Space Charge

Accumulation of charges in a particular region is referred to as space charge. The space in which the charges concentrate can be either free space or a dielectric. Further, this cloud of charges might be mobile or immobile in nature. Let us try to understand better with the help of examples.
Example 1: Now, consider the case where we have brought a p-type semiconductor in contact with an n-type semiconductor. As is well known, n-type semiconductor material has excess electrons while the p-type material is depleted of them. Thus, when these two kind of materials are brought in-contact, the electrons will start moving from n-type to p-type.

This causes the electrons and holes present near the junction to recombine with each other. As a result, a certain region around the junction will be depleted of mobile charge carriers. This region is nothing but the space charge region which has immobile ions (Figure 1a). space charge Example 2: Next, let us assume that we have an electron tube supplied with power. In such a situation, the electrons will be ejected from the cathode terminal and will start moving towards the anode.
However these electrons cannot reach their destination instantaneously i.e. they will take some amount of finite time to complete their journey. As a result, these electrons can accumulate near the cathode end of the device forming a cloud of negative charges. This leads to the formation of negative space charge region (Figure 1b) which can move under the influence of applied electric field.
Example 2 indicates that the basic reason which leads to the accumulation of charges is the fact that the rate of removal is less when compared to the rate of accumulation. That is, the cathode terminal emits more number of electrons than those which travel towards the anode. However, even trapping of charges, drift and diffusion can contribute for the occurrence of space charge region. Further if the polarity of the charges constituting the space charge is same as that of the electrode associated, then they are called homocharges. On the other hand, if their polarities differ from each other, then they are referred to as heterocharges.

Consequences of Space Charge

The space charge effect poses a challenge by affecting the conversion efficiency and the output power of thermionic converters. This is because when such an electron cloud is present around the metal surface, it poses an additional barrier for the electrons which are supposed to reach their final point. This inhibition for the movement of electrons is experienced in the form of repulsion for the emitted electrons from the electrons which are present in the cloud.
The space charge effect which occurs in the dielectrics also leads to the breakdown of electrical components like capacitors. This is because, when high voltages are applied, the electric charges emitted from the electrode get trapped within the gas surrounding it. The same effect is also seen to cause the failure of power cables which carry high voltages. However, space charge effect is also seen to be advantageous in certain scenarios. For example, the presence of space charge region creates a negative EMF on certain tubes which is analogous of providing a negative bias to it. This is inturn meritorious as it helps the engineers to have a better control over the process of amplification, thus improving its efficiency.
Yet another example noteworthy of being mentioned is that the space charge has a tendency to reduce the shot noise. This is because, basically the space charge affects the easy movement of charges along their path. This inturn reduces the number of charges which arrive randomly, thereby reducing their statistical variation which is nothing but the shot noise.


Closely Related Articles Amplifier Gain | Decibel or dB GainIntegrated Circuits | Types of ICRegulated Power SupplyLaser | Types and Components of LaserWork FunctionMobility of Charge CarrierWhat are Photo Electrons? Electron volt or eVEnergy Quanta | Development of Quantum Physics Schottky EffectHeisenberg Uncertainty PrincipleSchrodinger Wave Equation and Wave FunctionCyclotron Basic Construction and Working PrincipleSinusoidal Wave SignalCommon Emitter AmplifierRC Coupled AmplifierDifferential AmplifierWave Particle Duality PrincipleMore Related Articles Vacuum Diode History Working Principle and Types of Vacuum DiodePN Junction Diode and its CharacteristicsDiode | Working and Types of DiodeDiode CharacteristicsHalf Wave Diode RectifierFull Wave Diode RectifierDiode Bridge RectifierWhat is Zener Diode?Application of Zener DiodeLED or Light Emitting DiodePIN Photodiode | Avalanche PhotodiodeTunnel Diode and its ApplicationsGUNN DiodeVaractor DiodeLaser DiodeSchottky DiodePower DiodesDiode ResistanceDiode Current EquationIdeal DiodeReverse Recovery Time of DiodeDiode TestingMOSFET | Working Principle of p-channel n-channel MOSFETMOSFET CircuitsMOS Capacitor | MOS Capacitance C V CurveApplications of MOSFETMOSFET as a SwitchMOSFET CharacteristicsPower MOSFETHalf Wave RectifiersFull Wave RectifiersBridge RectifiersClamping CircuitTheory of SemiconductorIntrinsic SemiconductorExtrinsic SemiconductorsEnergy Bands of SiliconDonor and Acceptor Impurities in Semiconductor Conductivity of SemiconductorCurrent Density in Metal and Semiconductor Intrinsic Silicon and Extrinsic SiliconP Type SemiconductorN Type SemiconductorP N Junction Theory Behind P N JunctionForward and Reverse Bias of P N JunctionZener BreakdownAvalanche BreakdownHall Effect Applications of Hall EffectGallium Arsenide SemiconductorSilicon SemiconductorTypes of TransistorsBipolar Junction Transistor or BJTBiasing of Bipolar Junction Transistor or BJTTransistor BiasingTransistor CharacteristicsCurrent Components in a TransistorTransistor Manufacturing TechniquesApplications of Bipolar Junction Transistor or BJT | History of BJTTransistor as a SwitchTransistor as an AmplifierJFET or Junction Field Effect Transistorn-channel JFET and p-channel JFETApplications of Field Effect TransistorDIAC Construction Operation and Applications of DIACTRIAC Construction Operation and Applications of TRIACPhototransistorNew Articles Trees and Cotrees of Electric NetworkDifferentiatorIntegratorPhase Synchronizing Device or Controlled Switching DeviceDigital to Analog Converter or DACDifference Amplifier