Mosfet Ro

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.Neglect all but the mobile inversion charge (valid for deep inversion).For the MOSFET, the charge in the semiconductor is a linear function of position along the semiconductor side of the plate. Thus, varies from 0 to V. MOSFET in saturation is: ( )2 (1) iKv V v DDS=− + GS t λ Where the value λ is a MOSFET device parameter with units of 1/V (i.e., V-1). Typically, this value is small (thus the dependence on v DS is slight), ranging from 0.005 to 0.02 V-1. Often, the channel-length modulation parameter λ. 11/5/2004 MOSFET Output Resistance.doc 2/2 Jim Stiles The Univ. Of EECS 2 2 GS GS GS GS D dds DS vV GS t ds vV GS t ds ds o di iv dv Kv V v KV V v v r λ λ = = = =− =− = where r o is defined as the MOSFET output resistance: 2 1 1 o GS t D r KV V I λ λ = − = The small signal drain current i d of a MOSFET( biased at. In saturation region,it's small signal resistor,which is also called ro=1/gds.It's usually designed to be very large because we want to get a large gain In linear region,the resistance of a MOSFET is called Rds(on).it's designed to be very small because We use MOSFET as a switch.

A P-Channel MOSFET is a type of MOSFET in which the channel of the MOSFET is composed of a majority of holes as current carriers. When the MOSFET is activated and is on, the majority of the current flowing are holes moving through the channels.

This is in contrast to the other type of MOSFET, which are N-Channel MOSFETs, in which the majority ofcurrent carriers are electrons.

Before, we go over the construction of P-Channel MOSFETs, we must go over the 2 types that exist. There are 2 types of P-Channel MOSFETs, enhancement-type MOSFETs and depletion-type MOSFETs.

A depletion-type MOSFET is normally on (maximum current flows from source to drain) when no differencein voltage exists between the gate and source terminals. However, if a voltage is applied to its gate lead, the drain-source channel becomes more resistive, until the gate voltage is so high, the transistor completely shuts off. An enhancement-type MOSFET is the opposite. It is normally off when the gate-source voltage is 0V(VGS=0). However, if a voltage is applied to its gate lead, the drain-source channel becomesless resistive.

In this article, we will go over how both P-Channel enhancement-type and depletion-type MOSFETs are constructed and operate.

How P-Channel MOSFETs Are Constructed Internally

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An P-Channel MOSFET is made up of a P channel, which is a channel composed of a majority of hole current carriers. The gate terminals are made up of N-type material.

Depending on the voltage quantity and type (negative or positive)determines how the transistor operates and whether it turns on or off.

How a P-Channel Enhancement-type MOSFET Works

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How to Turn on a P-Channel Enhancement Type MOSFET

To turn on a P-Channel Enhancement-type MOSFET, apply a positive voltage VS to the source of the MOSFET and apply a negative voltage to the gate terminal of the MOSFET (the gate must be sufficiently more negative than the threshold voltage across the drain-source region(VGDS). This will allow a current to flow through the source-drain channel.

So with a sufficient positive voltage, VS, to the source and load, and sufficient negative voltage applied to the gate, the P-Channel Enhancement-type MOSFET is fully functional and is in the active 'ON' mode of operation.

How to Turn Off a P-Channel Enhancement Type MOSFET

To turn off a P-channel enhancement type MOSFET, there are 2 steps you can take. You can either cut off the bias positive voltage, VS, that powers the source. Or you can turn off the negative voltagegoing to the gate of the transistor.

How a P-Channel Depletion-type MOSFET Works

How to Turn on a P-Channel Depletion Type MOSFET

To turn on a P-Channel Depletion-Type MOSFET, for maximum operation, the gate voltage feeding the gate terminal should be 0V. With the gate voltage being 0V, the drain current is at is largest value and the transistor is in the active 'ON'region of conduction.

So, again, to turn on a P channel depletion-type MOSFET, positive voltage is applied to the source of the p-channel MOSFET. So we power the source terminal of the MOSFET with VS, a positive voltage supply. With a sufficient positive voltage, VS, and no voltage (0V) applied to the base, the P-channel Depletion-type MOSFET is in maximum operation and has the largest current.

How to Turn Off a P-Channel Depletion Type MOSFET


To turn off a P-channel MOSFET, there are 2 steps you can take. You can either cut off the bias positivevoltage, VDD, that powers the drain. Or you can apply a negative voltage to the gate. When a negativevoltage is applied to the gate, the current is reduced. As the gate voltage, VG, becomes more negative, the current lessens until cutoff, which is when then MOSFET is in the 'OFF' condition. This stops a large source-drain current.

So ,again, as negative voltage is applied to the gate terminal of the P channel depletion-type MOSFET, the MOSFET conducts less and less current across the source-drain terminal. When the gate voltage reaches a certain negative voltage threshold, it shuts the transistor off. Negative voltage shuts the transistor off. This is for a depletion-type P-channel MOSFET.

MOSFET transistors are used for both switching and amplifying applications. MOSFETs are perhaps the most popular transistors used today. Their high input impedance makes them draw very little input current, they are easy to make, can be made very small, and consume very little power.

Related Resources

How to Build a P-Channel MOSFET Switch Circuit
N-Channel MOSFET Basics
N Channel JFET Basics
P Channel JFET Basics
Types of Transistors


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Mosfet Rout

mosfet ro
Please help me with the following questions, they have confused me a lot
1) In the spice parameters file provided by mosis (for example ), What does K' refers to, it says here that K' is (uo*Cox/2) but when i use this value for my simulations I dont get a correct result, rather this K' value is closer to uo*cox.
2) If I have to find Ro for a MOS transistor, i use formula 1/(lambda*Id), where lambda is channel length modulation factor, how can I find lambda using this spice file.
I dont want to find exact Ro for each transistor, but would like to know the approximate value which I should use for hand calculations and basic analysis.