This should confirm a common-sense approach, that would say you can't increase total energy storage merely by reconnecting the same capacitors in different arrangements.

Conclusions

Parallel capacitors are safer and more reliable than series connections.
There is no advantage in total energy storage to choose one of these circuits over another. But! There may well be a time when you need lower capacitance (e.g. a faster timing pulse) than your parts on hand can provide.

Active device & Passive device

Active device:

A device that requires an external source of energy to be ready for operation and has an output that is a function of present and past input.
               e.g, Diodes, Transistors, PN and PIN junctions, ICs and varactors etc. we have to apply an external voltage usually called as Vcc, to these devices before placing in circuit network.




Passive device:

A device that does not require a source of energy for its operation.
             e.g, such as Resistors, Capacitors, Inductors, etc. We place these devices in the circuit, network without applying any external voltage.

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